Add waitForCondition Operation

docs/spec/waitForCondition.md

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+# Design: waitForCondition for Durable Execution Java SDK
+
+## Overview
+
+This design adds a `waitForCondition` operation to the Java Durable Execution SDK. The operation periodically checks a user-supplied condition function, using a configurable wait strategy to determine polling intervals and termination. It follows the same checkpoint-and-replay model as existing operations (`step`, `wait`, `invoke`) and mirrors the JavaScript SDK's `waitForCondition` implementation.
+
+## Architecture
+
+### How it works
+
+`waitForCondition` is implemented as a specialized step operation that uses the RETRY checkpoint action for polling iterations:
+
+1. User calls `ctx.waitForCondition(name, resultType, checkFunc, config)`
+2. A `WaitForConditionOperation` is created with a unique operation ID
+3. On first execution:
+   - Checkpoint START with subtype `WAIT_FOR_CONDITION`
+   - Execute the check function with `initialState` and a `StepContext`
+   - Call the wait strategy with the new state and attempt number
+   - If `stopPolling()`: checkpoint SUCCEED with the final state, return it
+   - If `continuePolling(delay)`: checkpoint RETRY with the state and delay, poll for READY, then loop
+   - If check function throws: checkpoint FAIL, propagate the error
+4. On replay:
+   - SUCCEEDED: return cached result (skip re-execution)
+   - FAILED: re-throw cached error
+   - PENDING: wait for READY transition, then resume polling
+   - STARTED/READY: resume execution from current attempt and state
+
+This matches the JS SDK's behavior where each polling iteration is a RETRY on the same STEP operation.
+
+### New Classes
+
+```
+sdk/src/main/java/software/amazon/lambda/durable/
+├── WaitForConditionConfig.java       # Config builder (waitStrategy, initialState, serDes)
+├── WaitForConditionWaitStrategy.java  # Functional interface: (T state, int attempt) → WaitForConditionDecision
+├── WaitForConditionDecision.java     # Sealed result: continuePolling(Duration) | stopPolling()
+├── WaitStrategies.java               # Factory with builder for common patterns
+├── operation/
+│   └── WaitForConditionOperation.java  # Operation implementation
+├── model/
+│   └── OperationSubType.java          # Add WAIT_FOR_CONDITION enum value
+```
+
+### Class Diagram
+
+```
+DurableContext
+  ├── waitForCondition(name, Class<T>, checkFunc, config) → T
+  ├── waitForCondition(name, TypeToken<T>, checkFunc, config) → T
+  ├── waitForConditionAsync(name, Class<T>, checkFunc, config) → DurableFuture<T>
+  └── waitForConditionAsync(name, TypeToken<T>, checkFunc, config) → DurableFuture<T>
+         │
+         ▼
+WaitForConditionOperation<T> extends BaseDurableOperation<T>
+  ├── start()           → checkpoint START, execute check loop
+  ├── replay(existing)  → handle SUCCEEDED/FAILED/PENDING/STARTED/READY
+  ├── get()             → block, deserialize result or throw
+  └── executeCheckLoop(currentState, attempt)
+         │
+         ├── calls checkFunc(state, stepContext) → newState
+         ├── calls waitStrategy.evaluate(newState, attempt) → WaitForConditionDecision
+         │     ├── stopPolling()         → checkpoint SUCCEED
+         │     └── continuePolling(delay) → checkpoint RETRY, poll, loop
+         └── on error → checkpoint FAIL
+```
+
+## Detailed Design
+
+### WaitForConditionWaitStrategy<T> (Functional Interface)
+
+```java
+@FunctionalInterface
+public interface WaitForConditionWaitStrategy<T> {
+    WaitForConditionDecision evaluate(T state, int attempt);
+}
+```
+
+- `state`: the current state returned by the check function
+- `attempt`: 1-based attempt number (first check is attempt 1)
+- Returns a `WaitForConditionDecision` indicating whether to continue or stop
+
+### WaitForConditionDecision
+
+```java
+public sealed interface WaitForConditionDecision {
+    record ContinuePolling(Duration delay) implements WaitForConditionDecision {}
+    record StopPolling() implements WaitForConditionDecision {}
+
+    static WaitForConditionDecision continuePolling(Duration delay) {
+        return new ContinuePolling(delay);
+    }
+
+    static WaitForConditionDecision stopPolling() {
+        return new StopPolling();
+    }
+}
+```
+
+Uses Java sealed interfaces for type safety. The `delay` in `ContinuePolling` must be >= 1 second (enforced at construction).
+
+### WaitStrategies (Factory)
+
+```java
+public final class WaitStrategies {
+    public static <T> Builder<T> builder(Predicate<T> shouldContinuePolling) { ... }
+
+    public static class Builder<T> {
+        // Defaults match JS SDK
+        private int maxAttempts = 60;
+        private Duration initialDelay = Duration.ofSeconds(5);
+        private Duration maxDelay = Duration.ofSeconds(300);
+        private double backoffRate = 1.5;
+        private JitterStrategy jitter = JitterStrategy.FULL;
+
+        public Builder<T> maxAttempts(int maxAttempts) { ... }
+        public Builder<T> initialDelay(Duration initialDelay) { ... }
+        public Builder<T> maxDelay(Duration maxDelay) { ... }
+        public Builder<T> backoffRate(double backoffRate) { ... }
+        public Builder<T> jitter(JitterStrategy jitter) { ... }
+        public WaitForConditionWaitStrategy<T> build() { ... }
+    }
+}
+```
+
+The built strategy:
+1. Calls `shouldContinuePolling.test(state)` — if false, returns `stopPolling()`
+2. Checks `attempt >= maxAttempts` — if true, throws `WaitForConditionException`
+3. Calculates delay: `min(initialDelay * backoffRate^(attempt-1), maxDelay)`
+4. Applies jitter using the existing `JitterStrategy` enum
+5. Ensures delay >= 1 second, rounds to nearest integer second
+6. Returns `continuePolling(delay)`
+
+### WaitForConditionConfig<T>
+
+```java
+public class WaitForConditionConfig<T> {
+    private final WaitForConditionWaitStrategy<T> waitStrategy;
+    private final T initialState;
+    private final SerDes serDes;  // nullable, falls back to DurableConfig default
+
+    public static <T> Builder<T> builder(WaitForConditionWaitStrategy<T> waitStrategy, T initialState) { ... }
+
+    public static class Builder<T> {
+        public Builder<T> serDes(SerDes serDes) { ... }
+        public WaitForConditionConfig<T> build() { ... }
+    }
+}
+```
+
+`waitStrategy` and `initialState` are required constructor parameters on the builder (not optional setters), so they can never be null.
+
+### WaitForConditionOperation<T>
+
+Extends `BaseDurableOperation<T>`. Key behaviors:
+
+- **start()**: Begins the check loop from `initialState` at attempt 0
+- **replay(existing)**: Handles all operation statuses (SUCCEEDED, FAILED, PENDING, STARTED, READY)
+- **executeCheckLoop(state, attempt)**: Core polling logic
+  - Creates a `StepContext` for the check function
+  - Executes check function in the user executor (same pattern as `StepOperation`)
+  - Serializes/deserializes state through SerDes (round-trip, matching JS SDK)
+  - Calls wait strategy with deserialized state
+  - Checkpoints RETRY with `NextAttemptDelaySeconds` or SUCCEED/FAIL
+- **get()**: Blocks on completion, deserializes result or throws exception
+
+All checkpoint updates use `OperationType.STEP` and `OperationSubType.WAIT_FOR_CONDITION`.
+
+### DurableContext API Methods
+
+```java
+// Sync methods (block until condition met)
+public <T> T waitForCondition(String name, Class<T> resultType,
+    Function<StepContext, T> checkFunc, WaitForConditionConfig<T> config)
+
+public <T> T waitForCondition(String name, TypeToken<T> typeToken,
+    Function<StepContext, T> checkFunc, WaitForConditionConfig<T> config)
+
+// Async methods (return DurableFuture immediately)
+public <T> DurableFuture<T> waitForConditionAsync(String name, Class<T> resultType,
+    Function<StepContext, T> checkFunc, WaitForConditionConfig<T> config)
+
+public <T> DurableFuture<T> waitForConditionAsync(String name, TypeToken<T> typeToken,
+    Function<StepContext, T> checkFunc, WaitForConditionConfig<T> config)
+```
+
+The check function signature is `Function<StepContext, T>` rather than `BiFunction<T, StepContext, T>` because the current state is managed internally by the operation. The check function receives the current state via the operation's internal loop — the `StepContext` provides logging and attempt info. Wait, actually looking at the JS SDK more carefully, the check function does receive the current state as a parameter: `(state: T, context) => Promise<T>`. So the Java signature should be `BiFunction<T, StepContext, T>`.
+
+Corrected signature:
+
+```java
+public <T> DurableFuture<T> waitForConditionAsync(String name, TypeToken<T> typeToken,
+    BiFunction<T, StepContext, T> checkFunc, WaitForConditionConfig<T> config)
+```
+
+### OperationSubType Addition
+
+```java
+public enum OperationSubType {
+    RUN_IN_CHILD_CONTEXT("RunInChildContext"),
+    MAP("Map"),
+    PARALLEL("Parallel"),
+    WAIT_FOR_CALLBACK("WaitForCallback"),
+    WAIT_FOR_CONDITION("WaitForCondition");  // NEW
+    ...
+}
+```
+
+### Error Handling
+
+- **Check function throws**: Checkpoint FAIL with serialized error, wrap in `WaitForConditionException`
+- **Max attempts exceeded**: `WaitStrategies`-built strategy throws `WaitForConditionException("waitForCondition exceeded maximum attempts (N)")`
+- **Custom strategy throws**: Propagated as-is (checkpoint FAIL)
+- **SerDes failure**: Wrapped in `SerDesException` (existing pattern)
+
+A new `WaitForConditionException` extends `DurableOperationException` for domain-specific errors.
+
+### Exception Class
+
+```java
+public class WaitForConditionException extends DurableOperationException {
+    public WaitForConditionException(String message) { ... }
+    public WaitForConditionException(Operation operation) { ... }
+}
+```
+
+## Testing Strategy
+
+### Unit Tests (sdk/src/test/)
+- `WaitForConditionDecisionTest`: verify `continuePolling`/`stopPolling` factory methods
+- `WaitStrategiesTest`: verify builder defaults, exponential backoff, jitter, max attempts
+- `WaitForConditionConfigTest`: verify builder validation
+- `WaitForConditionOperationTest`: verify start, replay, error handling
+
+### Integration Tests (sdk-integration-tests/)
+- `WaitForConditionIntegrationTest`: end-to-end with `LocalDurableTestRunner`, verify replay across invocations
+
+### Example Tests (examples/)
+- `WaitForConditionExample`: demonstrates polling with `WaitStrategies` factory
+- `WaitForConditionExampleTest`: verifies example with `LocalDurableTestRunner`
+
+### Testing Framework
+- JUnit 5 for all tests
+- jqwik for property-based tests (already available in the project's test dependencies — if not, we'll use JUnit 5 parameterized tests with random generators)
+
+## Correctness Properties
+
+### Property 1: WaitForConditionWaitStrategy contract — stopPolling terminates
+For any state `s` of type `T` and any attempt number `n >= 1`, if `waitStrategy.evaluate(s, n)` returns `StopPolling`, then `waitForCondition` completes with `s` as the result.
+
+**Validates: Requirements 1.5, 2.1**
+
+### Property 2: WaitStrategies factory — exponential backoff calculation
+For any `initialDelay d`, `backoffRate r >= 1`, `maxDelay m >= d`, and attempt `n >= 1` with jitter=NONE, the delay equals `min(d * r^(n-1), m)` rounded to the nearest integer second, with a minimum of 1 second.
+
+**Validates: Requirements 2.3, 2.4**
+
+### Property 3: WaitStrategies factory — max attempts enforcement
+For any `maxAttempts N >= 1` and any state where `shouldContinuePolling` returns true, calling the strategy with `attempt >= N` must throw `WaitForConditionException`.
+
+**Validates: Requirements 2.5**
+
+### Property 4: WaitForConditionConfig — required fields validation
+Building a `WaitForConditionConfig` without a `waitStrategy` or with a null `initialState` must always throw an exception, regardless of other configuration.
+
+**Validates: Requirements 3.2**
+
+### Property 5: WaitForConditionWaitStrategy receives correct state and attempt
+For any sequence of check function invocations, the wait strategy always receives the state returned by the most recent check function call and the correct 1-based attempt number.
+
+**Validates: Requirements 1.3, 2.1**
+
+### Property 6: Operation name validation
+For any string that violates `ParameterValidator.validateOperationName` rules, calling `waitForCondition` or `waitForConditionAsync` with that name must throw.
+
+**Validates: Requirements 5.4**
+
+### Property 7: Jitter bounds
+For any delay `d` and jitter strategy: NONE produces exactly `d`, FULL produces a value in `[0, d]` (clamped to min 1s), HALF produces a value in `[d/2, d]`.
+
+**Validates: Requirements 2.3**

examples/src/main/java/software/amazon/lambda/durable/examples/WaitForConditionExample.java

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+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+package software.amazon.lambda.durable.examples;
+
+import java.time.Duration;
+import software.amazon.lambda.durable.DurableContext;
+import software.amazon.lambda.durable.DurableHandler;
+import software.amazon.lambda.durable.WaitForConditionConfig;
+import software.amazon.lambda.durable.WaitStrategies;
+import software.amazon.lambda.durable.retry.JitterStrategy;
+
+/**
+ * Example demonstrating the waitForCondition operation.
+ *
+ * <p>Polls a counter until it reaches the length of the input name, then returns the final count.
+ */
+public class WaitForConditionExample extends DurableHandler<GreetingRequest, Integer> {
+
+    @Override
+    public Integer handleRequest(GreetingRequest input, DurableContext context) {
+        var targetCount = input.getName().length();
+
+        var strategy = WaitStrategies.<Integer>builder(state -> state < targetCount)
+                .initialDelay(Duration.ofSeconds(1))
+                .jitter(JitterStrategy.NONE)
+                .build();
+
+        var config = WaitForConditionConfig.<Integer>builder(strategy, 0).build();
+
+        return context.waitForCondition("count-to-name-length", Integer.class, (state, stepCtx) -> state + 1, config);
+    }
+}

examples/src/test/java/software/amazon/lambda/durable/examples/WaitForConditionExampleTest.java

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+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+package software.amazon.lambda.durable.examples;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import software.amazon.lambda.durable.model.ExecutionStatus;
+import software.amazon.lambda.durable.testing.LocalDurableTestRunner;
+
+class WaitForConditionExampleTest {
+
+    @Test
+    void testWaitForConditionExample() {
+        var handler = new WaitForConditionExample();
+        var runner = LocalDurableTestRunner.create(GreetingRequest.class, handler);
+
+        var result = runner.runUntilComplete(new GreetingRequest("Alice"));
+
+        assertEquals(ExecutionStatus.SUCCEEDED, result.getStatus());
+        assertEquals(5, result.getResult(Integer.class));
+    }
+}