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package org.dcache.util.backoff;
import java.util.concurrent.TimeUnit;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
/**
* Factory implementation for creating an {@link ExponentialBackoffAlgorithm}.
*
* @author arossi
*/
public class ExponentialBackoffAlgorithmFactory implements
IBackoffAlgorithmFactory {
/**
* Standard exponential backoff algorithm. Computes recursive function: <br>
* <br>
*
* W(0) = W[min] <br>
* W(n) = (W[min] + W(n-1) * 2)/2 <br>
* <br>
*
* where W[min] is the minimum delay time, and n is the attempt number. The
* algorithm can also be given a maximum delay (default is Long.MAX_VALUE).
* The minimum delay must be positive; the default is 1.
*
* <p>
* The default {@link TimeUnit} for both min and max is MINUTES.
*
* <p>
* {@link #quitAtMaxDelay} is false by default.
*
* <p>
* Note that the algorithm is stateful and therefore not thread-safe; hence
* the factory creates a new copy at each call to
* {@link ExponentialBackoffAlgorithmFactory#getAlgorithm()}.
*/
private class ExponentialBackoffAlgorithm implements IBackoffAlgorithm {
/**
* Derived
*/
private Long maxDelayInMillis;
private Long minDelayInMillis;
/**
* State
*/
private long previousDelay;
private int previousAttempts;
@Override
public long getWaitDuration() {
if (previousDelay == NO_WAIT) {
return previousDelay;
}
if (previousDelay == 0) {
previousDelay = minDelayInMillis;
} else {
previousDelay = (minDelayInMillis + 2 * previousDelay) / 2;
}
if (maxDelayInMillis != null) {
/*
* catch potential overflow (negative value)
*/
if (previousDelay > maxDelayInMillis || previousDelay <= 0) {
previousDelay = maxDelayInMillis;
}
if (quitAtMaxDelay && previousDelay == maxDelayInMillis) {
previousDelay = NO_WAIT;
}
}
if (maxAllowedAttempts != null
&& ++previousAttempts >= maxAllowedAttempts) {
previousDelay = NO_WAIT;
}
return previousDelay;
}
}
/**
* Injected
*/
private Long maxDelay;
private Long minDelay = 1L;
private TimeUnit maxUnit = TimeUnit.MINUTES;
private TimeUnit minUnit = TimeUnit.MINUTES;
private Integer maxAllowedAttempts;
private boolean quitAtMaxDelay;
@Override
public IBackoffAlgorithm getAlgorithm() {
ExponentialBackoffAlgorithm algorithm = new ExponentialBackoffAlgorithm();
algorithm.minDelayInMillis = minUnit.toMillis(minDelay);
if (maxDelay != null) {
algorithm.maxDelayInMillis = maxUnit.toMillis(maxDelay);
checkArgument(algorithm.maxDelayInMillis >= algorithm.minDelayInMillis);
}
return algorithm;
}
public void setMaxAllowedAttempts(Integer maxAllowedAttempts) {
this.maxAllowedAttempts = maxAllowedAttempts;
}
public void setMaxDelay(Long maxDelay) {
this.maxDelay = maxDelay;
}
public void setMaxUnit(TimeUnit maxUnit) {
checkNotNull(maxUnit);
this.maxUnit = maxUnit;
}
public void setMinDelay(Long minDelay) {
checkNotNull(minDelay);
checkArgument(minDelay > 0);
this.minDelay = minDelay;
}
public void setMinUnit(TimeUnit minUnit) {
checkNotNull(minUnit);
this.minUnit = minUnit;
}
public void setQuitAtMaxDelay(boolean quitAtMaxDelay) {
this.quitAtMaxDelay = quitAtMaxDelay;
}
}