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/*
* ====================================================================
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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.
* ====================================================================
*
* This software consists of voluntary contributions made by many
* individuals on behalf of the Apache Software Foundation. For more
* information on the Apache Software Foundation, please see
* <http://www.apache.org/>.
*
*/
package ch.boye.httpclientandroidlib.impl.client.cache;
import ch.boye.httpclientandroidlib.annotation.ThreadSafe;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
/**
* An implementation that backs off exponentially based on the number of
* consecutive failed attempts stored in the
* {@link AsynchronousValidationRequest}. It uses the following defaults:
* <pre>
* no delay in case it was never tried or didn't fail so far
* 6 secs delay for one failed attempt (= {@link #getInitialExpiryInMillis()})
* 60 secs delay for two failed attempts
* 10 mins delay for three failed attempts
* 100 mins delay for four failed attempts
* ~16 hours delay for five failed attempts
* 24 hours delay for six or more failed attempts (= {@link #getMaxExpiryInMillis()})
* </pre>
*
* The following equation is used to calculate the delay for a specific revalidation request:
* <pre>
* delay = {@link #getInitialExpiryInMillis()} * Math.pow({@link #getBackOffRate()}, {@link AsynchronousValidationRequest#getConsecutiveFailedAttempts()} - 1))
* </pre>
* The resulting delay won't exceed {@link #getMaxExpiryInMillis()}.
*
* @since 4.3
*/
@ThreadSafe
public class ExponentialBackOffSchedulingStrategy implements SchedulingStrategy {
public static final long DEFAULT_BACK_OFF_RATE = 10;
public static final long DEFAULT_INITIAL_EXPIRY_IN_MILLIS = TimeUnit.SECONDS.toMillis(6);
public static final long DEFAULT_MAX_EXPIRY_IN_MILLIS = TimeUnit.SECONDS.toMillis(86400);
private final long backOffRate;
private final long initialExpiryInMillis;
private final long maxExpiryInMillis;
private final ScheduledExecutorService executor;
/**
* Create a new scheduling strategy using a fixed pool of worker threads.
* @param cacheConfig the thread pool configuration to be used; not <code>null</code>
* @see ch.boye.httpclientandroidlib.impl.client.cache.CacheConfig#getAsynchronousWorkersMax()
* @see #DEFAULT_BACK_OFF_RATE
* @see #DEFAULT_INITIAL_EXPIRY_IN_MILLIS
* @see #DEFAULT_MAX_EXPIRY_IN_MILLIS
*/
public ExponentialBackOffSchedulingStrategy(final CacheConfig cacheConfig) {
this(cacheConfig,
DEFAULT_BACK_OFF_RATE,
DEFAULT_INITIAL_EXPIRY_IN_MILLIS,
DEFAULT_MAX_EXPIRY_IN_MILLIS);
}
/**
* Create a new scheduling strategy by using a fixed pool of worker threads and the
* given parameters to calculated the delay.
*
* @param cacheConfig the thread pool configuration to be used; not <code>null</code>
* @param backOffRate the back off rate to be used; not negative
* @param initialExpiryInMillis the initial expiry in milli seconds; not negative
* @param maxExpiryInMillis the upper limit of the delay in milli seconds; not negative
* @see ch.boye.httpclientandroidlib.impl.client.cache.CacheConfig#getAsynchronousWorkersMax()
* @see ExponentialBackOffSchedulingStrategy
*/
public ExponentialBackOffSchedulingStrategy(
final CacheConfig cacheConfig,
final long backOffRate,
final long initialExpiryInMillis,
final long maxExpiryInMillis) {
this(createThreadPoolFromCacheConfig(cacheConfig),
backOffRate,
initialExpiryInMillis,
maxExpiryInMillis);
}
private static ScheduledThreadPoolExecutor createThreadPoolFromCacheConfig(
final CacheConfig cacheConfig) {
final ScheduledThreadPoolExecutor scheduledThreadPoolExecutor = new ScheduledThreadPoolExecutor(
cacheConfig.getAsynchronousWorkersMax());
scheduledThreadPoolExecutor.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
return scheduledThreadPoolExecutor;
}
ExponentialBackOffSchedulingStrategy(
final ScheduledExecutorService executor,
final long backOffRate,
final long initialExpiryInMillis,
final long maxExpiryInMillis) {
this.executor = checkNotNull("executor", executor);
this.backOffRate = checkNotNegative("backOffRate", backOffRate);
this.initialExpiryInMillis = checkNotNegative("initialExpiryInMillis", initialExpiryInMillis);
this.maxExpiryInMillis = checkNotNegative("maxExpiryInMillis", maxExpiryInMillis);
}
public void schedule(
final AsynchronousValidationRequest revalidationRequest) {
checkNotNull("revalidationRequest", revalidationRequest);
final int consecutiveFailedAttempts = revalidationRequest.getConsecutiveFailedAttempts();
final long delayInMillis = calculateDelayInMillis(consecutiveFailedAttempts);
executor.schedule(revalidationRequest, delayInMillis, TimeUnit.MILLISECONDS);
}
public void close() {
executor.shutdown();
}
public long getBackOffRate() {
return backOffRate;
}
public long getInitialExpiryInMillis() {
return initialExpiryInMillis;
}
public long getMaxExpiryInMillis() {
return maxExpiryInMillis;
}
protected long calculateDelayInMillis(final int consecutiveFailedAttempts) {
if (consecutiveFailedAttempts > 0) {
final long delayInSeconds = (long) (initialExpiryInMillis *
Math.pow(backOffRate, consecutiveFailedAttempts - 1));
return Math.min(delayInSeconds, maxExpiryInMillis);
}
else {
return 0;
}
}
protected static <T> T checkNotNull(final String parameterName, final T value) {
if (value == null) {
throw new IllegalArgumentException(parameterName + " may not be null");
}
return value;
}
protected static long checkNotNegative(final String parameterName, final long value) {
if (value < 0) {
throw new IllegalArgumentException(parameterName + " may not be negative");
}
return value;
}
}
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