feat: add linear retry strategy

sdk/src/main/java/software/amazon/lambda/durable/retry/RetryStrategies.java

@@ -28,6 +28,9 @@ public static class Presets {
                 JitterStrategy.FULL // jitter
                 );
 
+        /** Linear retry strategy: 6 total attempts (1 initial + 5 retries) with 1s, 2s, 3s, 4s, and 5s delays. */
+        public static final RetryStrategy LINEAR = linearBackoff(6, Duration.ofSeconds(1), Duration.ofSeconds(1));
+
         /** No retry strategy - fails immediately on first error. Use this for operations that should not be retried. */
         public static final RetryStrategy NO_RETRY = (error, attempt) -> RetryDecision.fail();
     }
@@ -79,6 +82,33 @@ public static RetryStrategy exponentialBackoff(
         };
     }
 
+    /**
+     * Creates a linear backoff retry strategy.
+     *
+     * <p>The delay calculation follows the formula: delay = initialDelay + increment × (attempt-1)
+     *
+     * @param maxAttempts Maximum number of attempts (including initial attempt)
+     * @param initialDelay Initial delay before first retry
+     * @param increment Amount to add to the delay after each retry attempt
+     * @return RetryStrategy implementing linear backoff
+     */
+    public static RetryStrategy linearBackoff(int maxAttempts, Duration initialDelay, Duration increment) {
+        if (maxAttempts <= 0) {
+            throw new IllegalArgumentException("maxAttempts must be positive");
+        }
+        ParameterValidator.validateDuration(initialDelay, "initialDelay");
+        ParameterValidator.validateDuration(increment, "increment");
+
+        return (error, attempt) -> {
+            if (attempt >= maxAttempts) {
+                return RetryDecision.fail();
+            }
+
+            var delay = initialDelay.plus(increment.multipliedBy(attempt - 1));
+            return RetryDecision.retry(delay);
+        };
+    }
+
     /**
      * Creates a simple retry strategy that retries a fixed number of times with a fixed delay.
      *

sdk/src/test/java/software/amazon/lambda/durable/retry/RetryStrategiesTest.java

@@ -138,6 +138,104 @@ void testFixedDelayStrategy() {
         assertFalse(decision3.shouldRetry());
     }
 
+    @Test
+    void linearBackoff_withCustomDelays_shouldIncreaseByIncrement() {
+        var strategy = RetryStrategies.linearBackoff(5, Duration.ofSeconds(2), Duration.ofSeconds(3));
+
+        var decision1 = strategy.makeRetryDecision(new RuntimeException("test"), 1);
+        var decision2 = strategy.makeRetryDecision(new RuntimeException("test"), 2);
+        var decision3 = strategy.makeRetryDecision(new RuntimeException("test"), 3);
+        var decision4 = strategy.makeRetryDecision(new RuntimeException("test"), 4);
+
+        assertTrue(decision1.shouldRetry());
+        assertEquals(Duration.ofSeconds(2), decision1.delay());
+
+        assertTrue(decision2.shouldRetry());
+        assertEquals(Duration.ofSeconds(5), decision2.delay());
+
+        assertTrue(decision3.shouldRetry());
+        assertEquals(Duration.ofSeconds(8), decision3.delay());
+
+        assertTrue(decision4.shouldRetry());
+        assertEquals(Duration.ofSeconds(11), decision4.delay());
+    }
+
+    @Test
+    void linearPreset_shouldUseOneThroughFiveSecondDelays() {
+        var strategy = RetryStrategies.Presets.LINEAR;
+
+        for (int attempt = 1; attempt <= 5; attempt++) {
+            var decision = strategy.makeRetryDecision(new RuntimeException("test"), attempt);
+
+            assertTrue(decision.shouldRetry(), "Should retry on attempt " + attempt);
+            assertEquals(Duration.ofSeconds(attempt), decision.delay());
+        }
+
+        var finalDecision = strategy.makeRetryDecision(new RuntimeException("test"), 6);
+        assertFalse(finalDecision.shouldRetry());
+    }
+
+    @Test
+    void linearBackoff_shouldStopAtMaxAttempts() {
+        var strategy = RetryStrategies.linearBackoff(3, Duration.ofSeconds(1), Duration.ofSeconds(1));
+
+        var decision1 = strategy.makeRetryDecision(new RuntimeException("test"), 1);
+        var decision2 = strategy.makeRetryDecision(new RuntimeException("test"), 2);
+        var decision3 = strategy.makeRetryDecision(new RuntimeException("test"), 3);
+
+        assertTrue(decision1.shouldRetry());
+        assertTrue(decision2.shouldRetry());
+        assertFalse(decision3.shouldRetry());
+    }
+
+    @Test
+    void linearBackoff_withInvalidMaxAttempts_shouldThrow() {
+        var exception = assertThrows(
+                IllegalArgumentException.class,
+                () -> RetryStrategies.linearBackoff(0, Duration.ofSeconds(1), Duration.ofSeconds(1)));
+
+        assertTrue(exception.getMessage().contains("maxAttempts"));
+        assertTrue(exception.getMessage().contains("positive"));
+    }
+
+    @Test
+    void linearBackoff_withSubSecondInitialDelay_shouldThrow() {
+        var exception = assertThrows(
+                IllegalArgumentException.class,
+                () -> RetryStrategies.linearBackoff(3, Duration.ofMillis(500), Duration.ofSeconds(1)));
+
+        assertTrue(exception.getMessage().contains("initialDelay"));
+        assertTrue(exception.getMessage().contains("at least 1 second"));
+    }
+
+    @Test
+    void linearBackoff_withNullInitialDelay_shouldThrow() {
+        var exception = assertThrows(
+                IllegalArgumentException.class, () -> RetryStrategies.linearBackoff(3, null, Duration.ofSeconds(1)));
+
+        assertTrue(exception.getMessage().contains("initialDelay"));
+        assertTrue(exception.getMessage().contains("cannot be null"));
+    }
+
+    @Test
+    void linearBackoff_withSubSecondIncrement_shouldThrow() {
+        var exception = assertThrows(
+                IllegalArgumentException.class,
+                () -> RetryStrategies.linearBackoff(3, Duration.ofSeconds(1), Duration.ofMillis(500)));
+
+        assertTrue(exception.getMessage().contains("increment"));
+        assertTrue(exception.getMessage().contains("at least 1 second"));
+    }
+
+    @Test
+    void linearBackoff_withNullIncrement_shouldThrow() {
+        var exception = assertThrows(
+                IllegalArgumentException.class, () -> RetryStrategies.linearBackoff(3, Duration.ofSeconds(1), null));
+
+        assertTrue(exception.getMessage().contains("increment"));
+        assertTrue(exception.getMessage().contains("cannot be null"));
+    }
+
     @Test
     void testInvalidParameters() {
         assertThrows(