Retry asynchronous functions

This document describes how you can request asynchronous (non-HTTPS) background functions to retry on failure.

Semantics of retry

Cloud Functions guarantees at-least-once execution of an event-driven function for each event emitted by an event source. However, by default, if a function invocation terminates with an error, the function will not be invoked again, and the event will be dropped. When you enable retries on an event-driven function, Cloud Functions will retry a failed function invocation until it completes successfully or the retry window expires (by default, after 7 days).

When retries are not enabled for a function, which is the default, the function always reports that it executed successfully, and 200 OK response codes might appear in its logs. This occurs even if the function encountered an error. To make it clear when your function encounters an error, be sure to report errors appropriately.

Why event-driven functions fail to complete

On rare occasions, a function might exit prematurely due to an internal error, and by default the function might or might not be automatically retried.

More typically, an event-driven function may fail to successfully complete due to errors thrown in the function code itself. Some of the reasons this might happen are as follows:

• The function contains a bug and the runtime throws an exception.
• The function cannot reach a service endpoint, or times out while trying to reach the endpoint.
• The function intentionally throws an exception (for example, when a parameter fails validation).
• When functions written in Node.js return a rejected promise or pass a non-null value to a callback.

In any of the above cases, the function stops executing by default and the event is discarded. If you want to retry the function when an error occurs, you can change the default retry policy by setting the "retry on failure" property. This causes the event to be retried repeatedly for up to multiple days until the function successfully completes.

Enabling and disabling retries

Using the GCP Console

You can enable or disable retries in the GCP Console as follows:

1. Go to the Cloud Functions Overview page in the Cloud Platform Console.

2. Click Create function. Alternatively, click an existing function to go to its details page and click Edit.

3. Fill in the required fields for your function.

4. Ensure the Trigger field is set to an event-based trigger type, such as Cloud Pub/Sub or Cloud Storage.

5. Expand the advanced settings by clicking More.

6. Check or uncheck the box labeled Retry on failure.

In function code

With Cloud Functions for Firebase, you can enable retries in the code for a function. To do this for a background function such as functions.foo.onBar(myHandler);, use runWith and configure a failure policy:

functions.runWith({failurePolicy: true}).foo.onBar(myHandler);

Setting true as shown configures a function to retry on failure.

Best practices

This section describes best practices for using retries.

Use retry to handle transient errors

Because your function is retried continuously until successful execution, permanent errors like bugs should be eliminated from your code through testing before enabling retries. Retries are best used to handle intermittent/transient failures that have a high likelihood of resolution upon retrying, such as a flaky service endpoint or timeout.

Set an end condition to avoid infinite retry loops

It is best practice to protect your function against continuous looping when using retries. You can do this by including a well-defined end condition, before the function begins processing. Note that this technique only works if your function starts successfully and is able to evaluate the end condition.

A simple yet effective approach is to discard events with timestamps older than a certain time. This helps to avoid excessive executions when failures are either persistent or longer-lived than expected.

For example, this code snippet discards all events older than 10 seconds:

const eventAgeMs = Date.now() - Date.parse(event.timestamp);
const eventMaxAgeMs = 10000;
if (eventAgeMs > eventMaxAgeMs) {
  console.log(`Dropping event ${event} with age[ms]: ${eventAgeMs}`);
  callback();
  return;
}

Use catch with Promises

If your function has retries enabled, any unhandled error will trigger a retry. Make sure that your code captures any errors that should not result in a retry.

Here is an example of what you should do:

return doFooAsync().catch((err) => {
    if (isFatal(err)) {
        console.error(`Fatal error ${err}`);
    }
    return Promise.reject(err);
});

Make retryable event-driven functions idempotent

Event-driven functions that can be retried must be idempotent. Here are some general guidelines for making such a function idempotent:

• Many external APIs (such as Stripe) let you supply an idempotency key as a parameter. If you are using such an API, you should use the event ID as the idempotency key.
• Idempotency works well with at-least-once delivery, because it makes it safe to retry. So a general best practice for writing reliable code is to combine idempotency with retries.
• Make sure that your code is internally idempotent. For example:
  • Make sure that mutations can happen more than once without changing the outcome.
  • Query database state in a transaction before mutating the state.
  • Make sure that all side effects are themselves idempotent.
• Impose a transactional check outside the function, independent of the code. For example, persist state somewhere recording that a given event ID has already been processed.
• Deal with duplicate function calls out-of-band. For example, have a separate clean up process that cleans up after duplicate function calls.