/*
* 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 com.google.common.base.Preconditions;
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 as a factor of {@code initialMaxDelay} */
private final int maxExponentialFactor;
/** 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);
this.maxExponentialFactor = maxExponentialFactor;
this.initialMaxDelay = initialMaxDelay;
Preconditions.checkArgument(initialMaxDelay > 0, "initial 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;
}
/** 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.
int maxDelay = initialMaxDelay * maxExponentialFactor;
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;
}
}