feat: expand RetryPresets with fixed, linear, and slow strategies

- Add BackoffStrategy enum (EXPONENTIAL, LINEAR, FIXED) to config.py
- Add backoff_strategy field to RetryStrategyConfig (defaults to EXPONENTIAL)
- Delegate delay calculation to BackoffStrategy.calculate_base_delay()
- Add three new presets: fixed_wait, linear_backoff, slow
- Add comprehensive tests for new backoff strategies and presets

Resolves #328

Author: AdityaAudi

src/aws_durable_execution_sdk_python/config.py

@@ -453,6 +453,55 @@ def result(self, timeout_seconds: int | None = None) -> T:
         return self.future.result(timeout=timeout_seconds)
 
 
+# region Backoff
+
+
+class BackoffStrategy(StrEnum):
+    """
+    Backoff strategies determine how retry delay grows between attempts.
+
+    members:
+        :EXPONENTIAL: Delay grows exponentially: initial_delay * backoff_rate ^ (attempts - 1)
+        :LINEAR: Delay grows linearly: initial_delay * attempts_made
+        :FIXED: Delay stays constant: initial_delay
+    """
+
+    EXPONENTIAL = "EXPONENTIAL"
+    LINEAR = "LINEAR"
+    FIXED = "FIXED"
+
+    def calculate_base_delay(
+        self,
+        initial_delay_seconds: int,
+        backoff_rate: Numeric,
+        attempts_made: int,
+        max_delay_seconds: int,
+    ) -> float:
+        """Calculate base delay before jitter for the given attempt.
+
+        Args:
+            initial_delay_seconds: The initial delay in seconds.
+            backoff_rate: The rate at which delay grows (used by EXPONENTIAL).
+            attempts_made: Number of attempts already made (1-based).
+            max_delay_seconds: Maximum delay cap in seconds.
+
+        Returns:
+            The base delay in seconds, capped at max_delay_seconds.
+        """
+        match self:
+            case BackoffStrategy.FIXED:
+                base: float = float(initial_delay_seconds)
+            case BackoffStrategy.LINEAR:
+                base = float(initial_delay_seconds) * attempts_made
+            case _:  # default is EXPONENTIAL
+                base = initial_delay_seconds * (backoff_rate ** (attempts_made - 1))
+
+        return min(base, max_delay_seconds)
+
+
+# endregion Backoff
+
+
 # region Jitter
 
 

src/aws_durable_execution_sdk_python/retries.py

@@ -7,7 +7,11 @@
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING
 
-from aws_durable_execution_sdk_python.config import Duration, JitterStrategy
+from aws_durable_execution_sdk_python.config import (
+    BackoffStrategy,
+    Duration,
+    JitterStrategy,
+)
 
 if TYPE_CHECKING:
     from collections.abc import Callable
@@ -49,6 +53,7 @@ class RetryStrategyConfig:
         default_factory=lambda: Duration.from_minutes(5)
     )  # 5 minutes
     backoff_rate: Numeric = 2.0
+    backoff_strategy: BackoffStrategy = field(default=BackoffStrategy.EXPONENTIAL)
     jitter_strategy: JitterStrategy = field(default=JitterStrategy.FULL)
     retryable_errors: list[str | re.Pattern] | None = None
     retryable_error_types: list[type[Exception]] | None = None
@@ -103,10 +108,12 @@ def retry_strategy(error: Exception, attempts_made: int) -> RetryDecision:
         if not is_retryable_error_message and not is_retryable_error_type:
             return RetryDecision.no_retry()
 
-        # Calculate delay with exponential backoff
-        base_delay: float = min(
-            config.initial_delay_seconds * (config.backoff_rate ** (attempts_made - 1)),
-            config.max_delay_seconds,
+        # Calculate delay using configured backoff strategy
+        base_delay: float = config.backoff_strategy.calculate_base_delay(
+            initial_delay_seconds=config.initial_delay_seconds,
+            backoff_rate=config.backoff_rate,
+            attempts_made=attempts_made,
+            max_delay_seconds=config.max_delay_seconds,
         )
         # Apply jitter to get final delay
         delay_with_jitter: float = config.jitter_strategy.apply_jitter(base_delay)
@@ -172,3 +179,42 @@ def critical(cls) -> Callable[[Exception, int], RetryDecision]:
                 jitter_strategy=JitterStrategy.NONE,
             )
         )
+
+    @classmethod
+    def fixed_wait(cls) -> Callable[[Exception, int], RetryDecision]:
+        """Constant delay between retries with no backoff."""
+        return create_retry_strategy(
+            RetryStrategyConfig(
+                max_attempts=5,
+                initial_delay=Duration.from_seconds(5),
+                max_delay=Duration.from_minutes(5),
+                backoff_strategy=BackoffStrategy.FIXED,
+                jitter_strategy=JitterStrategy.NONE,
+            )
+        )
+
+    @classmethod
+    def linear_backoff(cls) -> Callable[[Exception, int], RetryDecision]:
+        """Linearly increasing delay between retries."""
+        return create_retry_strategy(
+            RetryStrategyConfig(
+                max_attempts=5,
+                initial_delay=Duration.from_seconds(5),
+                max_delay=Duration.from_minutes(5),
+                backoff_strategy=BackoffStrategy.LINEAR,
+                jitter_strategy=JitterStrategy.FULL,
+            )
+        )
+
+    @classmethod
+    def slow(cls) -> Callable[[Exception, int], RetryDecision]:
+        """Long delays for operations that need extended recovery time."""
+        return create_retry_strategy(
+            RetryStrategyConfig(
+                max_attempts=8,
+                initial_delay=Duration.from_seconds(30),
+                max_delay=Duration.from_minutes(10),
+                backoff_rate=2,
+                jitter_strategy=JitterStrategy.FULL,
+            )
+        )