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/*
 * Copyright 2002-2018 the original author or authors.
 *
 * 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
 *
 *      https://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 org.springframework.util.backoff;

import org.springframework.util.Assert;

/**
 * Implementation of {@link BackOff} that increases the back off period for each
 * retry attempt. When the interval has reached the {@link #setMaxInterval(long)
 * max interval}, it is no longer increased. Stops retrying once the
 * {@link #setMaxElapsedTime(long) max elapsed time} has been reached.
 *
 * 

Example: The default interval is {@value #DEFAULT_INITIAL_INTERVAL} ms, * the default multiplier is {@value #DEFAULT_MULTIPLIER}, and the default max * interval is {@value #DEFAULT_MAX_INTERVAL}. For 10 attempts the sequence will be * as follows: * *

 * request#     back off
 *
 *  1              2000
 *  2              3000
 *  3              4500
 *  4              6750
 *  5             10125
 *  6             15187
 *  7             22780
 *  8             30000
 *  9             30000
 * 10             30000
 * 
* *

Note that the default max elapsed time is {@link Long#MAX_VALUE}. Use * {@link #setMaxElapsedTime(long)} to limit the maximum length of time * that an instance should accumulate before returning * {@link BackOffExecution#STOP}. * * @author Stephane Nicoll * @since 4.1 */ public class ExponentialBackOff implements BackOff { /** * The default initial interval. */ public static final long DEFAULT_INITIAL_INTERVAL = 2000L; /** * The default multiplier (increases the interval by 50%). */ public static final double DEFAULT_MULTIPLIER = 1.5; /** * The default maximum back off time. */ public static final long DEFAULT_MAX_INTERVAL = 30000L; /** * The default maximum elapsed time. */ public static final long DEFAULT_MAX_ELAPSED_TIME = Long.MAX_VALUE; private long initialInterval = DEFAULT_INITIAL_INTERVAL; private double multiplier = DEFAULT_MULTIPLIER; private long maxInterval = DEFAULT_MAX_INTERVAL; private long maxElapsedTime = DEFAULT_MAX_ELAPSED_TIME; /** * Create an instance with the default settings. * @see #DEFAULT_INITIAL_INTERVAL * @see #DEFAULT_MULTIPLIER * @see #DEFAULT_MAX_INTERVAL * @see #DEFAULT_MAX_ELAPSED_TIME */ public ExponentialBackOff() { } /** * Create an instance with the supplied settings. * @param initialInterval the initial interval in milliseconds * @param multiplier the multiplier (should be greater than or equal to 1) */ public ExponentialBackOff(long initialInterval, double multiplier) { checkMultiplier(multiplier); this.initialInterval = initialInterval; this.multiplier = multiplier; } /** * The initial interval in milliseconds. */ public void setInitialInterval(long initialInterval) { this.initialInterval = initialInterval; } /** * Return the initial interval in milliseconds. */ public long getInitialInterval() { return this.initialInterval; } /** * The value to multiply the current interval by for each retry attempt. */ public void setMultiplier(double multiplier) { checkMultiplier(multiplier); this.multiplier = multiplier; } /** * Return the value to multiply the current interval by for each retry attempt. */ public double getMultiplier() { return this.multiplier; } /** * The maximum back off time. */ public void setMaxInterval(long maxInterval) { this.maxInterval = maxInterval; } /** * Return the maximum back off time. */ public long getMaxInterval() { return this.maxInterval; } /** * The maximum elapsed time in milliseconds after which a call to * {@link BackOffExecution#nextBackOff()} returns {@link BackOffExecution#STOP}. */ public void setMaxElapsedTime(long maxElapsedTime) { this.maxElapsedTime = maxElapsedTime; } /** * Return the maximum elapsed time in milliseconds after which a call to * {@link BackOffExecution#nextBackOff()} returns {@link BackOffExecution#STOP}. */ public long getMaxElapsedTime() { return this.maxElapsedTime; } @Override public BackOffExecution start() { return new ExponentialBackOffExecution(); } private void checkMultiplier(double multiplier) { Assert.isTrue(multiplier >= 1, () -> "Invalid multiplier '" + multiplier + "'. Should be greater than " + "or equal to 1. A multiplier of 1 is equivalent to a fixed interval."); } private class ExponentialBackOffExecution implements BackOffExecution { private long currentInterval = -1; private long currentElapsedTime = 0; @Override public long nextBackOff() { if (this.currentElapsedTime >= maxElapsedTime) { return STOP; } long nextInterval = computeNextInterval(); this.currentElapsedTime += nextInterval; return nextInterval; } private long computeNextInterval() { long maxInterval = getMaxInterval(); if (this.currentInterval >= maxInterval) { return maxInterval; } else if (this.currentInterval < 0) { long initialInterval = getInitialInterval(); this.currentInterval = (initialInterval < maxInterval ? initialInterval : maxInterval); } else { this.currentInterval = multiplyInterval(maxInterval); } return this.currentInterval; } private long multiplyInterval(long maxInterval) { long i = this.currentInterval; i *= getMultiplier(); return (i > maxInterval ? maxInterval : i); } @Override public String toString() { StringBuilder sb = new StringBuilder("ExponentialBackOff{"); sb.append("currentInterval=").append(this.currentInterval < 0 ? "n/a" : this.currentInterval + "ms"); sb.append(", multiplier=").append(getMultiplier()); sb.append('}'); return sb.toString(); } } }





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