| /* |
| * Copyright 2011 Google 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.google.ipc.invalidation.util; |
| |
| import java.util.Random; |
| |
| /** |
| * Class that generates successive intervals for random exponential backoff. Class tracks a |
| * "high water mark" which is doubled each time {@code getNextDelay} is called; each call to |
| * {@code getNextDelay} returns a value uniformly randomly distributed between 0 (inclusive) and the |
| * high water mark (exclusive). Note that this class does not dictate the time units for which the |
| * delay is computed. |
| * |
| */ |
| public class ExponentialBackoffDelayGenerator { |
| |
| /** Initial allowed delay time. */ |
| private final int initialMaxDelay; |
| |
| /** Maximum allowed delay time. */ |
| private final int maxDelay; |
| |
| /** Next delay time to use. */ |
| private int currentMaxDelay; |
| |
| /** If the first call to {@code getNextDelay} has been made after reset. */ |
| private boolean inRetryMode; |
| |
| private final Random random; |
| |
| /** |
| * Creates a generator with the given initial delay and the maximum delay (in terms of a factor of |
| * the initial delay). |
| */ |
| public ExponentialBackoffDelayGenerator(Random random, int initialMaxDelay, |
| int maxExponentialFactor) { |
| Preconditions.checkArgument(maxExponentialFactor > 0, "max factor must be positive"); |
| this.random = Preconditions.checkNotNull(random); |
| Preconditions.checkArgument(initialMaxDelay > 0, "initial delay must be positive"); |
| this.initialMaxDelay = initialMaxDelay; |
| this.maxDelay = initialMaxDelay * maxExponentialFactor; |
| Preconditions.checkState(maxDelay > 0, "max delay must be positive"); |
| reset(); |
| } |
| |
| /** |
| * A constructor to restore a generator from saved state. Creates a generator with the given |
| * initial delay and the maximum delay (in terms of a factor of the initial delay). |
| * |
| * @param currentMaxDelay saved current max delay |
| * @param inRetryMode saved in-retry-mode value |
| */ |
| protected ExponentialBackoffDelayGenerator(Random random, int initialMaxDelay, |
| int maxExponentialFactor, int currentMaxDelay, boolean inRetryMode) { |
| this(random, initialMaxDelay, maxExponentialFactor); |
| this.currentMaxDelay = currentMaxDelay; |
| this.inRetryMode = inRetryMode; |
| } |
| |
| /** Resets the exponential backoff generator to start delays at the initial delay. */ |
| public void reset() { |
| this.currentMaxDelay = initialMaxDelay; |
| this.inRetryMode = false; |
| } |
| |
| /** |
| * Resets the exponential backoff generator to start delays such that the specified number of |
| * retries have already been made. */ |
| public void resetWithNumRetries(int numRetries) { |
| Preconditions.checkArgument(numRetries >= 0); |
| reset(); |
| if (numRetries > 0) { |
| inRetryMode = true; |
| if (numRetries > Integer.SIZE) { |
| // Cap, otherwise Java will use the lower order 5 bits causing incorrect power of 2. |
| numRetries = Integer.SIZE; |
| } |
| currentMaxDelay = currentMaxDelay << (numRetries - 1); |
| if (currentMaxDelay <= 0 || currentMaxDelay > maxDelay) { |
| currentMaxDelay = maxDelay; |
| } |
| } |
| } |
| |
| /** Gets the next delay interval to use. */ |
| public int getNextDelay() { |
| int delay = 0; // After a reset, the delay is 0. |
| if (inRetryMode) { |
| |
| // Generate the delay in the range [1, currentMaxDelay]. |
| delay = random.nextInt(currentMaxDelay) + 1; |
| |
| // Adjust the max for the next run. |
| if (currentMaxDelay <= maxDelay) { // Guard against overflow. |
| currentMaxDelay *= 2; |
| if (currentMaxDelay > maxDelay) { |
| currentMaxDelay = maxDelay; |
| } |
| } |
| } |
| inRetryMode = true; |
| return delay; |
| } |
| |
| protected int getCurrentMaxDelay() { |
| return currentMaxDelay; |
| } |
| |
| protected boolean getInRetryMode() { |
| return inRetryMode; |
| } |
| } |