How Do Google Pub/Sub\'s Retry Policy Parameters Relate to the `github.com/cenkalti/backoff` Library?

Google Pub/Sub Retry Policy: Exponential Backoff Configuration

Cloud Pub/Sub recently introduced a RetryPolicy server-side feature that allows for configuring exponential backoff for message delivery attempts. The provided documentation explains that MinimumBackoff and MaximumBackoff parameters determine the initial and maximum wait periods between retries. However, the relationship between these parameters and the InitialInterval and MaxInterval defined in the github.com/cenkalti/backoff library is still unclear.

The ExponentialBackOff algorithm used by github.com/cenkalti/backoff defines the randomized retry interval as:

randomized interval = RetryInterval * (random value in range [1 - RandomizationFactor, 1 + RandomizationFactor])

where RetryInterval starts at InitialInterval and is capped by MaxInterval.

Do MinimumBackoff and MaximumBackoff map to InitialInterval and MaxInterval?

Yes. MinimumBackoff corresponds to the initial wait period, and MaximumBackoff corresponds to the largest amount of time allowed between retries. This is analogous to InitialInterval and MaxInterval in github.com/cenkalti/backoff.

Example Usage and Observations

The provided example program illustrates the exponential backoff behavior. With MinimumBackoff set to 5s and MaximumBackoff set to 60s, the time between retries increases as expected, up to the maximum of 60s.

Reducing MinimumBackoff and MaximumBackoff to 1s and 2s, respectively, results in more frequent retries with a consistent delay of around 3s. This suggests that the exponential backoff algorithm is making a "best effort" to stay within the specified maximum.

Randomization, Multiplier, and MaxElapsedTime

The example output does not exhibit visible randomization, and there is no explicit multiplier specified. The exponential backoff algorithm in Pub/Sub uses a similar formula to the one described in github.com/cenkalti/backoff.

Regarding MaxElapsedTime, Cloud Pub/Sub does not currently have an equivalent concept. However, you can implement a similar behavior by using Dead Letter Queues to stop retries after a specified number of attempts.

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