Repository: iluwatar/java-design-patterns
Issue: Implement Retry pattern with exponential backoff 
Labels: type: enhancement

In practice, we often retry with exponential backoff to avoid making traffic congestion/server load worse, would be nice to implement this option.

Also, I would like to implement this.

Issue comments:
Yes, I agree. Go ahead @awesomefei 
Hi, is this done?
Created pull request #797 in case it's still needed. I am new to open source, would really appreciate any feedback. I used this source to do it: https://cloud.google.com/storage/docs/exponential-backoff
Updated: #829 

Pull request review comments:

Patch that resolved the issue:
diff --git a/retry/src/main/java/com/iluwatar/retry/App.java b/retry/src/main/java/com/iluwatar/retry/App.java
index 376a64fbff97..20205bdf3973 100644
--- a/retry/src/main/java/com/iluwatar/retry/App.java
+++ b/retry/src/main/java/com/iluwatar/retry/App.java
@@ -71,6 +71,7 @@ public static void main(String[] args) throws Exception {
     noErrors();
     errorNoRetry();
     errorWithRetry();
+    errorWithRetryExponentialBackoff();
   }
 
   private static void noErrors() throws Exception {
@@ -102,4 +103,19 @@ private static void errorWithRetry() throws Exception {
         + "the result %s after a number of attempts %s", customerId, retry.attempts()
     ));
   }
+  
+  private static void errorWithRetryExponentialBackoff() throws Exception {
+    final RetryExponentialBackoff<String> retry = new RetryExponentialBackoff<>(
+        new FindCustomer("123", new CustomerNotFoundException("not found")),
+        6,  //6 attempts
+        30000, //30 s max delay between attempts
+        e -> CustomerNotFoundException.class.isAssignableFrom(e.getClass())
+        );
+    op = retry;
+    final String customerId = op.perform();
+    LOG.info(String.format(
+                "However, retrying the operation while ignoring a recoverable error will eventually yield "
+                        + "the result %s after a number of attempts %s", customerId, retry.attempts()
+        ));
+  }
 }
diff --git a/retry/src/main/java/com/iluwatar/retry/RetryExponentialBackoff.java b/retry/src/main/java/com/iluwatar/retry/RetryExponentialBackoff.java
new file mode 100644
index 000000000000..b24bebbce192
--- /dev/null
+++ b/retry/src/main/java/com/iluwatar/retry/RetryExponentialBackoff.java
@@ -0,0 +1,115 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014-2016 Ilkka SeppÃ¤lÃ¤
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package com.iluwatar.retry;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Predicate;
+
+/**
+ * Decorates {@link BusinessOperation business operation} with "retry" capabilities.
+ *
+ * @author George Aristy (george.aristy@gmail.com)
+ * @param <T> the remote op's return type
+ */
+public final class RetryExponentialBackoff<T> implements BusinessOperation<T> {
+  private final BusinessOperation<T> op;
+  private final int maxAttempts;
+  private final long maxDelay;
+  private final AtomicInteger attempts;
+  private final Predicate<Exception> test;
+  private final List<Exception> errors;
+
+    /**
+     * Ctor.
+     *
+     * @param op the {@link BusinessOperation} to retry
+     * @param maxAttempts number of times to retry
+     * @param ignoreTests tests to check whether the remote exception can be ignored. No exceptions
+     *     will be ignored if no tests are given
+     */
+  @SafeVarargs
+  public RetryExponentialBackoff(
+            BusinessOperation<T> op,
+            int maxAttempts,
+            long maxDelay,
+            Predicate<Exception>... ignoreTests
+  ) {
+    this.op = op;
+    this.maxAttempts = maxAttempts;
+    this.maxDelay = maxDelay;
+    this.attempts = new AtomicInteger();
+    this.test = Arrays.stream(ignoreTests).reduce(Predicate::or).orElse(e -> false);
+    this.errors = new ArrayList<>();
+  }
+
+    /**
+     * The errors encountered while retrying, in the encounter order.
+     *
+     * @return the errors encountered while retrying
+     */
+  public List<Exception> errors() {
+    return Collections.unmodifiableList(this.errors);
+  }
+
+    /**
+     * The number of retries performed.
+     *
+     * @return the number of retries performed
+     */
+  public int attempts() {
+    return this.attempts.intValue();
+  }
+
+  @Override
+  public T perform() throws BusinessException {
+    do {
+      try {
+        return this.op.perform();
+      } catch (BusinessException e) {
+        this.errors.add(e);
+
+        if (this.attempts.incrementAndGet() >= this.maxAttempts || !this.test.test(e)) {
+          throw e;
+        }
+
+        try {
+          Random rand = new Random();
+          long testDelay = (long) Math.pow(2, this.attempts()) * 1000 + rand.nextInt(1000);
+          long delay = testDelay < this.maxDelay ? testDelay : maxDelay;
+          Thread.sleep(delay);
+        } catch (InterruptedException f) {
+          //ignore
+        }
+      }
+    }
+    while (true);
+  }
+}
+


Test patch:
diff --git a/retry/src/test/java/com/iluwatar/retry/RetryExponentialBackoffTest.java b/retry/src/test/java/com/iluwatar/retry/RetryExponentialBackoffTest.java
new file mode 100644
index 000000000000..d14b1eef68a6
--- /dev/null
+++ b/retry/src/test/java/com/iluwatar/retry/RetryExponentialBackoffTest.java
@@ -0,0 +1,115 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014-2016 Ilkka SeppÃ¤lÃ¤
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package com.iluwatar.retry;
+
+import org.junit.jupiter.api.Test;
+import static org.hamcrest.CoreMatchers.hasItem;
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+/**
+ * Unit tests for {@link Retry}.
+ *
+ * @author George Aristy (george.aristy@gmail.com)
+ */
+public class RetryExponentialBackoffTest {
+    /**
+     * Should contain all errors thrown.
+     */
+  @Test
+  public void errors() throws Exception {
+    final BusinessException e = new BusinessException("unhandled");
+    final RetryExponentialBackoff<String> retry = new RetryExponentialBackoff<>(
+        () -> { 
+          throw e; 
+        },
+        2,
+        0
+    );
+    try {
+      retry.perform();
+    } catch (BusinessException ex) {
+      //ignore
+    }
+
+    assertThat(
+        retry.errors(),
+        hasItem(e)
+    );
+  }
+
+    /**
+     * No exceptions will be ignored, hence final number of attempts should be 1 even if we're asking
+     * it to attempt twice.
+     */
+  @Test
+  public void attempts() {
+    final BusinessException e = new BusinessException("unhandled");
+    final RetryExponentialBackoff<String> retry = new RetryExponentialBackoff<>(
+        () -> { 
+          throw e; 
+        },
+        2,
+        0
+    );
+    try {
+      retry.perform();
+    } catch (BusinessException ex) {
+      //ignore
+    }
+
+    assertThat(
+        retry.attempts(),
+        is(1)
+    );
+  }
+
+    /**
+     * Final number of attempts should be equal to the number of attempts asked because we are
+     * asking it to ignore the exception that will be thrown.
+     */
+  @Test
+  public void ignore() throws Exception {
+    final BusinessException e = new CustomerNotFoundException("customer not found");
+    final RetryExponentialBackoff<String> retry = new RetryExponentialBackoff<>(
+        () -> { 
+          throw e; 
+        },
+        2,
+        0,
+        ex -> CustomerNotFoundException.class.isAssignableFrom(ex.getClass())
+    );
+    try {
+      retry.perform();
+    } catch (BusinessException ex) {
+      //ignore
+    }
+
+    assertThat(
+        retry.attempts(),
+        is(2)
+    );
+  }
+}