diff --git a/onion-architecture/README.md b/onion-architecture/README.md
new file mode 100644
index 000000000000..1a7fc394638d
--- /dev/null
+++ b/onion-architecture/README.md
@@ -0,0 +1,292 @@
+---
+title: "Onion Architecture in Java: A Layered Approach to Building Maintainable and Testable Applications"
+shortTitle: Onion Architecture
+description: "Learn how the Onion Architecture pattern promotes maintainability, testability, and separation of concerns in Java applications. Explore examples, benefits, and best practices."
+category: Architectural
+language: en
+tag:
+  - Decoupling
+  - Enterprise patterns
+  - Integration
+  - Microservices
+  - Scalability
+  - Security
+---
+## Intent of Microservices API Gateway Design Pattern
+
+In this project, the implementation demonstrates **Onion Architecture** with clear dependency direction: infrastructure and application depend on domain, but domain is independent.
+
+The central intent is to keep business rules in `domain`, orchestrate use cases in `application`, and isolate delivery/persistence/API details in `infrastructure`.
+
+Current implementation highlights:
+
+* `domain`: `Person`, `Category`, `PersonRepository`, `DomainException`
+* `application`: `SavePersonUseCase`, `GetPersonUseCase`, DTO records
+* `infrastructure`: Spring Boot REST controller, JPA entities, repository adapter, bean wiring
+
+## Also known as
+
+* Ports and Adapters Architecture
+* Hexagonal-style layering (conceptually related)
+* Dependency-rule-first architecture
+
+## Detailed Explanation of Onion Architecture Pattern with Real-World Examples
+
+Real-world example
+
+> Imagine a people-management service where business validation must stay consistent no matter how data is stored or exposed. In this codebase, `Person` and `Category` enforce invariants (e.g., age >= 18, required email/category), use cases coordinate behavior, and infrastructure adapts HTTP + JPA concerns. This allows the persistence or web layer to evolve without changing core domain rules.
+
+In plain words
+
+> The project keeps business logic in the center and treats frameworks as replaceable details around it.
+
+Wikipedia says
+
+> Onion Architecture is a software architecture pattern that emphasizes separation of concerns and dependency inversion by organizing code in concentric layers, with the domain model at the center.
+
+Sequence diagram
+
+![Onion Architecture class diagram](./etc/onion-architecture.png)
+
+Request flow in this implementation:
+
+1. Client calls REST endpoint in `PersonController` (`/api/persons`, `/api/persons/{id}`)
+2. Controller delegates to `SavePersonUseCase` or `GetPersonUseCase`
+3. Use case interacts with `PersonRepository` abstraction from `domain`
+4. `PersonRepositoryAdapter` maps domain <-> JPA and delegates to `SpringDataPersonRepository`
+5. Response DTO (`PersonResponse`) is returned to the client
+
+## Programmatic Example of Onion Architecture in Java
+
+This repository exposes a simple person API backed by use cases and domain models.
+
+Controller (infrastructure layer):
+
+```java
+@RestController
+@RequestMapping("/api")
+public class PersonController {
+
+    @GetMapping("/persons/{id}")
+    public ResponseEntity<PersonResponse> getPerson(@PathVariable Long id) {
+        var person = getPersonUseCase.execute(id);
+        return ResponseEntity.ok(person);
+    }
+
+    @GetMapping("/persons")
+    public ResponseEntity<List<PersonResponse>> getAllPersons() {
+        var persons = getPersonUseCase.executeAll();
+        return ResponseEntity.ok(persons);
+    }
+
+    @PostMapping("/persons")
+    public ResponseEntity<PersonResponse> savePerson(@RequestBody SavePersonCommand command) {
+        try {
+            var savedPerson = savePersonUseCase.execute(command);
+            return ResponseEntity.status(HttpStatus.OK).body(savedPerson);
+        } catch (DomainException e) {
+            return ResponseEntity.status(HttpStatus.BAD_REQUEST).build();
+        }
+    }
+}
+```
+
+Use case (application layer):
+
+```java
+public class SavePersonUseCase {
+
+    private final PersonRepository repository;
+
+    public PersonResponse execute(SavePersonCommand command) {
+        var category = new Category(command.categoryId(), command.categoryType());
+        var person = new Person(
+                null,
+                command.firstName(),
+                command.lastName(),
+                command.age(),
+                command.phoneNumber(),
+                command.email(),
+                category);
+
+        var savedPerson = repository.save(person);
+        return new PersonResponse(
+                savedPerson.getId(),
+                savedPerson.getFirstName(),
+                savedPerson.getLastName(),
+                savedPerson.getAge(),
+                savedPerson.getPhoneNumber(),
+                savedPerson.getEmail(),
+                savedPerson.getCategory().getId(),
+                savedPerson.getCategory().getType()
+        );
+    }
+}
+```
+
+Domain validation (domain layer):
+
+```java
+public class Person {
+    public Person(Long id, String firstName, String lastName, int age,
+                  String phoneNumber, String email, Category category) {
+        validateNames(firstName, lastName);
+        validateAge(age);
+        validatePhone(phoneNumber);
+        validateEmail(email);
+        validateCategory(category);
+        // assign fields...
+    }
+}
+```
+
+Repository adapter (infrastructure -> domain port):
+
+```java
+@Repository
+public class PersonRepositoryAdapter implements PersonRepository {
+
+    private final SpringDataPersonRepository repository;
+
+    @Override
+    public Optional<Person> findById(Long id) {
+        return repository.findById(id).map(this::mapToDomain);
+    }
+
+    @Override
+    public Person save(Person person) {
+        JpaPersonEntity savedEntity = repository.save(mapToEntity(person));
+        return mapToDomain(savedEntity);
+    }
+}
+```
+
+- **Maven 3.6.0** or higher
+
+### Build Steps
+
+1. **Navigate to the onion-architecture module directory:**
+   ```bash
+   cd java-design-patterns/onion-architecture
+   ```
+
+2. **Build all modules:**
+   ```bash
+   mvn clean package
+   ```
+   This will compile the `domain`, `application`, and `infrastructure` modules and package them into a Spring Boot executable JAR.
+
+3. **Run the Spring Boot application:**
+   ```bash
+   mvn -pl infrastructure spring-boot:run
+   ```
+   Alternatively, after building, run the JAR directly:
+   ```bash
+   java -jar infrastructure/target/infrastructure-1.26.0-SNAPSHOT.jar
+   ```
+
+### Accessing the API
+
+The application exposes REST endpoints at `http://localhost:8080/api`:
+There is a Postman collection available in the `etc/postman` folder for testing the API.
+
+- **Get all persons:**
+  ```bash
+  GET http://localhost:8080/api/persons
+  ```
+
+- **Get person by ID:**
+  ```bash
+  GET http://localhost:8080/api/persons/{id}
+  ```
+
+- **Create a new person:**
+  ```bash
+  POST http://localhost:8080/api/persons
+  Content-Type: application/json
+
+  {
+    "firstName": "John",
+    "lastName": "Doe",
+    "age": 30,
+    "phoneNumber": "555-1234",
+    "email": "john.doe@example.com",
+    "address": "123 Main St",
+    "categoryId": 1,
+    "categoryType": "individual"
+  }
+  ```
+
+### Run Tests
+
+To execute unit tests across all modules:
+
+```bash
+mvn clean test
+```
+
+To run tests for a specific module:
+
+```bash
+mvn -pl domain test
+mvn -pl application test
+mvn -pl infrastructure test
+```
+
+### Database
+
+The application uses an **H2 in-memory database** for demonstration purposes. Configuration is in `infrastructure/src/main/resources/application.properties`:
+
+- **JDBC URL:** `jdbc:h2:mem:testdb`
+- **Username:** `sa`
+- **Password:** `password`
+
+Sample data is initialized from `infrastructure/src/main/resources/data.sql` on application startup.
+
+## When to Use the Onion Architecture Pattern in Java
+
+* When domain rules must be stable and independent from frameworks.
+* When you want use cases to be testable without HTTP or database setup.
+* When infrastructure details (web, JPA, database) should be replaceable.
+* When dependency direction must be enforced from outer layers toward the domain core.
+
+## Onion Architecture Pattern Java Tutorials
+
+* [Clean Architecture with Spring Boot (Baeldung)](https://www.baeldung.com/spring-boot-clean-architecture)
+* [Hexagonal Architecture Explained (Cockburn)](https://alistair.cockburn.us/hexagonal-architecture)
+* [Spring Data JPA Reference](https://docs.spring.io/spring-data/jpa/reference/)
+
+## Benefits and Trade-offs of Microservices API Gateway Pattern
+
+Benefits:
+
+* Business validations are centralized in domain constructors (`Person`, `Category`).
+* Use cases stay independent from Spring, JPA, and transport concerns.
+* Repository abstraction (`PersonRepository`) keeps application logic persistence-agnostic.
+* Testability is strong across layers (domain, use case, adapter, controller tests).
+
+Trade-offs:
+
+* Additional mapping code between domain models, DTOs, and JPA entities.
+* More classes and modules than a simple CRUD-by-controller approach.
+* Requires discipline to avoid leaking infrastructure concerns into domain/application.
+
+## Real-World Applications of Microservices API Gateway Pattern in Java
+
+* People/contact management services with strict data validation.
+* Internal platforms where multiple delivery mechanisms (REST, batch, messaging) can share the same core domain.
+* Systems that need incremental infrastructure evolution while preserving business logic.
+
+## Related Java Design Patterns
+
+* [Repository](https://martinfowler.com/eaaCatalog/repository.html) - `PersonRepository` defines the domain-facing persistence contract.
+* [Adapter](https://refactoring.guru/design-patterns/adapter) - `PersonRepositoryAdapter` bridges domain model and Spring Data JPA.
+* [Dependency Injection](https://docs.spring.io/spring-framework/reference/core/beans/dependencies/factory-collaborators.html) - `ApplicationConfig` wires use case beans with repository implementations.
+
+## References and Credits
+
+* Project modules: `domain`, `application`, `infrastructure`
+* Java 21 + Maven multi-module setup
+* Spring Boot 3.3 (`spring-boot-starter-web`, `spring-boot-starter-data-jpa`, H2)
+* Layer-focused tests in each module validating domain invariants and use case behavior
+
diff --git a/onion-architecture/application/pom.xml b/onion-architecture/application/pom.xml
new file mode 100644
index 000000000000..eccdaae2869a
--- /dev/null
+++ b/onion-architecture/application/pom.xml
@@ -0,0 +1,66 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>onion-architecture</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>application</artifactId>
+    <name>Application</name>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.iluwatar</groupId>
+            <artifactId>domain</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <!-- JUnit 5 for testing -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- Mockito for mocking -->
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-junit-jupiter</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>
diff --git a/onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/PersonResponse.java b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/PersonResponse.java
new file mode 100644
index 000000000000..1586f30e48a1
--- /dev/null
+++ b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/PersonResponse.java
@@ -0,0 +1,37 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.dto;
+
+public record PersonResponse(
+    Long id,
+    String firstName,
+    String lastName,
+    int age,
+    String phoneNumber,
+    String email,
+    Long categoryId,
+    String categoryType) {}
diff --git a/onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/SavePersonCommand.java b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/SavePersonCommand.java
new file mode 100644
index 000000000000..649db88a4ab5
--- /dev/null
+++ b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/SavePersonCommand.java
@@ -0,0 +1,37 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.dto;
+
+public record SavePersonCommand(
+    String firstName,
+    String lastName,
+    int age,
+    String phoneNumber,
+    String email,
+    String address,
+    Long categoryId,
+    String categoryType) {}
diff --git a/onion-architecture/application/src/main/java/com/iluwatar/onion/application/usecase/GetPersonUseCase.java b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/usecase/GetPersonUseCase.java
new file mode 100644
index 000000000000..abae6a32196a
--- /dev/null
+++ b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/usecase/GetPersonUseCase.java
@@ -0,0 +1,75 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.usecase;
+
+import com.iluwatar.onion.application.dto.PersonResponse;
+import com.iluwatar.onion.domain.repository.PersonRepository;
+import java.util.Collection;
+import java.util.List;
+
+public class GetPersonUseCase {
+
+  private final PersonRepository repository;
+
+  public GetPersonUseCase(PersonRepository repository) {
+    this.repository = repository;
+  }
+
+  public PersonResponse execute(Long id) {
+    var person =
+        repository
+            .findById(id)
+            .orElseThrow(() -> new RuntimeException("Person not found with id: " + id));
+
+    return new PersonResponse(
+        person.getId(),
+        person.getFirstName(),
+        person.getLastName(),
+        person.getAge(),
+        person.getPhoneNumber(),
+        person.getEmail(),
+        person.getCategory().getId(),
+        person.getCategory().getType());
+  }
+
+  public List<PersonResponse> executeAll() {
+    return repository.findAll().stream()
+        .flatMap(Collection::stream)
+        .map(
+            person ->
+                new PersonResponse(
+                    person.getId(),
+                    person.getFirstName(),
+                    person.getLastName(),
+                    person.getAge(),
+                    person.getPhoneNumber(),
+                    person.getEmail(),
+                    person.getCategory().getId(),
+                    person.getCategory().getType()))
+        .toList();
+  }
+}
diff --git a/onion-architecture/application/src/main/java/com/iluwatar/onion/application/usecase/SavePersonUseCase.java b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/usecase/SavePersonUseCase.java
new file mode 100644
index 000000000000..8d63e2612fab
--- /dev/null
+++ b/onion-architecture/application/src/main/java/com/iluwatar/onion/application/usecase/SavePersonUseCase.java
@@ -0,0 +1,67 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.usecase;
+
+import com.iluwatar.onion.application.dto.PersonResponse;
+import com.iluwatar.onion.application.dto.SavePersonCommand;
+import com.iluwatar.onion.domain.model.Category;
+import com.iluwatar.onion.domain.model.Person;
+import com.iluwatar.onion.domain.repository.PersonRepository;
+
+public class SavePersonUseCase {
+
+  private final PersonRepository repository;
+
+  public SavePersonUseCase(PersonRepository repository) {
+    this.repository = repository;
+  }
+
+  public PersonResponse execute(SavePersonCommand command) {
+    var category = new Category(command.categoryId(), command.categoryType());
+    var person =
+        new Person(
+            null,
+            command.firstName(),
+            command.lastName(),
+            command.age(),
+            command.phoneNumber(),
+            command.email(),
+            category);
+
+    var savedPerson = repository.save(person);
+
+    return new PersonResponse(
+        savedPerson.getId(),
+        savedPerson.getFirstName(),
+        savedPerson.getLastName(),
+        savedPerson.getAge(),
+        savedPerson.getPhoneNumber(),
+        savedPerson.getEmail(),
+        savedPerson.getCategory().getId(),
+        savedPerson.getCategory().getType());
+  }
+}
diff --git a/onion-architecture/application/src/test/java/com/iluwatar/onion/application/usecase/GetPersonUseCaseTest.java b/onion-architecture/application/src/test/java/com/iluwatar/onion/application/usecase/GetPersonUseCaseTest.java
new file mode 100644
index 000000000000..f9de119c2309
--- /dev/null
+++ b/onion-architecture/application/src/test/java/com/iluwatar/onion/application/usecase/GetPersonUseCaseTest.java
@@ -0,0 +1,193 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.usecase;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+import com.iluwatar.onion.application.dto.PersonResponse;
+import com.iluwatar.onion.domain.model.Category;
+import com.iluwatar.onion.domain.model.Person;
+import com.iluwatar.onion.domain.repository.PersonRepository;
+import java.util.List;
+import java.util.Optional;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class GetPersonUseCaseTest {
+
+  @Mock private PersonRepository personRepository;
+
+  private GetPersonUseCase getPersonUseCase;
+
+  @BeforeEach
+  void setUp() {
+    getPersonUseCase = new GetPersonUseCase(personRepository);
+  }
+
+  @Nested
+  @DisplayName("Execute by ID - Happy Path")
+  class ExecuteByIdHappyPath {
+
+    @Test
+    @DisplayName("Should return PersonResponse when person exists")
+    void shouldReturnPersonResponseWhenPersonExists() {
+      // Arrange
+      var person =
+          new Person(
+              1L,
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "john.doe@example.com",
+              new Category(1L, "Professional"));
+
+      when(personRepository.findById(1L)).thenReturn(Optional.of(person));
+
+      // Act
+      var response = getPersonUseCase.execute(1L);
+
+      // Assert
+      assertNotNull(response);
+      assertEquals(1L, response.id());
+      assertEquals("John", response.firstName());
+      assertEquals("Doe", response.lastName());
+      assertEquals(25, response.age());
+      assertEquals("+1234567890", response.phoneNumber());
+      assertEquals("john.doe@example.com", response.email());
+      assertEquals(1L, response.categoryId());
+      assertEquals("Professional", response.categoryType());
+
+      verify(personRepository, times(1)).findById(1L);
+    }
+  }
+
+  @Nested
+  @DisplayName("Execute by ID - Error Cases")
+  class ExecuteByIdErrorCases {
+
+    @Test
+    @DisplayName("Should throw RuntimeException when person not found")
+    void shouldThrowExceptionWhenPersonNotFound() {
+      // Arrange
+      when(personRepository.findById(999L)).thenReturn(Optional.empty());
+
+      // Act & Assert
+      var exception = assertThrows(RuntimeException.class, () -> getPersonUseCase.execute(999L));
+      assertTrue(exception.getMessage().contains("Person not found with id: 999"));
+
+      verify(personRepository, times(1)).findById(999L);
+    }
+
+    @Test
+    @DisplayName("Should propagate exception when repository throws exception")
+    void shouldPropagateExceptionWhenRepositoryFails() {
+      // Arrange
+      when(personRepository.findById(anyLong())).thenThrow(new RuntimeException("Database error"));
+
+      // Act & Assert
+      var exception = assertThrows(RuntimeException.class, () -> getPersonUseCase.execute(1L));
+      assertTrue(exception.getMessage().contains("Database error"));
+
+      verify(personRepository, times(1)).findById(1L);
+    }
+  }
+
+  @Nested
+  @DisplayName("Execute All - Happy Path")
+  class ExecuteAllHappyPath {
+
+    @Test
+    @DisplayName("Should return list of PersonResponses")
+    void shouldReturnListOfPersonResponses() {
+      // Arrange
+      var person1 =
+          new Person(
+              1L,
+              "John",
+              "Doe",
+              30,
+              "+9876543255",
+              "john.doe@example.com",
+              new Category(2L, "Personal"));
+
+      var person2 =
+          new Person(
+              2L,
+              "Jane",
+              "Smith",
+              25,
+              "+9876543210",
+              "jane.smith@example.com",
+              new Category(2L, "Personal"));
+
+      when(personRepository.findAll()).thenReturn(Optional.of(List.of(person1, person2)));
+
+      // Act
+      var responses = getPersonUseCase.executeAll();
+
+      // Assert
+      assertNotNull(responses);
+      assertEquals(2, responses.size());
+
+      PersonResponse response1 = responses.get(0);
+      assertEquals(1L, response1.id());
+      assertEquals("John", response1.firstName());
+      assertEquals("Doe", response1.lastName());
+
+      PersonResponse response2 = responses.get(1);
+      assertEquals(2L, response2.id());
+      assertEquals("Jane", response2.firstName());
+      assertEquals("Smith", response2.lastName());
+
+      verify(personRepository, times(1)).findAll();
+    }
+
+    @Test
+    @DisplayName("Should return empty list when no persons found")
+    void shouldReturnEmptyListWhenNoPersonsFound() {
+      // Arrange
+      when(personRepository.findAll()).thenReturn(Optional.of(List.of()));
+
+      // Act
+      List<PersonResponse> responses = getPersonUseCase.executeAll();
+
+      // Assert
+      assertNotNull(responses);
+      assertTrue(responses.isEmpty());
+
+      verify(personRepository, times(1)).findAll();
+    }
+  }
+}
diff --git a/onion-architecture/application/src/test/java/com/iluwatar/onion/application/usecase/SavePersonUseCaseTest.java b/onion-architecture/application/src/test/java/com/iluwatar/onion/application/usecase/SavePersonUseCaseTest.java
new file mode 100644
index 000000000000..434147ae6495
--- /dev/null
+++ b/onion-architecture/application/src/test/java/com/iluwatar/onion/application/usecase/SavePersonUseCaseTest.java
@@ -0,0 +1,253 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.usecase;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+import com.iluwatar.onion.application.dto.SavePersonCommand;
+import com.iluwatar.onion.domain.exception.DomainException;
+import com.iluwatar.onion.domain.model.Category;
+import com.iluwatar.onion.domain.model.Person;
+import com.iluwatar.onion.domain.repository.PersonRepository;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Captor;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class SavePersonUseCaseTest {
+
+  @Mock private PersonRepository personRepository;
+
+  @Captor private ArgumentCaptor<Person> personCaptor;
+
+  private SavePersonUseCase savePersonUseCase;
+
+  @BeforeEach
+  void setUp() {
+    savePersonUseCase = new SavePersonUseCase(personRepository);
+  }
+
+  @Nested
+  @DisplayName("Execute - Happy Path")
+  class ExecuteHappyPath {
+
+    @Test
+    @DisplayName("Should save person and return PersonResponse")
+    void shouldSavePersonAndReturnResponse() {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "john.doe@example.com",
+              "123 Main St",
+              1L,
+              "Professional");
+
+      var category = new Category(1L, "Professional");
+      var savedPerson =
+          new Person(
+              100L, // ID generated by database
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "john.doe@example.com",
+              category);
+
+      when(personRepository.save(any(Person.class))).thenReturn(savedPerson);
+
+      // Act
+      var response = savePersonUseCase.execute(command);
+
+      // Assert
+      assertNotNull(response);
+      assertEquals(100L, response.id());
+      assertEquals("John", response.firstName());
+      assertEquals("Doe", response.lastName());
+      assertEquals(25, response.age());
+      assertEquals("+1234567890", response.phoneNumber());
+      assertEquals("john.doe@example.com", response.email());
+      assertEquals(1L, response.categoryId());
+      assertEquals("Professional", response.categoryType());
+
+      // Verify repository interaction
+      verify(personRepository, times(1)).save(any(Person.class));
+    }
+
+    @Test
+    @DisplayName("Should pass correct Person object to repository")
+    void shouldPassCorrectPersonToRepository() {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "Jane",
+              "Smith",
+              30,
+              "+9876543210",
+              "jane.smith@example.com",
+              "456 Oak Ave",
+              2L,
+              "Personal");
+
+      var category = new Category(2L, "Personal");
+      var savedPerson =
+          new Person(200L, "Jane", "Smith", 30, "+9876543210", "jane.smith@example.com", category);
+
+      when(personRepository.save(any(Person.class))).thenReturn(savedPerson);
+
+      // Act
+      savePersonUseCase.execute(command);
+
+      // Assert - Verify what was passed to repository
+      verify(personRepository).save(personCaptor.capture());
+      var capturedPerson = personCaptor.getValue();
+
+      assertNull(capturedPerson.getId()); // New person should have null ID
+      assertEquals("Jane", capturedPerson.getFirstName());
+      assertEquals("Smith", capturedPerson.getLastName());
+      assertEquals(30, capturedPerson.getAge());
+      assertEquals("+9876543210", capturedPerson.getPhoneNumber());
+      assertEquals("jane.smith@example.com", capturedPerson.getEmail());
+      assertEquals("Personal", capturedPerson.getCategory().getType());
+    }
+  }
+
+  @Nested
+  @DisplayName("Execute - Validation Failures")
+  class ExecuteValidationFailures {
+
+    @Test
+    @DisplayName("Should throw DomainException when age is less than 18")
+    void shouldThrowExceptionWhenAgeIsInvalid() {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "Young",
+              "Person",
+              17, // Invalid age
+              "+1234567890",
+              "young@example.com",
+              "123 Main St",
+              1L,
+              "Student");
+
+      // Act & Assert
+      var exception = assertThrows(DomainException.class, () -> savePersonUseCase.execute(command));
+      assertTrue(exception.getMessage().contains("Age cannot be less than 18"));
+
+      // Verify repository was never called
+      verify(personRepository, never()).save(any(Person.class));
+    }
+
+    @Test
+    @DisplayName("Should throw DomainException when email is empty")
+    void shouldThrowExceptionWhenEmailIsEmpty() {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "", // Empty email
+              "123 Main St",
+              1L,
+              "Professional");
+
+      // Act & Assert
+      var exception = assertThrows(DomainException.class, () -> savePersonUseCase.execute(command));
+      assertTrue(exception.getMessage().contains("Email cannot be null or empty"));
+
+      verify(personRepository, never()).save(any(Person.class));
+    }
+
+    @Test
+    @DisplayName("Should throw DomainException when category type is invalid")
+    void shouldThrowExceptionWhenCategoryTypeIsInvalid() {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "john@example.com",
+              "123 Main St",
+              1L,
+              "" // Empty category type
+              );
+
+      // Act & Assert
+      var exception = assertThrows(DomainException.class, () -> savePersonUseCase.execute(command));
+      assertTrue(exception.getMessage().contains("Type is null or empty"));
+
+      verify(personRepository, never()).save(any(Person.class));
+    }
+  }
+
+  @Nested
+  @DisplayName("Execute - Repository Exceptions")
+  class ExecuteRepositoryExceptions {
+
+    @Test
+    @DisplayName("Should propagate exception when repository fails")
+    void shouldPropagateExceptionWhenRepositoryFails() {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "john.doe@example.com",
+              "123 Main St",
+              1L,
+              "Professional");
+
+      when(personRepository.save(any(Person.class)))
+          .thenThrow(new RuntimeException("Database connection failed"));
+
+      // Act & Assert
+      var exception =
+          assertThrows(RuntimeException.class, () -> savePersonUseCase.execute(command));
+      assertTrue(exception.getMessage().contains("Database connection failed"));
+
+      verify(personRepository, times(1)).save(any(Person.class));
+    }
+  }
+}
diff --git a/onion-architecture/domain/pom.xml b/onion-architecture/domain/pom.xml
new file mode 100644
index 000000000000..d1efc50def12
--- /dev/null
+++ b/onion-architecture/domain/pom.xml
@@ -0,0 +1,47 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>onion-architecture</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>domain</artifactId>
+    <name>Domain</name>
+
+    <dependencies>
+        <!-- JUnit 5 for testing -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>
diff --git a/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/exception/DomainException.java b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/exception/DomainException.java
new file mode 100644
index 000000000000..5e0831a57c02
--- /dev/null
+++ b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/exception/DomainException.java
@@ -0,0 +1,34 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.domain.exception;
+
+public class DomainException extends RuntimeException {
+
+  public DomainException(String message) {
+    super(message);
+  }
+}
diff --git a/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/model/Category.java b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/model/Category.java
new file mode 100644
index 000000000000..8380f4e849ee
--- /dev/null
+++ b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/model/Category.java
@@ -0,0 +1,51 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.domain.model;
+
+import com.iluwatar.onion.domain.exception.DomainException;
+
+public class Category {
+
+  private final Long id;
+  private final String type;
+
+  public Category(Long id, String type) {
+    if (type == null || type.isEmpty()) {
+      throw new DomainException("Type is null or empty. Category type is required.");
+    }
+    this.id = id;
+    this.type = type;
+  }
+
+  public Long getId() {
+    return id;
+  }
+
+  public String getType() {
+    return type;
+  }
+}
diff --git a/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/model/Person.java b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/model/Person.java
new file mode 100644
index 000000000000..55eced0be29a
--- /dev/null
+++ b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/model/Person.java
@@ -0,0 +1,121 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.domain.model;
+
+import com.iluwatar.onion.domain.exception.DomainException;
+
+public class Person {
+
+  private final Long id;
+  private final String firstName;
+  private final String lastName;
+  private final int age;
+  private final String phoneNumber;
+  private final String email;
+  private final Category category;
+
+  public Person(
+      Long id,
+      String firstName,
+      String lastName,
+      int age,
+      String phoneNumber,
+      String email,
+      Category category) {
+    validateNames(firstName, lastName);
+    validateAge(age);
+    validatePhone(phoneNumber);
+    validateEmail(email);
+    validateCategory(category);
+
+    this.id = id;
+    this.firstName = firstName;
+    this.lastName = lastName;
+    this.age = age;
+    this.phoneNumber = phoneNumber;
+    this.email = email;
+    this.category = category;
+  }
+
+  private void validateNames(String firstName, String lastName) {
+    if (firstName == null || lastName == null) {
+      throw new DomainException("First name and last name cannot be null.");
+    }
+  }
+
+  private void validateAge(int age) {
+    if (age < 18) {
+      throw new DomainException("Age cannot be less than 18.");
+    }
+  }
+
+  private void validatePhone(String phone) {
+    if (phone == null || phone.isEmpty()) {
+      throw new DomainException("Phone number cannot be null or empty.");
+    }
+  }
+
+  private void validateEmail(String email) {
+    if (email == null || email.isEmpty()) {
+      throw new DomainException("Email cannot be null or empty.");
+    }
+  }
+
+  private void validateCategory(Category category) {
+    if (category == null || category.getType().isEmpty()) {
+      throw new DomainException("Category cannot be null or empty.");
+    }
+  }
+
+  public Long getId() {
+    return id;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+
+  public int getAge() {
+    return age;
+  }
+
+  public String getPhoneNumber() {
+    return phoneNumber;
+  }
+
+  public String getEmail() {
+    return email;
+  }
+
+  public Category getCategory() {
+    return category;
+  }
+}
diff --git a/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/repository/PersonRepository.java b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/repository/PersonRepository.java
new file mode 100644
index 000000000000..c14aa24a8ef6
--- /dev/null
+++ b/onion-architecture/domain/src/main/java/com/iluwatar/onion/domain/repository/PersonRepository.java
@@ -0,0 +1,45 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.domain.repository;
+
+import com.iluwatar.onion.domain.model.Person;
+import java.util.List;
+import java.util.Optional;
+
+public interface PersonRepository {
+  Optional<Person> findById(Long id);
+
+  Optional<Person> findByFirstName(String firstName);
+
+  Optional<Person> findByLastName(String lastName);
+
+  Optional<List<Person>> findAll();
+
+  Person save(Person person);
+
+  boolean deleteById(Long id);
+}
diff --git a/onion-architecture/domain/src/test/java/com/iluwatar/onion/domain/model/CategoryTest.java b/onion-architecture/domain/src/test/java/com/iluwatar/onion/domain/model/CategoryTest.java
new file mode 100644
index 000000000000..82e88285c35b
--- /dev/null
+++ b/onion-architecture/domain/src/test/java/com/iluwatar/onion/domain/model/CategoryTest.java
@@ -0,0 +1,75 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.domain.model;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import com.iluwatar.onion.domain.exception.DomainException;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class CategoryTest {
+
+  @Test
+  @DisplayName("Should create category with valid data")
+  void shouldCreateCategoryWithValidData() {
+    // Arrange & Act
+    var category = new Category(1L, "Professional");
+
+    // Assert
+    assertNotNull(category);
+    assertEquals(1L, category.getId());
+    assertEquals("Professional", category.getType());
+  }
+
+  @Test
+  @DisplayName("Should create category with null id")
+  void shouldCreateCategoryWithNullId() {
+    // Arrange & Act
+    var category = new Category(null, "Personal");
+
+    // Assert
+    assertNull(category.getId());
+    assertEquals("Personal", category.getType());
+  }
+
+  @Test
+  @DisplayName("Should throw exception when type is null")
+  void shouldThrowExceptionWhenTypeIsNull() {
+    // Act & Assert
+    var exception = assertThrows(DomainException.class, () -> new Category(1L, null));
+    assertTrue(exception.getMessage().contains("Type is null or empty"));
+  }
+
+  @Test
+  @DisplayName("Should throw exception when type is empty")
+  void shouldThrowExceptionWhenTypeIsEmpty() {
+    // Act & Assert
+    var exception = assertThrows(DomainException.class, () -> new Category(1L, ""));
+    assertTrue(exception.getMessage().contains("Type is null or empty"));
+  }
+}
diff --git a/onion-architecture/domain/src/test/java/com/iluwatar/onion/domain/model/PersonTest.java b/onion-architecture/domain/src/test/java/com/iluwatar/onion/domain/model/PersonTest.java
new file mode 100644
index 000000000000..8f5bef0da1a6
--- /dev/null
+++ b/onion-architecture/domain/src/test/java/com/iluwatar/onion/domain/model/PersonTest.java
@@ -0,0 +1,179 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.domain.model;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import com.iluwatar.onion.domain.exception.DomainException;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+class PersonTest {
+
+  private final Category validCategory = new Category(1L, "Professional");
+
+  @Nested
+  @DisplayName("Person Creation - Valid Cases")
+  class ValidPersonCreation {
+
+    @Test
+    @DisplayName("Should create person with valid data")
+    void shouldCreatePersonWithValidData() {
+      // Arrange & Act
+      var person =
+          new Person(1L, "John", "Doe", 25, "+1234567890", "john.doe@example.com", validCategory);
+
+      // Assert
+      assertNotNull(person);
+      assertEquals(1L, person.getId());
+      assertEquals("John", person.getFirstName());
+      assertEquals("Doe", person.getLastName());
+      assertEquals(25, person.getAge());
+      assertEquals("+1234567890", person.getPhoneNumber());
+      assertEquals("john.doe@example.com", person.getEmail());
+      assertEquals(validCategory, person.getCategory());
+    }
+
+    @Test
+    @DisplayName("Should create person with minimum valid age (18)")
+    void shouldCreatePersonWithMinimumValidAge() {
+      // Arrange & Act
+      var person =
+          new Person(
+              null,
+              "Jane",
+              "Smith",
+              18, // minimum valid age
+              "+9876543210",
+              "jane@example.com",
+              validCategory);
+
+      // Assert
+      assertEquals(18, person.getAge());
+    }
+  }
+
+  @Nested
+  @DisplayName("Person Creation - Invalid Cases")
+  class InvalidPersonCreation {
+
+    @Test
+    @DisplayName("Should throw exception when first name is null")
+    void shouldThrowExceptionWhenFirstNameIsNull() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () ->
+                  new Person(
+                      1L, null, "Doe", 25, "+1234567890", "john@example.com", validCategory));
+      assertTrue(exception.getMessage().contains("First name and last name cannot be null"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when last name is null")
+    void shouldThrowExceptionWhenLastNameIsNull() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () ->
+                  new Person(
+                      1L, "John", null, 25, "+1234567890", "john@example.com", validCategory));
+      assertTrue(exception.getMessage().contains("First name and last name cannot be null"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when age is less than 18")
+    void shouldThrowExceptionWhenAgeIsLessThan18() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () ->
+                  new Person(
+                      1L, "John", "Doe", 17, "+1234567890", "john@example.com", validCategory));
+      assertTrue(exception.getMessage().contains("Age cannot be less than 18"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when phone number is null")
+    void shouldThrowExceptionWhenPhoneIsNull() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () -> new Person(1L, "John", "Doe", 25, null, "john@example.com", validCategory));
+      assertTrue(exception.getMessage().contains("Phone number cannot be null or empty"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when phone number is empty")
+    void shouldThrowExceptionWhenPhoneIsEmpty() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () -> new Person(1L, "John", "Doe", 25, "", "john@example.com", validCategory));
+      assertTrue(exception.getMessage().contains("Phone number cannot be null or empty"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when email is null")
+    void shouldThrowExceptionWhenEmailIsNull() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () -> new Person(1L, "John", "Doe", 25, "+1234567890", null, validCategory));
+      assertTrue(exception.getMessage().contains("Email cannot be null or empty"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when email is empty")
+    void shouldThrowExceptionWhenEmailIsEmpty() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () -> new Person(1L, "John", "Doe", 25, "+1234567890", "", validCategory));
+      assertTrue(exception.getMessage().contains("Email cannot be null or empty"));
+    }
+
+    @Test
+    @DisplayName("Should throw exception when category is null")
+    void shouldThrowExceptionWhenCategoryIsNull() {
+      // Act & Assert
+      var exception =
+          assertThrows(
+              DomainException.class,
+              () -> new Person(1L, "John", "Doe", 25, "+1234567890", "john@example.com", null));
+      assertTrue(exception.getMessage().contains("Category cannot be null or empty"));
+    }
+  }
+}
diff --git a/onion-architecture/etc/onion-architecture.png b/onion-architecture/etc/onion-architecture.png
new file mode 100644
index 000000000000..82f34e3d7fd1
Binary files /dev/null and b/onion-architecture/etc/onion-architecture.png differ
diff --git a/onion-architecture/etc/onion-architecture.puml b/onion-architecture/etc/onion-architecture.puml
new file mode 100644
index 000000000000..0e00429de2e6
--- /dev/null
+++ b/onion-architecture/etc/onion-architecture.puml
@@ -0,0 +1,237 @@
+@startuml onion-architecture
+
+skinparam packageStyle rectangle
+skinparam shadowing false
+skinparam defaultFontName Arial
+skinparam defaultFontSize 11
+skinparam classAttributeIconSize 0
+skinparam ArrowColor #444444
+skinparam ArrowThickness 1.2
+
+skinparam package {
+  BackgroundColor<<domain>>         #FFF9C4
+  BorderColor<<domain>>             #F9A825
+  FontColor<<domain>>               #5D4037
+  FontStyle<<domain>>               bold
+}
+
+skinparam package {
+  BackgroundColor<<application>>    #C8E6C9
+  BorderColor<<application>>        #388E3C
+  FontColor<<application>>          #1B5E20
+  FontStyle<<application>>          bold
+}
+
+skinparam package {
+  BackgroundColor<<infrastructure>> #BBDEFB
+  BorderColor<<infrastructure>>     #1976D2
+  FontColor<<infrastructure>>       #0D47A1
+  FontStyle<<infrastructure>>       bold
+}
+
+' ─────────────────────────────────────────────────────────────────────────────
+' DOMAIN LAYER (innermost)
+' ─────────────────────────────────────────────────────────────────────────────
+package "Domain Layer" <<domain>> {
+
+  package "model" {
+    class Person {
+      - id : Long
+      - firstName : String
+      - lastName : String
+      - age : int
+      - phoneNumber : String
+      - email : String
+      - category : Category
+      + getId() : Long
+      + getFirstName() : String
+      + getLastName() : String
+      + getAge() : int
+      + getPhoneNumber() : String
+      + getEmail() : String
+      + getCategory() : Category
+    }
+
+    class Category {
+      - id : Long
+      - type : String
+      + getId() : Long
+      + getType() : String
+    }
+  }
+
+  package "repository" {
+    interface PersonRepository {
+      + findById(id : Long) : Optional<Person>
+      + findByFirstName(firstName : String) : Optional<Person>
+      + findByLastName(lastName : String) : Optional<Person>
+      + findAll() : Optional<List<Person>>
+      + save(person : Person) : Person
+      + deleteById(id : Long) : boolean
+    }
+  }
+
+  package "exception" {
+    class DomainException {
+      + DomainException(message : String)
+    }
+  }
+}
+
+' ─────────────────────────────────────────────────────────────────────────────
+' APPLICATION LAYER
+' ─────────────────────────────────────────────────────────────────────────────
+package "Application Layer" <<application>> {
+
+  package "usecase" {
+    class SavePersonUseCase {
+      - repository : PersonRepository
+      + SavePersonUseCase(repository : PersonRepository)
+      + execute(command : SavePersonCommand) : PersonResponse
+    }
+
+    class GetPersonUseCase {
+      - repository : PersonRepository
+      + GetPersonUseCase(repository : PersonRepository)
+      + execute(id : Long) : PersonResponse
+      + executeAll() : List<PersonResponse>
+    }
+  }
+
+  package "dto" {
+    class SavePersonCommand <<record>> {
+      firstName : String
+      lastName : String
+      age : int
+      phoneNumber : String
+      email : String
+      address : String
+      categoryId : Long
+      categoryType : String
+    }
+
+    class PersonResponse <<record>> {
+      id : Long
+      firstName : String
+      lastName : String
+      age : int
+      phoneNumber : String
+      email : String
+      categoryId : Long
+      categoryType : String
+    }
+  }
+}
+
+' ─────────────────────────────────────────────────────────────────────────────
+' INFRASTRUCTURE LAYER (outermost)
+' ─────────────────────────────────────────────────────────────────────────────
+package "Infrastructure Layer" <<infrastructure>> {
+
+  package "web" {
+    class PersonController {
+      - savePersonUseCase : SavePersonUseCase
+      - getPersonUseCase : GetPersonUseCase
+      + getPerson(id : Long) : ResponseEntity<PersonResponse>
+      + getAllPersons() : ResponseEntity<List<PersonResponse>>
+      + savePerson(command : SavePersonCommand) : ResponseEntity<PersonResponse>
+    }
+  }
+
+  package "persistence" {
+    class PersonRepositoryAdapter {
+      - repository : SpringDataPersonRepository
+      + findById(id : Long) : Optional<Person>
+      + findByFirstName(firstName : String) : Optional<Person>
+      + findByLastName(lastName : String) : Optional<Person>
+      + findAll() : Optional<List<Person>>
+      + save(person : Person) : Person
+      + deleteById(id : Long) : boolean
+    }
+
+    interface SpringDataPersonRepository {
+      + findByFirstName(firstName : String) : Optional<JpaPersonEntity>
+      + findByLastName(lastName : String) : Optional<JpaPersonEntity>
+    }
+
+    class JpaPersonEntity {
+      - id : Long
+      - firstName : String
+      - lastName : String
+      - age : int
+      - phoneNumber : String
+      - email : String
+      - category : JpaCategoryEntity
+    }
+
+    class JpaCategoryEntity {
+      - id : Long
+      - type : String
+    }
+  }
+
+  package "config" {
+    class ApplicationConfig <<@Configuration>> {
+      + savePersonUseCase(repo : PersonRepository) : SavePersonUseCase
+      + getPersonUseCase(repo : PersonRepository) : GetPersonUseCase
+    }
+
+    class Application <<@SpringBootApplication>> {
+      + main(args : String[])
+    }
+  }
+}
+
+' ─────────────────────────────────────────────────────────────────────────────
+' DOMAIN: internal relationships
+' ─────────────────────────────────────────────────────────────────────────────
+Person "1" *-- "1" Category : has
+Person ..> DomainException : <<throws>>
+Category ..> DomainException : <<throws>>
+
+' ─────────────────────────────────────────────────────────────────────────────
+' APPLICATION → DOMAIN
+' ─────────────────────────────────────────────────────────────────────────────
+SavePersonUseCase --> PersonRepository : uses
+SavePersonUseCase ..> Person : creates
+SavePersonUseCase ..> Category : creates
+SavePersonUseCase ..> PersonResponse : returns
+SavePersonUseCase ..> SavePersonCommand : receives
+
+GetPersonUseCase --> PersonRepository : uses
+GetPersonUseCase ..> PersonResponse : returns
+
+' ─────────────────────────────────────────────────────────────────────────────
+' INFRASTRUCTURE → APPLICATION / DOMAIN
+' ─────────────────────────────────────────────────────────────────────────────
+PersonController --> SavePersonUseCase : delegates to
+PersonController --> GetPersonUseCase : delegates to
+PersonController ..> DomainException : catches
+
+PersonRepositoryAdapter ..|> PersonRepository : implements
+PersonRepositoryAdapter --> SpringDataPersonRepository : uses
+PersonRepositoryAdapter ..> JpaPersonEntity : maps
+PersonRepositoryAdapter ..> JpaCategoryEntity : maps
+PersonRepositoryAdapter ..> Person : maps to / from
+PersonRepositoryAdapter ..> Category : maps to / from
+
+SpringDataPersonRepository --> JpaPersonEntity : manages
+JpaPersonEntity "1" *-- "1" JpaCategoryEntity : contains
+
+ApplicationConfig ..> SavePersonUseCase : <<@Bean>>
+ApplicationConfig ..> GetPersonUseCase : <<@Bean>>
+ApplicationConfig --> PersonRepository : injects
+
+legend right
+  <b>Layer dependency rule</b>
+  Infrastructure  →  Application  →  Domain
+  Outer layers depend on inner layers.
+  Domain has NO external dependencies.
+  ----
+  <color:#F9A825>■</color> Domain Layer
+  <color:#388E3C>■</color> Application Layer
+  <color:#1976D2>■</color> Infrastructure Layer
+endlegend
+
+@enduml
+
diff --git a/onion-architecture/etc/postman/onion-architecture.postman_collection.json b/onion-architecture/etc/postman/onion-architecture.postman_collection.json
new file mode 100644
index 000000000000..ed0cbd5c58f5
--- /dev/null
+++ b/onion-architecture/etc/postman/onion-architecture.postman_collection.json
@@ -0,0 +1,83 @@
+{
+  "info": {
+    "_postman_id": "79aca374-7080-46c1-938f-89b327a84d6a",
+    "name": "onion-architecture",
+    "schema": "https://schema.getpostman.com/json/collection/v2.1.0/collection.json",
+    "_exporter_id": "51221021",
+    "_collection_link": "https://go.postman.co/collection/51221021-79aca374-7080-46c1-938f-89b327a84d6a?source=collection_link"
+  },
+  "item": [
+    {
+      "name": "savePerson",
+      "request": {
+        "method": "POST",
+        "header": [],
+        "body": {
+          "mode": "raw",
+          "raw": "{\r\n        \"number1\": 5,\r\n        \"number2\": 3,\r\n        \"operation\": \"multiply\"\r\n      }"
+        },
+        "url": {
+          "raw": "http://localhost:8080/api/persons",
+          "protocol": "http",
+          "host": [
+            "localhost"
+          ],
+          "port": "8080",
+          "path": [
+            "api",
+            "persons"
+          ]
+        }
+      },
+      "response": []
+    },
+    {
+      "name": "getPersons",
+      "protocolProfileBehavior": {
+        "disableBodyPruning": true
+      },
+      "request": {
+        "method": "GET",
+        "header": [],
+        "body": {
+          "mode": "raw",
+          "raw": "{\r\n        \"number1\": 5,\r\n        \"number2\": 3,\r\n        \"operation\": \"multiply\"\r\n      }"
+        },
+        "url": {
+          "raw": "http://localhost:8080/api/persons",
+          "protocol": "http",
+          "host": [
+            "localhost"
+          ],
+          "port": "8080",
+          "path": [
+            "api",
+            "persons"
+          ]
+        }
+      },
+      "response": []
+    },
+    {
+      "name": "getPersonById",
+      "request": {
+        "method": "GET",
+        "header": [],
+        "url": {
+          "raw": "http://localhost:8080/api/persons/1",
+          "protocol": "http",
+          "host": [
+            "localhost"
+          ],
+          "port": "8080",
+          "path": [
+            "api",
+            "persons",
+            "1"
+          ]
+        }
+      },
+      "response": []
+    }
+  ]
+}
\ No newline at end of file
diff --git a/onion-architecture/infrastructure/pom.xml b/onion-architecture/infrastructure/pom.xml
new file mode 100644
index 000000000000..d26e2a7ef69d
--- /dev/null
+++ b/onion-architecture/infrastructure/pom.xml
@@ -0,0 +1,87 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>onion-architecture</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>infrastructure</artifactId>
+    <name>Infrastructure</name>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.iluwatar</groupId>
+            <artifactId>domain</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <dependency>
+            <groupId>com.iluwatar</groupId>
+            <artifactId>application</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <!-- REST Controller -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-web</artifactId>
+        </dependency>
+
+        <!-- JPA + Spring Data JPA-->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-jpa</artifactId>
+        </dependency>
+
+        <!-- H2 Database -->
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <scope>runtime</scope>
+        </dependency>
+
+        <!-- Spring Boot Test -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
+            </plugin>
+        </plugins>
+    </build>
+</project>
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/Application.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/Application.java
new file mode 100644
index 000000000000..c86c95740599
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/Application.java
@@ -0,0 +1,38 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class Application {
+
+  public static void main(String[] args) {
+    SpringApplication.run(Application.class, args);
+  }
+}
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/config/ApplicationConfig.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/config/ApplicationConfig.java
new file mode 100644
index 000000000000..7794c91e3bcf
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/config/ApplicationConfig.java
@@ -0,0 +1,47 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.config;
+
+import com.iluwatar.onion.application.usecase.GetPersonUseCase;
+import com.iluwatar.onion.application.usecase.SavePersonUseCase;
+import com.iluwatar.onion.domain.repository.PersonRepository;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+public class ApplicationConfig {
+
+  @Bean
+  public SavePersonUseCase savePersonUseCase(PersonRepository repository) {
+    return new SavePersonUseCase(repository);
+  }
+
+  @Bean
+  public GetPersonUseCase getPersonUseCase(PersonRepository repository) {
+    return new GetPersonUseCase(repository);
+  }
+}
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/JpaCategoryEntity.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/JpaCategoryEntity.java
new file mode 100644
index 000000000000..aafd242992be
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/JpaCategoryEntity.java
@@ -0,0 +1,55 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.persistence;
+
+import jakarta.persistence.*;
+
+@Entity
+@Table(name = "category")
+public class JpaCategoryEntity {
+
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private Long id;
+
+  private String type;
+
+  public JpaCategoryEntity() {}
+
+  public JpaCategoryEntity(Long id, String type) {
+    this.id = id;
+    this.type = type;
+  }
+
+  public Long getId() {
+    return id;
+  }
+
+  public String getType() {
+    return type;
+  }
+}
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/JpaPersonEntity.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/JpaPersonEntity.java
new file mode 100644
index 000000000000..bf71b5306c92
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/JpaPersonEntity.java
@@ -0,0 +1,93 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.persistence;
+
+import jakarta.persistence.*;
+
+@Entity
+@Table(name = "person")
+public class JpaPersonEntity {
+
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private Long id;
+
+  private String firstName;
+  private String lastName;
+  private int age;
+  private String phoneNumber;
+  private String email;
+
+  @ManyToOne private JpaCategoryEntity category;
+
+  public JpaPersonEntity() {}
+
+  public JpaPersonEntity(
+      Long id,
+      String firstName,
+      String lastName,
+      int age,
+      String phoneNumber,
+      String email,
+      JpaCategoryEntity category) {
+    this.id = id;
+    this.firstName = firstName;
+    this.lastName = lastName;
+    this.age = age;
+    this.phoneNumber = phoneNumber;
+    this.email = email;
+    this.category = category;
+  }
+
+  public Long getId() {
+    return id;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+
+  public int getAge() {
+    return age;
+  }
+
+  public String getPhoneNumber() {
+    return phoneNumber;
+  }
+
+  public String getEmail() {
+    return email;
+  }
+
+  public JpaCategoryEntity getCategory() {
+    return category;
+  }
+}
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/PersonRepositoryAdapter.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/PersonRepositoryAdapter.java
new file mode 100644
index 000000000000..4c4aa267493a
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/PersonRepositoryAdapter.java
@@ -0,0 +1,102 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.persistence;
+
+import com.iluwatar.onion.domain.model.Category;
+import com.iluwatar.onion.domain.model.Person;
+import com.iluwatar.onion.domain.repository.PersonRepository;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+import org.springframework.stereotype.Repository;
+
+@Repository
+public class PersonRepositoryAdapter implements PersonRepository {
+
+  private final SpringDataPersonRepository repository;
+
+  public PersonRepositoryAdapter(SpringDataPersonRepository repository) {
+    this.repository = repository;
+  }
+
+  @Override
+  public Optional<Person> findById(Long id) {
+    return repository.findById(id).map(this::mapToDomain);
+  }
+
+  @Override
+  public Optional<Person> findByFirstName(String firstName) {
+    return repository.findByFirstName(firstName).map(this::mapToDomain);
+  }
+
+  @Override
+  public Optional<Person> findByLastName(String lastName) {
+    return repository.findByLastName(lastName).map(this::mapToDomain);
+  }
+
+  @Override
+  public Optional<List<Person>> findAll() {
+    return repository.findAll().stream()
+        .map(this::mapToDomain)
+        .collect(Collectors.collectingAndThen(Collectors.toList(), Optional::of));
+  }
+
+  @Override
+  public Person save(Person person) {
+    JpaPersonEntity entity = mapToEntity(person);
+    JpaPersonEntity savedEntity = repository.save(entity);
+    return mapToDomain(savedEntity);
+  }
+
+  @Override
+  public boolean deleteById(Long id) {
+    repository.deleteById(id);
+    return repository.findById(id).isEmpty();
+  }
+
+  private JpaPersonEntity mapToEntity(Person person) {
+    return new JpaPersonEntity(
+        person.getId(),
+        person.getFirstName(),
+        person.getLastName(),
+        person.getAge(),
+        person.getPhoneNumber(),
+        person.getEmail(),
+        new JpaCategoryEntity(person.getCategory().getId(), person.getCategory().getType()));
+  }
+
+  private Person mapToDomain(JpaPersonEntity entity) {
+    return new Person(
+        entity.getId(),
+        entity.getFirstName(),
+        entity.getLastName(),
+        entity.getAge(),
+        entity.getPhoneNumber(),
+        entity.getEmail(),
+        new Category(entity.getCategory().getId(), entity.getCategory().getType()));
+  }
+}
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/SpringDataPersonRepository.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/SpringDataPersonRepository.java
new file mode 100644
index 000000000000..35f86515c70b
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/persistence/SpringDataPersonRepository.java
@@ -0,0 +1,37 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.persistence;
+
+import java.util.Optional;
+import org.springframework.data.jpa.repository.JpaRepository;
+
+public interface SpringDataPersonRepository extends JpaRepository<JpaPersonEntity, Long> {
+
+  Optional<JpaPersonEntity> findByFirstName(String firstName);
+
+  Optional<JpaPersonEntity> findByLastName(String lastName);
+}
diff --git a/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/web/PersonController.java b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/web/PersonController.java
new file mode 100644
index 000000000000..417e749f41a6
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/java/com/iluwatar/onion/infrastructure/web/PersonController.java
@@ -0,0 +1,72 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.web;
+
+import com.iluwatar.onion.application.dto.PersonResponse;
+import com.iluwatar.onion.application.dto.SavePersonCommand;
+import com.iluwatar.onion.application.usecase.GetPersonUseCase;
+import com.iluwatar.onion.application.usecase.SavePersonUseCase;
+import com.iluwatar.onion.domain.exception.DomainException;
+import java.util.List;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+
+@RestController
+@RequestMapping("/api")
+public class PersonController {
+
+  private final SavePersonUseCase savePersonUseCase;
+  private final GetPersonUseCase getPersonUseCase;
+
+  public PersonController(SavePersonUseCase savePersonUseCase, GetPersonUseCase getPersonUseCase) {
+    this.savePersonUseCase = savePersonUseCase;
+    this.getPersonUseCase = getPersonUseCase;
+  }
+
+  @GetMapping("/persons/{id}")
+  public ResponseEntity<PersonResponse> getPerson(@PathVariable("id") Long id) {
+    var person = getPersonUseCase.execute(id);
+    return ResponseEntity.ok(person);
+  }
+
+  @GetMapping("/persons")
+  public ResponseEntity<List<PersonResponse>> getAllPersons() {
+    var persons = getPersonUseCase.executeAll();
+    return ResponseEntity.ok(persons);
+  }
+
+  @PostMapping("/persons")
+  public ResponseEntity<PersonResponse> savePerson(@RequestBody SavePersonCommand command) {
+    try {
+      var savedPerson = savePersonUseCase.execute(command);
+      return ResponseEntity.status(HttpStatus.OK).body(savedPerson);
+    } catch (DomainException e) {
+      return ResponseEntity.status(HttpStatus.BAD_REQUEST).build();
+    }
+  }
+}
diff --git a/onion-architecture/infrastructure/src/main/resources/application.properties b/onion-architecture/infrastructure/src/main/resources/application.properties
new file mode 100644
index 000000000000..6a3ed156f558
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/resources/application.properties
@@ -0,0 +1,19 @@
+# You can also specify the server port in the application.properties file.
+# Uncomment the following line and set the desired port number to change
+# the default port (8080) to a different value.
+#server.port=8080
+
+spring.datasource.url=jdbc:h2:mem:testdb
+spring.datasource.driverClassName=org.h2.Driver
+spring.datasource.username=sa
+spring.datasource.password=password
+spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
+
+# Force Spring Boot to always run SQL initialization scripts on startup
+spring.sql.init.mode=always
+
+# If using Spring Data JPA / Hibernate, defer data insertion until AFTER entities are generated
+spring.jpa.defer-datasource-initialization=true
+
+# Keep Hibernate's DDL generation active (or set to 'update')
+spring.jpa.hibernate.ddl-auto=create-drop
diff --git a/onion-architecture/infrastructure/src/main/resources/data.sql b/onion-architecture/infrastructure/src/main/resources/data.sql
new file mode 100644
index 000000000000..6291dd984e31
--- /dev/null
+++ b/onion-architecture/infrastructure/src/main/resources/data.sql
@@ -0,0 +1,4 @@
+INSERT INTO category (id, type) VALUES (1, 'Teenage');
+INSERT INTO category (id, type) VALUES (2, 'Young Adult');
+INSERT INTO category (id, type) VALUES (3, 'Adult');
+INSERT INTO category (id, type) VALUES (4, 'Senior');
\ No newline at end of file
diff --git a/onion-architecture/infrastructure/src/test/java/com/iluwatar/onion/infrastructure/persistence/PersonRepositoryAdapterTest.java b/onion-architecture/infrastructure/src/test/java/com/iluwatar/onion/infrastructure/persistence/PersonRepositoryAdapterTest.java
new file mode 100644
index 000000000000..49fd2816f172
--- /dev/null
+++ b/onion-architecture/infrastructure/src/test/java/com/iluwatar/onion/infrastructure/persistence/PersonRepositoryAdapterTest.java
@@ -0,0 +1,332 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.persistence;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+import com.iluwatar.onion.domain.model.Category;
+import com.iluwatar.onion.domain.model.Person;
+import java.util.List;
+import java.util.Optional;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Captor;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class PersonRepositoryAdapterTest {
+
+  @Mock private SpringDataPersonRepository springDataRepository;
+
+  @Captor private ArgumentCaptor<JpaPersonEntity> entityCaptor;
+
+  private PersonRepositoryAdapter personRepositoryAdapter;
+  private JpaPersonEntity jpaPersonEntity;
+
+  @BeforeEach
+  void setUp() {
+    personRepositoryAdapter = new PersonRepositoryAdapter(springDataRepository);
+
+    // Setup test data
+    var jpaCategory = new JpaCategoryEntity(1L, "Professional");
+    jpaPersonEntity =
+        new JpaPersonEntity(
+            1L, "John", "Doe", 25, "+1234567890", "john.doe@example.com", jpaCategory);
+  }
+
+  @Nested
+  @DisplayName("Find By ID")
+  class FindById {
+
+    @Test
+    @DisplayName("Should return Person when entity exists")
+    void shouldReturnPersonWhenEntityExists() {
+      // Arrange
+      when(springDataRepository.findById(1L)).thenReturn(Optional.of(jpaPersonEntity));
+
+      // Act
+      var result = personRepositoryAdapter.findById(1L);
+
+      // Assert
+      assertTrue(result.isPresent());
+      Person person = result.get();
+      assertEquals(1L, person.getId());
+      assertEquals("John", person.getFirstName());
+      assertEquals("Doe", person.getLastName());
+      assertEquals(25, person.getAge());
+      assertEquals("+1234567890", person.getPhoneNumber());
+      assertEquals("john.doe@example.com", person.getEmail());
+      assertEquals(1L, person.getCategory().getId());
+      assertEquals("Professional", person.getCategory().getType());
+
+      verify(springDataRepository, times(1)).findById(1L);
+    }
+
+    @Test
+    @DisplayName("Should return empty Optional when entity not found")
+    void shouldReturnEmptyWhenEntityNotFound() {
+      // Arrange
+      when(springDataRepository.findById(999L)).thenReturn(Optional.empty());
+
+      // Act
+      var result = personRepositoryAdapter.findById(999L);
+
+      // Assert
+      assertTrue(result.isEmpty());
+      verify(springDataRepository, times(1)).findById(999L);
+    }
+  }
+
+  @Nested
+  @DisplayName("Find By First Name")
+  class FindByFirstName {
+
+    @Test
+    @DisplayName("Should return Person when entity with first name exists")
+    void shouldReturnPersonWhenEntityExists() {
+      // Arrange
+      when(springDataRepository.findByFirstName("John")).thenReturn(Optional.of(jpaPersonEntity));
+
+      // Act
+      var result = personRepositoryAdapter.findByFirstName("John");
+
+      // Assert
+      assertTrue(result.isPresent());
+      assertEquals("John", result.get().getFirstName());
+      verify(springDataRepository, times(1)).findByFirstName("John");
+    }
+
+    @Test
+    @DisplayName("Should return empty Optional when no entity found")
+    void shouldReturnEmptyWhenNotFound() {
+      // Arrange
+      when(springDataRepository.findByFirstName("Unknown")).thenReturn(Optional.empty());
+
+      // Act
+      var result = personRepositoryAdapter.findByFirstName("Unknown");
+
+      // Assert
+      assertTrue(result.isEmpty());
+      verify(springDataRepository, times(1)).findByFirstName("Unknown");
+    }
+  }
+
+  @Nested
+  @DisplayName("Find By Last Name")
+  class FindByLastName {
+
+    @Test
+    @DisplayName("Should return Person when entity with last name exists")
+    void shouldReturnPersonWhenEntityExists() {
+      // Arrange
+      when(springDataRepository.findByLastName("Doe")).thenReturn(Optional.of(jpaPersonEntity));
+
+      // Act
+      var result = personRepositoryAdapter.findByLastName("Doe");
+
+      // Assert
+      assertTrue(result.isPresent());
+      assertEquals("Doe", result.get().getLastName());
+      verify(springDataRepository, times(1)).findByLastName("Doe");
+    }
+  }
+
+  @Nested
+  @DisplayName("Find All")
+  class FindAll {
+
+    @Test
+    @DisplayName("Should return list of Persons when entities exist")
+    void shouldReturnListOfPersons() {
+      // Arrange
+      var category2 = new JpaCategoryEntity(2L, "Personal");
+      var person2 =
+          new JpaPersonEntity(
+              2L, "Jane", "Smith", 30, "+9876543210", "jane@example.com", category2);
+
+      when(springDataRepository.findAll()).thenReturn(List.of(jpaPersonEntity, person2));
+
+      // Act
+      var result = personRepositoryAdapter.findAll();
+
+      // Assert
+      assertTrue(result.isPresent());
+      var persons = result.get();
+      assertEquals(2, persons.size());
+      assertEquals("John", persons.get(0).getFirstName());
+      assertEquals("Jane", persons.get(1).getFirstName());
+
+      verify(springDataRepository, times(1)).findAll();
+    }
+
+    @Test
+    @DisplayName("Should return empty list when no entities exist")
+    void shouldReturnEmptyListWhenNoEntities() {
+      // Arrange
+      when(springDataRepository.findAll()).thenReturn(List.of());
+
+      // Act
+      var result = personRepositoryAdapter.findAll();
+
+      // Assert
+      assertTrue(result.isPresent());
+      assertTrue(result.get().isEmpty());
+      verify(springDataRepository, times(1)).findAll();
+    }
+  }
+
+  @Nested
+  @DisplayName("Save")
+  class Save {
+
+    @Test
+    @DisplayName("Should save person and return domain model")
+    void shouldSavePersonAndReturnDomainModel() {
+      // Arrange
+      var personToSave =
+          new Person(
+              null, // New person without ID
+              "Jane",
+              "Smith",
+              28,
+              "+1111111111",
+              "jane@example.com",
+              new Category(2L, "Personal"));
+
+      var savedCategory = new JpaCategoryEntity(2L, "Personal");
+      var savedEntity =
+          new JpaPersonEntity(
+              100L, // ID assigned by database
+              "Jane",
+              "Smith",
+              28,
+              "+1111111111",
+              "jane@example.com",
+              savedCategory);
+
+      when(springDataRepository.save(any(JpaPersonEntity.class))).thenReturn(savedEntity);
+
+      // Act
+      var result = personRepositoryAdapter.save(personToSave);
+
+      // Assert
+      assertNotNull(result);
+      assertEquals(100L, result.getId());
+      assertEquals("Jane", result.getFirstName());
+      assertEquals("Smith", result.getLastName());
+      assertEquals(28, result.getAge());
+
+      verify(springDataRepository, times(1)).save(any(JpaPersonEntity.class));
+    }
+
+    @Test
+    @DisplayName("Should correctly map domain model to JPA entity")
+    void shouldCorrectlyMapDomainToEntity() {
+      // Arrange
+      var personToSave =
+          new Person(
+              null,
+              "Bob",
+              "Johnson",
+              35,
+              "+2222222222",
+              "bob@example.com",
+              new Category(3L, "Business"));
+
+      var savedEntity =
+          new JpaPersonEntity(
+              200L,
+              "Bob",
+              "Johnson",
+              35,
+              "+2222222222",
+              "bob@example.com",
+              new JpaCategoryEntity(3L, "Business"));
+
+      when(springDataRepository.save(any(JpaPersonEntity.class))).thenReturn(savedEntity);
+
+      // Act
+      personRepositoryAdapter.save(personToSave);
+
+      // Assert - Verify what was passed to Spring Data repository
+      verify(springDataRepository).save(entityCaptor.capture());
+      JpaPersonEntity capturedEntity = entityCaptor.getValue();
+
+      assertEquals("Bob", capturedEntity.getFirstName());
+      assertEquals("Johnson", capturedEntity.getLastName());
+      assertEquals(35, capturedEntity.getAge());
+      assertEquals("+2222222222", capturedEntity.getPhoneNumber());
+      assertEquals("bob@example.com", capturedEntity.getEmail());
+      assertEquals(3L, capturedEntity.getCategory().getId());
+      assertEquals("Business", capturedEntity.getCategory().getType());
+    }
+  }
+
+  @Nested
+  @DisplayName("Delete By ID")
+  class DeleteById {
+
+    @Test
+    @DisplayName("Should return true when person is successfully deleted")
+    void shouldReturnTrueWhenDeleted() {
+      // Arrange
+      doNothing().when(springDataRepository).deleteById(1L);
+      when(springDataRepository.findById(1L)).thenReturn(Optional.empty());
+
+      // Act
+      boolean result = personRepositoryAdapter.deleteById(1L);
+
+      // Assert
+      assertTrue(result);
+      verify(springDataRepository, times(1)).deleteById(1L);
+      verify(springDataRepository, times(1)).findById(1L);
+    }
+
+    @Test
+    @DisplayName("Should return false when person still exists after deletion")
+    void shouldReturnFalseWhenStillExists() {
+      // Arrange
+      doNothing().when(springDataRepository).deleteById(1L);
+      when(springDataRepository.findById(1L)).thenReturn(Optional.of(jpaPersonEntity));
+
+      // Act
+      boolean result = personRepositoryAdapter.deleteById(1L);
+
+      // Assert
+      assertFalse(result);
+      verify(springDataRepository, times(1)).deleteById(1L);
+      verify(springDataRepository, times(1)).findById(1L);
+    }
+  }
+}
diff --git a/onion-architecture/infrastructure/src/test/java/com/iluwatar/onion/infrastructure/web/PersonControllerTest.java b/onion-architecture/infrastructure/src/test/java/com/iluwatar/onion/infrastructure/web/PersonControllerTest.java
new file mode 100644
index 000000000000..9e0369b985c8
--- /dev/null
+++ b/onion-architecture/infrastructure/src/test/java/com/iluwatar/onion/infrastructure/web/PersonControllerTest.java
@@ -0,0 +1,348 @@
+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.infrastructure.web;
+
+import static org.hamcrest.Matchers.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.iluwatar.onion.application.dto.PersonResponse;
+import com.iluwatar.onion.application.dto.SavePersonCommand;
+import com.iluwatar.onion.application.usecase.GetPersonUseCase;
+import com.iluwatar.onion.application.usecase.SavePersonUseCase;
+import com.iluwatar.onion.domain.exception.DomainException;
+import java.util.List;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
+import org.springframework.http.MediaType;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.springframework.test.web.servlet.MockMvc;
+
+@WebMvcTest(PersonController.class)
+class PersonControllerTest {
+
+  @Autowired private MockMvc mockMvc;
+
+  @Autowired private ObjectMapper objectMapper;
+
+  @MockitoBean private SavePersonUseCase savePersonUseCase;
+
+  @MockitoBean private GetPersonUseCase getPersonUseCase;
+
+  @Nested
+  @DisplayName("GET /api/persons/{id}")
+  class GetPersonById {
+
+    @Test
+    @DisplayName("Should return person when person exists")
+    void shouldReturnPersonWhenExists() throws Exception {
+      // Arrange
+      var response =
+          new PersonResponse(
+              1L, "John", "Doe", 25, "+1234567890", "john.doe@example.com", 1L, "Professional");
+
+      when(getPersonUseCase.execute(1L)).thenReturn(response);
+
+      // Act & Assert
+      mockMvc
+          .perform(get("/api/persons/1").contentType(MediaType.APPLICATION_JSON))
+          .andExpect(status().isOk())
+          .andExpect(content().contentType(MediaType.APPLICATION_JSON))
+          .andExpect(jsonPath("$.id").value(1))
+          .andExpect(jsonPath("$.firstName").value("John"))
+          .andExpect(jsonPath("$.lastName").value("Doe"))
+          .andExpect(jsonPath("$.age").value(25))
+          .andExpect(jsonPath("$.phoneNumber").value("+1234567890"))
+          .andExpect(jsonPath("$.email").value("john.doe@example.com"))
+          .andExpect(jsonPath("$.categoryId").value(1))
+          .andExpect(jsonPath("$.categoryType").value("Professional"));
+
+      verify(getPersonUseCase, times(1)).execute(1L);
+    }
+
+    @Test
+    @DisplayName("Should handle different person IDs")
+    void shouldHandleDifferentPersonIds() throws Exception {
+      // Arrange
+      var response =
+          new PersonResponse(
+              42L, "Jane", "Smith", 30, "+9876543210", "jane@example.com", 2L, "Personal");
+
+      when(getPersonUseCase.execute(42L)).thenReturn(response);
+
+      // Act & Assert
+      mockMvc
+          .perform(get("/api/persons/42").contentType(MediaType.APPLICATION_JSON))
+          .andExpect(status().isOk())
+          .andExpect(jsonPath("$.id").value(42))
+          .andExpect(jsonPath("$.firstName").value("Jane"));
+
+      verify(getPersonUseCase, times(1)).execute(42L);
+    }
+  }
+
+  @Nested
+  @DisplayName("GET /api/persons")
+  class GetAllPersons {
+
+    @Test
+    @DisplayName("Should return list of persons")
+    void shouldReturnListOfPersons() throws Exception {
+      // Arrange
+      var person1 =
+          new PersonResponse(
+              1L, "John", "Doe", 25, "+1234567890", "john@example.com", 1L, "Professional");
+      var person2 =
+          new PersonResponse(
+              2L, "Jane", "Smith", 30, "+9876543210", "jane@example.com", 2L, "Personal");
+
+      var persons = List.of(person1, person2);
+      when(getPersonUseCase.executeAll()).thenReturn(persons);
+
+      // Act & Assert
+      mockMvc
+          .perform(get("/api/persons").contentType(MediaType.APPLICATION_JSON))
+          .andExpect(status().isOk())
+          .andExpect(content().contentType(MediaType.APPLICATION_JSON))
+          .andExpect(jsonPath("$", hasSize(2)))
+          .andExpect(jsonPath("$[0].id").value(1))
+          .andExpect(jsonPath("$[0].firstName").value("John"))
+          .andExpect(jsonPath("$[1].id").value(2))
+          .andExpect(jsonPath("$[1].firstName").value("Jane"));
+
+      verify(getPersonUseCase, times(1)).executeAll();
+    }
+
+    @Test
+    @DisplayName("Should return empty list when no persons exist")
+    void shouldReturnEmptyListWhenNoPersons() throws Exception {
+      // Arrange
+      when(getPersonUseCase.executeAll()).thenReturn(List.of());
+
+      // Act & Assert
+      mockMvc
+          .perform(get("/api/persons").contentType(MediaType.APPLICATION_JSON))
+          .andExpect(status().isOk())
+          .andExpect(content().contentType(MediaType.APPLICATION_JSON))
+          .andExpect(jsonPath("$", hasSize(0)));
+
+      verify(getPersonUseCase, times(1)).executeAll();
+    }
+
+    @Test
+    @DisplayName("Should handle large list of persons")
+    void shouldHandleLargeListOfPersons() throws Exception {
+      // Arrange
+      var persons =
+          List.of(
+              new PersonResponse(1L, "Person1", "Last1", 25, "+1", "p1@example.com", 1L, "Cat1"),
+              new PersonResponse(2L, "Person2", "Last2", 26, "+2", "p2@example.com", 1L, "Cat1"),
+              new PersonResponse(3L, "Person3", "Last3", 27, "+3", "p3@example.com", 1L, "Cat1"));
+
+      when(getPersonUseCase.executeAll()).thenReturn(persons);
+
+      // Act & Assert
+      mockMvc
+          .perform(get("/api/persons").contentType(MediaType.APPLICATION_JSON))
+          .andExpect(status().isOk())
+          .andExpect(jsonPath("$", hasSize(3)));
+
+      verify(getPersonUseCase, times(1)).executeAll();
+    }
+  }
+
+  @Nested
+  @DisplayName("POST /api/persons")
+  class SavePerson {
+
+    @Test
+    @DisplayName("Should create person with valid data")
+    void shouldCreatePersonWithValidData() throws Exception {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "John",
+              "Doe",
+              25,
+              "+1234567890",
+              "john.doe@example.com",
+              "123 Main St",
+              1L,
+              "Professional");
+
+      var response =
+          new PersonResponse(
+              1L, "John", "Doe", 25, "+1234567890", "john.doe@example.com", 1L, "Professional");
+
+      when(savePersonUseCase.execute(any(SavePersonCommand.class))).thenReturn(response);
+
+      // Act & Assert
+      mockMvc
+          .perform(
+              post("/api/persons")
+                  .contentType(MediaType.APPLICATION_JSON)
+                  .content(objectMapper.writeValueAsString(command)))
+          .andExpect(status().isOk())
+          .andExpect(content().contentType(MediaType.APPLICATION_JSON))
+          .andExpect(jsonPath("$.id").value(1))
+          .andExpect(jsonPath("$.firstName").value("John"))
+          .andExpect(jsonPath("$.lastName").value("Doe"))
+          .andExpect(jsonPath("$.age").value(25))
+          .andExpect(jsonPath("$.phoneNumber").value("+1234567890"))
+          .andExpect(jsonPath("$.email").value("john.doe@example.com"))
+          .andExpect(jsonPath("$.categoryId").value(1))
+          .andExpect(jsonPath("$.categoryType").value("Professional"));
+
+      verify(savePersonUseCase, times(1)).execute(any(SavePersonCommand.class));
+    }
+
+    @Test
+    @DisplayName("Should handle validation errors from use case")
+    void shouldPropagateValidationErrors() throws Exception {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "Young",
+              "Person",
+              17, // Invalid age
+              "+1234567890",
+              "young@example.com",
+              "123 Main St",
+              1L,
+              "Student");
+
+      when(savePersonUseCase.execute(any(SavePersonCommand.class)))
+          .thenThrow(new DomainException("Age cannot be less than 18"));
+
+      // Act & Assert
+      mockMvc
+          .perform(
+              post("/api/persons")
+                  .contentType(MediaType.APPLICATION_JSON)
+                  .content(objectMapper.writeValueAsString(command)))
+          .andExpect(status().is4xxClientError());
+
+      verify(savePersonUseCase, times(1)).execute(any(SavePersonCommand.class));
+    }
+
+    @Test
+    @DisplayName("Should handle malformed JSON")
+    void shouldHandleMalformedJson() throws Exception {
+      // Act & Assert
+      mockMvc
+          .perform(
+              post("/api/persons")
+                  .contentType(MediaType.APPLICATION_JSON)
+                  .content("{invalid json}"))
+          .andExpect(status().isBadRequest());
+
+      verify(savePersonUseCase, never()).execute(any(SavePersonCommand.class));
+    }
+
+    @Test
+    @DisplayName("Should accept all required fields in command")
+    void shouldAcceptAllRequiredFields() throws Exception {
+      // Arrange
+      var command =
+          new SavePersonCommand(
+              "Complete",
+              "Person",
+              30,
+              "+1111111111",
+              "complete@example.com",
+              "456 Oak Ave",
+              5L,
+              "Business");
+
+      var response =
+          new PersonResponse(
+              10L, "Complete", "Person", 30, "+1111111111", "complete@example.com", 5L, "Business");
+
+      when(savePersonUseCase.execute(any(SavePersonCommand.class))).thenReturn(response);
+
+      // Act & Assert
+      mockMvc
+          .perform(
+              post("/api/persons")
+                  .contentType(MediaType.APPLICATION_JSON)
+                  .content(objectMapper.writeValueAsString(command)))
+          .andExpect(status().isOk())
+          .andExpect(jsonPath("$.categoryType").value("Business"));
+
+      verify(savePersonUseCase, times(1)).execute(any(SavePersonCommand.class));
+    }
+  }
+
+  @Nested
+  @DisplayName("Controller Integration")
+  class ControllerIntegration {
+
+    @Test
+    @DisplayName("Should handle multiple requests sequentially")
+    void shouldHandleMultipleRequestsSequentially() throws Exception {
+      // Arrange
+      var getResponse =
+          new PersonResponse(
+              1L, "John", "Doe", 25, "+1234567890", "john@example.com", 1L, "Professional");
+
+      var savePersonCommand =
+          new SavePersonCommand(
+              "Jane", "Smith", 30, "+9876543210", "jane@example.com", "456 Oak", 2L, "Personal");
+
+      var savePersonResponse =
+          new PersonResponse(
+              2L, "Jane", "Smith", 30, "+9876543210", "jane@example.com", 2L, "Personal");
+
+      when(getPersonUseCase.execute(1L)).thenReturn(getResponse);
+      when(savePersonUseCase.execute(any(SavePersonCommand.class))).thenReturn(savePersonResponse);
+
+      // Act & Assert - GET request
+      mockMvc
+          .perform(get("/api/persons/1"))
+          .andExpect(status().isOk())
+          .andExpect(jsonPath("$.firstName").value("John"));
+
+      // Act & Assert - POST request
+      mockMvc
+          .perform(
+              post("/api/persons")
+                  .contentType(MediaType.APPLICATION_JSON)
+                  .content(objectMapper.writeValueAsString(savePersonCommand)))
+          .andExpect(status().isOk())
+          .andExpect(jsonPath("$.firstName").value("Jane"));
+
+      verify(getPersonUseCase, times(1)).execute(1L);
+      verify(savePersonUseCase, times(1)).execute(any(SavePersonCommand.class));
+    }
+  }
+}
diff --git a/onion-architecture/pom.xml b/onion-architecture/pom.xml
new file mode 100644
index 000000000000..07df2a1224c1
--- /dev/null
+++ b/onion-architecture/pom.xml
@@ -0,0 +1,49 @@
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>onion-architecture</artifactId>
+    <packaging>pom</packaging>
+
+    <properties>
+        <junit-jupiter.version>5.10.0</junit-jupiter.version>
+        <mockito.version>5.23.0</mockito.version>
+        <assertj.version>3.24.2</assertj.version>
+    </properties>
+
+    <modules>
+        <module>domain</module>
+        <module>application</module>
+        <module>infrastructure</module>
+    </modules>
+</project>
diff --git a/pom.xml b/pom.xml
index 7835110c008e..36f3b56ff0c4 100644
--- a/pom.xml
+++ b/pom.xml
@@ -106,7 +106,7 @@
     <module>converter</module>
     <module>curiously-recurring-template-pattern</module>
     <module>currying</module>
-      <module>dao-factory</module>
+    <module>dao-factory</module>
     <module>data-access-object</module>
     <module>data-bus</module>
     <module>data-locality</module>
@@ -252,6 +252,7 @@
     <module>backpressure</module>
     <module>actor-model</module>
     <module>rate-limiting-pattern</module>
+    <module>onion-architecture</module>
   </modules>
   <repositories>
     <repository>
@@ -346,6 +347,11 @@
         <artifactId>mockito-core</artifactId>
         <version>${mockito.version}</version>
       </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-junit-jupiter</artifactId>
+            <version>${mockito.version}</version>
+        </dependency>
       <dependency>
         <groupId>org.mongodb</groupId>
         <artifactId>bson</artifactId>