com.google.api.client.http
Class ExponentialBackOffPolicy

java.lang.Object
  com.google.api.client.http.ExponentialBackOffPolicy

All Implemented Interfaces:

BackOffPolicy

public class ExponentialBackOffPolicy
extends Object
implements BackOffPolicy

Implementation of BackOffPolicy that increases the back off period for each retry attempt using a randomization function that grows exponentially.

getNextBackOffMillis() is calculated using the following formula:

 randomized_interval =
     retry_interval * (random value in range [1 - randomization_factor, 1 + randomization_factor])
 

In other words getNextBackOffMillis() will range between the randomization factor percentage below and above the retry interval. For example, using 2 seconds as the base retry interval and 0.5 as the randomization factor, the actual back off period used in the next retry attempt will be between 1 and 3 seconds.

Note: max_interval caps the retry_interval and not the randomized_interval.

If the time elapsed since an ExponentialBackOffPolicy instance is created goes past the max_elapsed_time then the method getNextBackOffMillis() starts returning BackOffPolicy.STOP. The elapsed time can be reset by calling reset().

Example: The default retry_interval is .5 seconds, default randomization_factor is 0.5, default multiplier is 1.5 and the default max_interval is 1 minute. For 10 requests the sequence will be (values in seconds) and assuming we go over the max_elapsed_time on the 10th request:

 request#     retry_interval     randomized_interval

 1             0.5                [0.25,   0.75]
 2             0.75               [0.375,  1.125]
 3             1.125              [0.562,  1.687]
 4             1.687              [0.8435, 2.53]
 5             2.53               [1.265,  3.795]
 6             3.795              [1.897,  5.692]
 7             5.692              [2.846,  8.538]
 8             8.538              [4.269, 12.807]
 9            12.807              [6.403, 19.210]
 10           19.210              BackOffPolicy.STOP
 

Implementation is not thread-safe.

Since:

1.7

Author:

Ravi Mistry

Nested Class Summary

static class

ExponentialBackOffPolicy.Builder Builder for ExponentialBackOffPolicy.

Field Summary

static int	DEFAULT_INITIAL_INTERVAL_MILLIS The default initial interval value in milliseconds (0.5 seconds).
static int	DEFAULT_MAX_ELAPSED_TIME_MILLIS The default maximum elapsed time in milliseconds (15 minutes).
static int	DEFAULT_MAX_INTERVAL_MILLIS The default maximum back off time in milliseconds (1 minute).
static double	DEFAULT_MULTIPLIER The default multiplier value (1.5 which is 50% increase per back off).
static double	DEFAULT_RANDOMIZATION_FACTOR The default randomization factor (0.5 which results in a random period ranging between 50% below and 50% above the retry interval).

Fields inherited from interface com.google.api.client.http.BackOffPolicy

STOP

Constructor Summary

ExponentialBackOffPolicy()
Creates an instance of ExponentialBackOffPolicy using default values.

Method Summary

static ExponentialBackOffPolicy.Builder	builder() Returns an instance of a new builder.
int	getCurrentIntervalMillis() Returns the current retry interval in milliseconds.
long	getElapsedTimeMillis() Returns the elapsed time in milliseconds since an ExponentialBackOffPolicy instance is created and is reset when reset() is called.
int	getInitialIntervalMillis() Returns the initial retry interval in milliseconds.
int	getMaxElapsedTimeMillis() Returns the maximum elapsed time in milliseconds.
int	getMaxIntervalMillis() Returns the maximum value of the back off period in milliseconds.
double	getMultiplier() Returns the value to multiply the current interval with for each retry attempt.
long	getNextBackOffMillis() Gets the number of milliseconds to wait before retrying an HTTP request.
double	getRandomizationFactor() Returns the randomization factor to use for creating a range around the retry interval.
boolean	isBackOffRequired(int statusCode) Determines if back off is required based on the specified status code.
void	reset() Sets the interval back to the initial retry interval and restarts the timer.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_INITIAL_INTERVAL_MILLIS

public static final int DEFAULT_INITIAL_INTERVAL_MILLIS

The default initial interval value in milliseconds (0.5 seconds).

See Also:

Constant Field Values

DEFAULT_RANDOMIZATION_FACTOR

public static final double DEFAULT_RANDOMIZATION_FACTOR

The default randomization factor (0.5 which results in a random period ranging between 50% below and 50% above the retry interval).

See Also:

Constant Field Values

DEFAULT_MULTIPLIER

public static final double DEFAULT_MULTIPLIER

The default multiplier value (1.5 which is 50% increase per back off).

See Also:

Constant Field Values

DEFAULT_MAX_INTERVAL_MILLIS

public static final int DEFAULT_MAX_INTERVAL_MILLIS

The default maximum back off time in milliseconds (1 minute).

See Also:

Constant Field Values

DEFAULT_MAX_ELAPSED_TIME_MILLIS

public static final int DEFAULT_MAX_ELAPSED_TIME_MILLIS

The default maximum elapsed time in milliseconds (15 minutes).

See Also:

Constant Field Values

Constructor Detail

ExponentialBackOffPolicy

public ExponentialBackOffPolicy()

Creates an instance of ExponentialBackOffPolicy using default values. To override the defaults use builder().

• initialIntervalMillis is defaulted to DEFAULT_INITIAL_INTERVAL_MILLIS
• randomizationFactor is defaulted to DEFAULT_RANDOMIZATION_FACTOR
• multiplier is defaulted to DEFAULT_MULTIPLIER
• maxIntervalMillis is defaulted to DEFAULT_MAX_INTERVAL_MILLIS
• maxElapsedTimeMillis is defaulted in DEFAULT_MAX_ELAPSED_TIME_MILLIS

Method Detail

isBackOffRequired

public boolean isBackOffRequired(int statusCode)

Determines if back off is required based on the specified status code.

The idea is that the servers are only temporarily unavailable, and they should not be overwhelmed when they are trying to get back up.

The default implementation requires back off for 500 and 503 status codes. Subclasses may override if different status codes are required.

Specified by:

isBackOffRequired in interface BackOffPolicy

Parameters:

statusCode - HTTP status code

reset

public final void reset()

Sets the interval back to the initial retry interval and restarts the timer.

Specified by:

reset in interface BackOffPolicy

getNextBackOffMillis

public long getNextBackOffMillis()
                          throws IOException

Gets the number of milliseconds to wait before retrying an HTTP request. If BackOffPolicy.STOP is returned, no retries should be made.

This method calculates the next back off interval using the formula: randomized_interval = retry_interval +/- (randomization_factor * retry_interval)

Subclasses may override if a different algorithm is required.

Specified by:

getNextBackOffMillis in interface BackOffPolicy

Returns:

the number of milliseconds to wait when backing off requests, or BackOffPolicy.STOP if no more retries should be made

Throws:

IOException

getInitialIntervalMillis

public final int getInitialIntervalMillis()

Returns the initial retry interval in milliseconds.

getRandomizationFactor

public final double getRandomizationFactor()

Returns the randomization factor to use for creating a range around the retry interval.

A randomization factor of 0.5 results in a random period ranging between 50% below and 50% above the retry interval.

getCurrentIntervalMillis

public final int getCurrentIntervalMillis()

Returns the current retry interval in milliseconds.

getMultiplier

public final double getMultiplier()

Returns the value to multiply the current interval with for each retry attempt.

getMaxIntervalMillis

public final int getMaxIntervalMillis()

Returns the maximum value of the back off period in milliseconds. Once the current interval reaches this value it stops increasing.

getMaxElapsedTimeMillis

public final int getMaxElapsedTimeMillis()

Returns the maximum elapsed time in milliseconds.

If the time elapsed since an ExponentialBackOffPolicy instance is created goes past the max_elapsed_time then the method getNextBackOffMillis() starts returning BackOffPolicy.STOP. The elapsed time can be reset by calling reset().

getElapsedTimeMillis

public final long getElapsedTimeMillis()

Returns the elapsed time in milliseconds since an ExponentialBackOffPolicy instance is created and is reset when reset() is called.

The elapsed time is computed using System.nanoTime().

builder

public static ExponentialBackOffPolicy.Builder builder()

Returns an instance of a new builder.