How does Google Pub/Sub\'s RetryPolicy with Exponential Backoff handle minimum, maximum backoff and randomization?

How the Exponential Backoff in Google Pub/Sub's RetryPolicy Works

Google Pub/Sub recently introduced a RetryPolicy feature to enhance error handling in server-side operations. The configuration options for this policy include MinimumBackoff and MaximumBackoff parameters.

MinimumBackoff and MaximumBackoff Configuration

The MinimumBackoff parameter specifies the initial wait period before the first retry. The MaximumBackoff parameter defines the maximum amount of time allowed between subsequent retries. These parameters correspond to the InitialInterval and MaxInterval in the exponential backoff algorithm implemented in github.com/cenkalti/backoff.

Exponential Backoff Algorithm

In exponential backoff, each randomized interval is calculated using the formula:

retryInterval = InitialInterval * (random value in range [1 - RandomizationFactor, 1 + RandomizationFactor])

where the InitialInterval is MinimumBackoff, and the random value introduces randomization in the retry delay. The retryInterval is capped by MaximumBackoff.

Example Program

The provided program demonstrates the behavior of the exponential backoff algorithm with different MinimumBackoff and MaximumBackoff values.

Observations from Program Output

• Default Backoff: With MinimumBackoff set to 5s and MaximumBackoff to 60s, the time between retries increases gradually, with an average interval of approximately 10s. This indicates a RandomizationFactor close to 1.
• Smaller Backoff: With MinimumBackoff and MaximumBackoff set to 1s and 2s respectively, the time between retries is consistently around 3s. This suggests that the MaximumBackoff limit takes precedence in this case.

Randomization and Multiplier

The exponential backoff algorithm used by RetryPolicy includes randomization to ensure that retries are not always made at the same interval. However, the provided program's output does not show a clear pattern of exponential growth in the retry intervals, suggesting that the Multiplier parameter, which would double the retry interval with each iteration, is not in use.

MaxElapsedTime

Unlike the exponential backoff implementation in github.com/cenkalti/backoff, RetryPolicy does not have an equivalent MaxElapsedTime parameter. This means that retries will continue indefinitely if the server is unavailable, unless the application handles and limits retries independently.

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