mongodb · nhachicha · Dec 3, 2025
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+/*
+ * Copyright 2008-present MongoDB, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.mongodb.internal;
+
+import com.mongodb.annotations.NotThreadSafe;
+
+import java.util.concurrent.ThreadLocalRandom;
+
+/**
+ * Implements exponential backoff with jitter for retry scenarios.
+ * Formula: delayMS = jitter * min(maxBackoffMs, baseBackoffMs * growthFactor^retryCount)
+ * where jitter is random value [0, 1).
+ *
+ * <p>This class provides factory methods for common use cases:
+ * <ul>
+ *   <li>{@link #forTransactionRetry()} - For withTransaction retries (5ms base, 500ms max, 1.5 growth)</li>
+ *   <li>{@link #forCommandRetry()} - For command retries with overload (100ms base, 10000ms max, 2.0 growth)</li>
+ * </ul>
+ */
+@NotThreadSafe
+public final class ExponentialBackoff {
+    // Transaction retry constants (per spec)
+    private static final double TRANSACTION_BASE_BACKOFF_MS = 5.0;
+    private static final double TRANSACTION_MAX_BACKOFF_MS = 500.0;
+    private static final double TRANSACTION_BACKOFF_GROWTH = 1.5;
+
+    // Command retry constants (per spec)
+    private static final double COMMAND_BASE_BACKOFF_MS = 100.0;
+    private static final double COMMAND_MAX_BACKOFF_MS = 10000.0;
+    private static final double COMMAND_BACKOFF_GROWTH = 2.0;
+
+    private final double baseBackoffMs;
+    private final double maxBackoffMs;
+    private final double growthFactor;
+    private int retryCount = 0;
+
+    /**
+     * Creates an exponential backoff instance with specified parameters.
+     *
+     * @param baseBackoffMs Initial backoff in milliseconds
+     * @param maxBackoffMs  Maximum backoff cap in milliseconds
+     * @param growthFactor  Exponential growth factor (e.g., 1.5 or 2.0)
+     */
+    public ExponentialBackoff(final double baseBackoffMs, final double maxBackoffMs, final double growthFactor) {
+        this.baseBackoffMs = baseBackoffMs;
+        this.maxBackoffMs = maxBackoffMs;
+        this.growthFactor = growthFactor;
+    }
+
+    /**
+     * Creates a backoff instance configured for withTransaction retries.
+     * Uses: 5ms base, 500ms max, 1.5 growth factor.
+     *
+     * @return ExponentialBackoff configured for transaction retries
+     */
+    public static ExponentialBackoff forTransactionRetry() {
+        return new ExponentialBackoff(
+                TRANSACTION_BASE_BACKOFF_MS,
+                TRANSACTION_MAX_BACKOFF_MS,
+                TRANSACTION_BACKOFF_GROWTH
+        );
+    }
+
+    /**
+     * Creates a backoff instance configured for command retries during overload.
+     * Uses: 100ms base, 10000ms max, 2.0 growth factor.
+     *
+     * @return ExponentialBackoff configured for command retries
+     */
+    public static ExponentialBackoff forCommandRetry() {
+        return new ExponentialBackoff(
+                COMMAND_BASE_BACKOFF_MS,
+                COMMAND_MAX_BACKOFF_MS,
+                COMMAND_BACKOFF_GROWTH
+        );
+    }
+
+    /**
+     * Calculate next backoff delay with jitter.
+     *
+     * @return delay in milliseconds
+     */
+    public long calculateDelayMs() {
+        double jitter = ThreadLocalRandom.current().nextDouble();
+        double exponentialBackoff = baseBackoffMs * Math.pow(growthFactor, retryCount);
+        double cappedBackoff = Math.min(exponentialBackoff, maxBackoffMs);
+        retryCount++;
+        return Math.round(jitter * cappedBackoff);
+    }
+
+    /**
+     * Apply backoff delay by sleeping current thread.
+     *
+     * @throws InterruptedException if thread is interrupted during sleep
+     */
+    public void applyBackoff() throws InterruptedException {
+        long delayMs = calculateDelayMs();
+        if (delayMs > 0) {
+            Thread.sleep(delayMs);
+        }
+    }
+
+    /**
+     * Check if applying backoff would exceed the retry time limit.
+     * @param startTimeMs start time of retry attempts
+     * @param maxRetryTimeMs maximum retry time allowed
+     * @return true if backoff would exceed limit, false otherwise
+     */
+//    public boolean wouldExceedTimeLimit(final long startTimeMs, final long maxRetryTimeMs) {
+//        long elapsedMs = ClientSessionClock.INSTANCE.now() - startTimeMs;
+//        // Peek at next delay without incrementing counter
+//        double exponentialBackoff = baseBackoffMs * Math.pow(growthFactor, retryCount);
+//        double cappedBackoff = Math.min(exponentialBackoff, maxBackoffMs);
+//        long maxPossibleDelay = Math.round(cappedBackoff); // worst case with jitter=1
+//        return elapsedMs + maxPossibleDelay > maxRetryTimeMs;
+//    }
+
+    /**
+     * Reset retry counter for new sequence of retries.
+     */
+    public void reset() {
+        retryCount = 0;
+    }
+
+    /**
+     * Get current retry count for testing.
+     *
+     * @return current retry count
+     */
+    public int getRetryCount() {
+        return retryCount;
+    }
+
+    /**
+     * Get the base backoff in milliseconds.
+     *
+     * @return base backoff
+     */
+    public double getBaseBackoffMs() {
+        return baseBackoffMs;
+    }
+
+    /**
+     * Get the maximum backoff in milliseconds.
+     *
+     * @return maximum backoff
+     */
+    public double getMaxBackoffMs() {
+        return maxBackoffMs;
+    }
+
+    /**
+     * Get the growth factor.
+     *
+     * @return growth factor
+     */
+    public double getGrowthFactor() {
+        return growthFactor;
+    }
+}
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+/*
+ * Copyright 2008-present MongoDB, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.mongodb.internal;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ExponentialBackoffTest {
+
+    @Test
+    void testTransactionRetryBackoff() {
+        ExponentialBackoff backoff = ExponentialBackoff.forTransactionRetry();
+
+        // Verify configuration
+        assertEquals(5.0, backoff.getBaseBackoffMs());
+        assertEquals(500.0, backoff.getMaxBackoffMs());
+        assertEquals(1.5, backoff.getGrowthFactor());
+
+        // First retry (i=0): delay = jitter * min(5 * 1.5^0, 500) = jitter * 5
+        // Since jitter is random [0,1), the delay should be between 0 and 5ms
+        long delay1 = backoff.calculateDelayMs();
+        assertTrue(delay1 >= 0 && delay1 <= 5, "First delay should be 0-5ms, got: " + delay1);
+
+        // Second retry (i=1): delay = jitter * min(5 * 1.5^1, 500) = jitter * 7.5
+        long delay2 = backoff.calculateDelayMs();
+        assertTrue(delay2 >= 0 && delay2 <= 8, "Second delay should be 0-8ms, got: " + delay2);
+
+        // Third retry (i=2): delay = jitter * min(5 * 1.5^2, 500) = jitter * 11.25
+        long delay3 = backoff.calculateDelayMs();
+        assertTrue(delay3 >= 0 && delay3 <= 12, "Third delay should be 0-12ms, got: " + delay3);
+
+        // Verify the retry count is incrementing properly
+        assertEquals(3, backoff.getRetryCount());
+    }
+
+    @Test
+    void testTransactionRetryBackoffRespectsMaximum() {
+        ExponentialBackoff backoff = ExponentialBackoff.forTransactionRetry();
+
+        // Advance to a high retry count where backoff would exceed 500ms without capping
+        for (int i = 0; i < 20; i++) {
+            backoff.calculateDelayMs();
+        }
+
+        // Even at high retry counts, delay should never exceed 500ms
+        for (int i = 0; i < 5; i++) {
+            long delay = backoff.calculateDelayMs();
+            assertTrue(delay >= 0 && delay <= 500, "Delay should be capped at 500ms, got: " + delay);
+        }
+    }
+
+    @Test
+    void testTransactionRetryBackoffSequenceWithExpectedValues() {
+        // Test that the backoff sequence follows the expected pattern with growth factor 1.5
+        // Expected sequence (without jitter): 5, 7.5, 11.25, 16.875, 25.3125, 37.96875, 56.953125, ...
+        // With jitter, actual values will be between 0 and these maxima
+
+        ExponentialBackoff backoff = ExponentialBackoff.forTransactionRetry();
+
+        double[] expectedMaxValues = {5.0, 7.5, 11.25, 16.875, 25.3125, 37.96875, 56.953125, 85.4296875,
+                128.14453125, 192.21679688, 288.32519531, 432.48779297, 500.0};
+
+        for (int i = 0; i < expectedMaxValues.length; i++) {
+            long delay = backoff.calculateDelayMs();
+            assertTrue(delay >= 0 && delay <= Math.round(expectedMaxValues[i]),
+                    String.format("Retry %d: delay should be 0-%d ms, got: %d", i, Math.round(expectedMaxValues[i]), delay));
+        }
+    }
+
+    @Test
+    void testCommandRetryBackoff() {
+        ExponentialBackoff backoff = ExponentialBackoff.forCommandRetry();
+
+        // Verify configuration
+        assertEquals(100.0, backoff.getBaseBackoffMs());
+        assertEquals(10000.0, backoff.getMaxBackoffMs());
+        assertEquals(2.0, backoff.getGrowthFactor());
+
+        // Test sequence with growth factor 2.0
+        // Expected max delays: 100, 200, 400, 800, 1600, 3200, 6400, 10000 (capped)
+        long delay1 = backoff.calculateDelayMs();
+        assertTrue(delay1 >= 0 && delay1 <= 100, "First delay should be 0-100ms, got: " + delay1);
+
+        long delay2 = backoff.calculateDelayMs();
+        assertTrue(delay2 >= 0 && delay2 <= 200, "Second delay should be 0-200ms, got: " + delay2);
+
+        long delay3 = backoff.calculateDelayMs();
+        assertTrue(delay3 >= 0 && delay3 <= 400, "Third delay should be 0-400ms, got: " + delay3);
+
+        long delay4 = backoff.calculateDelayMs();
+        assertTrue(delay4 >= 0 && delay4 <= 800, "Fourth delay should be 0-800ms, got: " + delay4);
+
+        long delay5 = backoff.calculateDelayMs();
+        assertTrue(delay5 >= 0 && delay5 <= 1600, "Fifth delay should be 0-1600ms, got: " + delay5);
+    }
+
+    @Test
+    void testCommandRetryBackoffRespectsMaximum() {
+        ExponentialBackoff backoff = ExponentialBackoff.forCommandRetry();
+
+        // Advance to where exponential would exceed 10000ms
+        for (int i = 0; i < 10; i++) {
+            backoff.calculateDelayMs();
+        }
+
+        // Even at high retry counts, delay should never exceed 10000ms
+        for (int i = 0; i < 5; i++) {
+            long delay = backoff.calculateDelayMs();
+            assertTrue(delay >= 0 && delay <= 10000, "Delay should be capped at 10000ms, got: " + delay);
+        }
+    }
+
+    @Test
+    void testCustomBackoff() {
+        // Test with custom parameters
+        ExponentialBackoff backoff = new ExponentialBackoff(50.0, 2000.0, 1.8);
+
+        assertEquals(50.0, backoff.getBaseBackoffMs());
+        assertEquals(2000.0, backoff.getMaxBackoffMs());
+        assertEquals(1.8, backoff.getGrowthFactor());
+
+        // First delay: 0-50ms
+        long delay1 = backoff.calculateDelayMs();
+        assertTrue(delay1 >= 0 && delay1 <= 50, "First delay should be 0-50ms, got: " + delay1);
+
+        // Second delay: 0-90ms (50 * 1.8)
+        long delay2 = backoff.calculateDelayMs();
+        assertTrue(delay2 >= 0 && delay2 <= 90, "Second delay should be 0-90ms, got: " + delay2);
+    }
+
+    @Test
+    void testReset() {
+        ExponentialBackoff backoff = ExponentialBackoff.forTransactionRetry();
+
+        // Perform some retries
+        backoff.calculateDelayMs();
+        backoff.calculateDelayMs();
+        assertEquals(2, backoff.getRetryCount());
+
+        // Reset and verify counter is back to 0
+        backoff.reset();
+        assertEquals(0, backoff.getRetryCount());
+
+        // First delay after reset should be in the initial range again
+        long delay = backoff.calculateDelayMs();
+        assertTrue(delay >= 0 && delay <= 5, "First delay after reset should be 0-5ms, got: " + delay);
+    }
+
+//    @Test
+//    void testWouldExceedTimeLimitTransactionRetry() {
+//        ExponentialBackoff backoff = ExponentialBackoff.forTransactionRetry();
+//        long startTime = ClientSessionClock.INSTANCE.now();
+//
+//        // Initially, should not exceed time limit
+//        assertFalse(backoff.wouldExceedTimeLimit(startTime, 120000));
+//
+//        // With very little time remaining (4ms), first backoff (up to 5ms) would exceed
+//        long nearLimitTime = startTime - 119996; // 4ms remaining
+//        assertTrue(backoff.wouldExceedTimeLimit(nearLimitTime, 120000));
+//    }
+
+//    @Test
+//    void testWouldExceedTimeLimitCommandRetry() {
+//        ExponentialBackoff backoff = ExponentialBackoff.forCommandRetry();
+//        long startTime = ClientSessionClock.INSTANCE.now();
+//
+//        // Initially, should not exceed time limit
+//        assertFalse(backoff.wouldExceedTimeLimit(startTime, 10000));
+//
+//        // With 99ms remaining, first backoff (up to 100ms) would exceed
+//        long nearLimitTime = startTime - 9901; // 99ms remaining
+//        assertTrue(backoff.wouldExceedTimeLimit(nearLimitTime, 10000));
+//    }
+
+    @Test
+    void testCommandRetrySequenceMatchesSpec() {
+        // Test that command retry follows spec: 100ms * 2^i capped at 10000ms
+        ExponentialBackoff backoff = ExponentialBackoff.forCommandRetry();
+
+        double[] expectedMaxValues = {100.0, 200.0, 400.0, 800.0, 1600.0, 3200.0, 6400.0, 10000.0, 10000.0};
+
+        for (int i = 0; i < expectedMaxValues.length; i++) {
+            long delay = backoff.calculateDelayMs();
+            double expectedMax = expectedMaxValues[i];
+            assertTrue(delay >= 0 && delay <= Math.round(expectedMax),
+                    String.format("Retry %d: delay should be 0-%d ms, got: %d", i, Math.round(expectedMax), delay));
+        }
+    }
+}