CH11: Application.md

CHAPTER 11: APPLICATION

Application layer separates Domain Moder from the clients that query or change it's state. It connects all external points to the domain, like: html forms, API clients, command line, frameworks, UI, etc.

Application Service - by example

In order to register a new user following steps must be done:

• get email/login from the client
• check if the email is already in use
• create a new user
• add this new user to the existing user set
• return the user we've just created

Requests / DTO

namespace Application;

class SignUpUserRequest
{
    private $email, $password;
    public function __construct($email, $password) { ... }
    public function email() { return $this->email; }
    public function password() { erturn $this->password(); }
}

Build your DTO / Request in your controller. It might come form the request or form, doesn't matter.

Application Service

namespace Applciation;

class SignUpUserService
{
    private $userRepo;
    public function __construct(UserRepository $ur) { ... }
    
    public function execute(SignUpUserRequest $request)
    {
        $email = $request->email();
        $passwd = $request->password();
        
        $user = $this->userRepository->ofEmail($email); // LOOK! it returns Aggregate,
                                            // I guess Domain Serive should be injected like repo, but dunno..
        
        if (!$user) {
            throw new UserAlreadyExistsException();
        }
        
        $this->userRepo->add(new User($this->userRepo->nextIdentity(), $email, $passwd));
    }
}

Summarization and conclusion

• DTO / Requests:
  • build DTO / Requests within controller (from request or Form)
  • to create DTO use primitives - don't complicate data design
    • that gives you easy way to serialize and deserialize object event to log it or message away
  • do not put logic inside
  • DTO is an data structures not object - DON'T TEST THEM
• Application Service:
  • keep application service thin, use it only to coordinate task on the model
  • one class per each scenario, but if you wish you could use "common" service desing like UserRepository and create more that one method
  • return data to controller as DTO - never domain!
  • you can use external class called asembler to convert DTO into domain and vice versa (therefore app service would not be responsible for that) - code reusage
  • data transformers ???
• Domain Events MUST BE configured before any Application Service fires
• you can execute application service by a command bus

NOTES

Command buses and message systems are used outside the domain model!!! Domain model only published events but all the rest is handled in app layer or infrastructure layer!