Enable event-driven function retries

This document describes how to enable retrying for CloudEvents functions, also known as as event-driven functions. Automatic retrying is not available for HTTP functions.

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 might fail to successfully complete due to errors thrown in the function code itself. The reasons this might happen include:

The function contains a bug and the runtime throws an exception.
The function cannot reach a service endpoint, or times out while trying to do so.
The function intentionally throws an exception (for example, when a parameter fails validation).
A Node.js function returns a rejected promise, or passes a non-null value to a callback.

In any of the above cases, the function will stop executing and return an error. Event triggers producing the messages have retry policies that you can customize to meet the needs of your function.

Semantics of retry

Cloud Run functions provides at-least-once execution of an event-driven function for each event emitted by an event source. The way you configure retries depends on how you created your function:

Functions created in the Google Cloud console or with the Cloud Run Admin API requires you to separately create and manage the event triggers. Triggers have default retry behaviors that you can customize to suit the needs of your function.
Functions created with the Cloud Functions v2 API will implicitly create the necessary event triggers, for example Pub/Sub topics or Eventarc triggers. By default, the retries are disabled for these triggers and can be re-enabled using the Cloud Functions v2 API.

Event driven functions created with Cloud Run

Functions created in the Google Cloud console or with the Cloud Run Admin API requires you to separately create and manage the event triggers. We strongly recommend that you review the default behavior of each trigger type:

Eventarc retry policy has a default message retention of 24 hours with an exponential backoff delay. Refer to the Eventarc documentation on retry events.
Pub/Sub defaults to using the immediate redelivery policy for all subscriptions. Refer to the Pub/Sub documentation on handling message failures and retry requests.

Event driven functions created with Cloud Functions v2 API

Functions created using the Cloud Functions v2 API; for example, using the Cloud Functions gcloud CLI, the REST API, or Terraform, will create and manage event triggers on your behalf. By default, if a function invocation terminates with an error, the function is not invoked again and the event is dropped. When you enable retries on an event-driven function, Cloud Run functions retries a failed function invocation until it completes successfully or the retry window expires.

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.

Enable or disable retries

To enable or disable retries, you can either use the gcloud command-line tool or the Google Cloud console. By default, retries are disabled.

Configure retries from the `gcloud` command-line tool

To enable retries using the gcloud command-line tool, include the --retry flag when deploying your function:

gcloud functions deploy FUNCTION_NAME --retry FLAGS...

To disable retries, re-deploy the function without the --retry flag:

gcloud functions deploy FUNCTION_NAME FLAGS...

Configure retries from the console

If you're creating a new function:

From the Create Function screen, under Trigger and choose the type of event to act as a trigger for your function.
Select the Retry on failure checkbox to enable retries.

If you're updating an existing function:

From the Cloud Run functions Overview page, click the name of the function you're updating to open its Function details screen, then choose Edit from the menu bar to display Trigger pane.
Select or clear the Retry on failure checkbox to enable or disable retries.

Retry window

This retry window expires after 24 hours. Cloud Run functions retries newly created event-driven functions using an exponential backoff strategy, with an increasing backoff of between 10 and 600 seconds.

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 or 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:

Node.js

const functions = require('@google-cloud/functions-framework');

/**
 * Cloud Event Function that only executes within
 * a certain time period after the triggering event
 *
 * @param {object} event The Cloud Functions event.
 * @param {function} callback The callback function.
 */
functions.cloudEvent('avoidInfiniteRetries', (event, callback) => {
  const eventAge = Date.now() - Date.parse(event.time);
  const eventMaxAge = 10000;

  // Ignore events that are too old
  if (eventAge > eventMaxAge) {
    console.log(`Dropping event ${event} with age ${eventAge} ms.`);
    callback();
    return;
  }

  // Do what the function is supposed to do
  console.log(`Processing event ${event} with age ${eventAge} ms.`);

  // Retry failed function executions
  const failed = false;
  if (failed) {
    callback('some error');
  } else {
    callback();
  }
});

Python

from datetime import datetime, timezone

# The 'python-dateutil' package must be included in requirements.txt.
from dateutil import parser

import functions_framework


@functions_framework.cloud_event
def avoid_infinite_retries(cloud_event):
    """Cloud Event Function that only executes within a certain
    time period after the triggering event.

    Args:
        cloud_event: The cloud event associated with the current trigger
    Returns:
        None; output is written to Stackdriver Logging
    """
    timestamp = cloud_event["time"]

    event_time = parser.parse(timestamp)
    event_age = (datetime.now(timezone.utc) - event_time).total_seconds()
    event_age_ms = event_age * 1000

    # Ignore events that are too old
    max_age_ms = 10000
    if event_age_ms > max_age_ms:
        print("Dropped {} (age {}ms)".format(cloud_event["id"], event_age_ms))
        return "Timeout"

    # Do what the function is supposed to do
    print("Processed {} (age {}ms)".format(cloud_event["id"], event_age_ms))
    return  # To retry the execution, raise an exception here

Go


// Package tips contains tips for writing Cloud Functions in Go.
package tips

import (
	"context"
	"fmt"
	"log"
	"time"

	"github.com/GoogleCloudPlatform/functions-framework-go/functions"
	"github.com/cloudevents/sdk-go/v2/event"
)

func init() {
	functions.CloudEvent("FiniteRetryPubSub", FiniteRetryPubSub)
}

// MessagePublishedData contains the full Pub/Sub message
// See the documentation for more details:
// https://cloud.google.com/eventarc/docs/cloudevents#pubsub
type MessagePublishedData struct {
	Message PubSubMessage
}

// PubSubMessage is the payload of a Pub/Sub event.
// See the documentation for more details:
// https://cloud.google.com/pubsub/docs/reference/rest/v1/PubsubMessage
type PubSubMessage struct {
	Data []byte `json:"data"`
}

// FiniteRetryPubSub demonstrates how to avoid inifinite retries.
func FiniteRetryPubSub(ctx context.Context, e event.Event) error {
	var msg MessagePublishedData
	if err := e.DataAs(&msg); err != nil {
		return fmt.Errorf("event.DataAs: %w", err)
	}

	// Ignore events that are too old.
	expiration := e.Time().Add(10 * time.Second)
	if time.Now().After(expiration) {
		log.Printf("event timeout: halting retries for expired event '%q'", e.ID())
		return nil
	}

	// Add your message processing logic.
	return processTheMessage(msg)
}

Java


import com.google.cloud.functions.CloudEventsFunction;
import io.cloudevents.CloudEvent;
import java.time.Duration;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.util.logging.Logger;

public class RetryTimeout implements CloudEventsFunction {
  private static final Logger logger = Logger.getLogger(RetryTimeout.class.getName());
  private static final long MAX_EVENT_AGE = 10_000;

  /**
   * Cloud Event Function that only executes within
   * a certain time period after the triggering event
   */
  @Override
  public void accept(CloudEvent event) throws Exception {
    ZonedDateTime utcNow = ZonedDateTime.now(ZoneOffset.UTC);
    ZonedDateTime timestamp = event.getTime().atZoneSameInstant(ZoneOffset.UTC);

    long eventAge = Duration.between(timestamp, utcNow).toMillis();

    // Ignore events that are too old
    if (eventAge > MAX_EVENT_AGE) {
      logger.info(String.format("Dropping event with timestamp %s.", timestamp));
      return;
    }

    // Process events that are recent enough
    // To retry this invocation, throw an exception here
    logger.info(String.format("Processing event with timestamp %s.", timestamp));
  }
}

C#

using CloudNative.CloudEvents;
using Google.Cloud.Functions.Framework;
using Google.Events.Protobuf.Cloud.PubSub.V1;
using Microsoft.Extensions.Logging;
using System;
using System.Threading;
using System.Threading.Tasks;

namespace TimeBoundedRetries;

public class Function : ICloudEventFunction<MessagePublishedData>
{
    private static readonly TimeSpan MaxEventAge = TimeSpan.FromSeconds(10);
    private readonly ILogger _logger;

    // Note: for additional testability, use an injectable clock abstraction.
    public Function(ILogger<Function> logger) =>
        _logger = logger;

    public Task HandleAsync(CloudEvent cloudEvent, MessagePublishedData data, CancellationToken cancellationToken)
    {
        string textData = data.Message.TextData;

        DateTimeOffset utcNow = DateTimeOffset.UtcNow;

        // Every PubSub CloudEvent will contain a timestamp.
        DateTimeOffset timestamp = cloudEvent.Time.Value;
        DateTimeOffset expiry = timestamp + MaxEventAge;

        // Ignore events that are too old.
        if (utcNow > expiry)
        {
            _logger.LogInformation("Dropping PubSub message '{text}'", textData);
            return Task.CompletedTask;
        }

        // Process events that are recent enough.
        // If this processing throws an exception, the message will be retried until either
        // processing succeeds or the event becomes too old and is dropped by the code above.
        _logger.LogInformation("Processing PubSub message '{text}'", textData);
        return Task.CompletedTask;
    }
}

Ruby

require "functions_framework"

FunctionsFramework.cloud_event "avoid_infinite_retries" do |event|
  # Use the event timestamp to determine the event age.
  event_age_secs = Time.now - event.time.to_time
  event_age_ms = (event_age_secs * 1000).to_i

  max_age_ms = 10_000
  if event_age_ms > max_age_ms
    # Ignore events that are too old.
    logger.info "Dropped #{event.id} (age #{event_age_ms}ms)"

  else
    # Do what the function is supposed to do.
    logger.info "Handling #{event.id} (age #{event_age_ms}ms)..."
    failed = true

    # Raise an exception to signal failure and trigger a retry.
    raise "I failed!" if failed
  end
end

PHP


/**
 * This function shows an example method for avoiding infinite retries in
 * Google Cloud Functions. By default, functions configured to automatically
 * retry execution on failure will be retried indefinitely - causing an
 * infinite loop. To avoid this, we stop retrying executions (by not throwing
 * exceptions) for any events that are older than a predefined threshold.
 */

use Google\CloudFunctions\CloudEvent;

function avoidInfiniteRetries(CloudEvent $event): void
{
    $log = fopen(getenv('LOGGER_OUTPUT') ?: 'php://stderr', 'wb');

    $eventId = $event->getId();

    // The maximum age of events to process.
    $maxAge = 10; // 10 seconds

    // The age of the event being processed.
    $eventAge = time() - strtotime($event->getTime());

    // Ignore events that are too old
    if ($eventAge > $maxAge) {
        fwrite($log, 'Dropping event ' . $eventId . ' with age ' . $eventAge . ' seconds' . PHP_EOL);
        return;
    }

    // Do what the function is supposed to do
    fwrite($log, 'Processing event: ' . $eventId . ' with age ' . $eventAge . ' seconds' . PHP_EOL);

    // infinite_retries failed function executions
    $failed = true;
    if ($failed) {
        throw new Exception('Event ' . $eventId . ' failed; retrying...');
    }
}

Distinguish between functions that can be retried and fatal errors

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

Node.js

const functions = require('@google-cloud/functions-framework');

/**
 * Register a Cloud Event Function that demonstrates
 * how to toggle retries using a promise
 *
 * @param {object} event The Cloud Event for the function trigger.
 */
functions.cloudEvent('retryPromise', cloudEvent => {
  // The Pub/Sub event payload is passed as the CloudEvent's data payload.
  // See the documentation for more details:
  // https://cloud.google.com/eventarc/docs/cloudevents#pubsub
  const base64PubsubMessage = cloudEvent.data.message.data;
  const jsonString = Buffer.from(base64PubsubMessage, 'base64').toString();

  const tryAgain = JSON.parse(jsonString).retry;

  if (tryAgain) {
    throw new Error('Retrying...');
  } else {
    console.error('Not retrying...');
    return Promise.resolve();
  }
});

/**
 * Cloud Event Function that demonstrates
 * how to toggle retries using a callback
 *
 * @param {object} event The Cloud Event for the function trigger.
 * @param {function} callback The callback function.
 */
functions.cloudEvent('retryCallback', (cloudEvent, callback) => {
  // The Pub/Sub event payload is passed as the CloudEvent's data payload.
  // See the documentation for more details:
  // https://cloud.google.com/eventarc/docs/cloudevents#pubsub
  const base64PubsubMessage = cloudEvent.data.message.data;
  const jsonString = Buffer.from(base64PubsubMessage, 'base64').toString();

  const tryAgain = JSON.parse(jsonString).retry;
  const err = new Error('Error!');

  if (tryAgain) {
    console.error('Retrying:', err);
    callback(err);
  } else {
    console.error('Not retrying:', err);
    callback();
  }
});

Python

import base64
import json

import functions_framework
from google.cloud import error_reporting


error_client = error_reporting.Client()


@functions_framework.cloud_event
def retry_or_not(cloud_event):
    """Cloud Event Function that demonstrates how to toggle retries.

    Args:
        cloud_event: The cloud event with a Pub/Sub data payload
    Returns:
        None; output is written to Stackdriver Logging
    """

    # The Pub/Sub event payload is passed as the CloudEvent's data payload.
    # See the documentation for more details:
    # https://cloud.google.com/eventarc/docs/cloudevents#pubsub
    encoded_pubsub_message = cloud_event.data["message"]["data"]

    # Retry based on a user-defined parameter
    try_again = json.loads(base64.b64decode(encoded_pubsub_message).decode())["retry"]

    try:
        raise RuntimeError("I failed you")
    except RuntimeError:
        error_client.report_exception()
        if try_again:
            raise  # Raise the exception and try again
        else:
            pass  # Swallow the exception and don't retry

Go

package tips

import (
	"context"
	"errors"
	"fmt"
	"log"

	"github.com/GoogleCloudPlatform/functions-framework-go/functions"
	"github.com/cloudevents/sdk-go/v2/event"
)

func init() {
	functions.CloudEvent("RetryPubSub", RetryPubSub)
}

// MessagePublishedData contains the full Pub/Sub message
// See the documentation for more details:
// https://cloud.google.com/eventarc/docs/cloudevents#pubsub
type MessagePublishedData struct {
	Message PubSubMessage
}

// PubSubMessage is the payload of a Pub/Sub event.
// See the documentation for more details:
// https://cloud.google.com/pubsub/docs/reference/rest/v1/PubsubMessage
type PubSubMessage struct {
	Data []byte `json:"data"`
}

// RetryPubSub demonstrates how to toggle using retries.
func RetryPubSub(ctx context.Context, e event.Event) error {
	var msg MessagePublishedData
	if err := e.DataAs(&msg); err != nil {
		return fmt.Errorf("event.DataAs: %w", err)
	}

	name := string(msg.Message.Data)
	if name == "" {
		name = "World"
	}

	// A misconfigured client will stay broken until the function is redeployed.
	client, err := MisconfiguredDataClient()
	if err != nil {
		log.Printf("MisconfiguredDataClient (retry denied):  %v", err)
		// A nil return indicates that the function does not need a retry.
		return nil
	}

	// Runtime error might be resolved with a new attempt.
	if err = FailedWriteOperation(client, name); err != nil {
		log.Printf("FailedWriteOperation (retry expected): %v", err)
		// A non-nil return indicates that a retry is needed.
		return err
	}

	return nil
}

Java


import com.google.cloud.functions.CloudEventsFunction;
import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import functions.eventpojos.PubSubBody;
import io.cloudevents.CloudEvent;
import java.nio.charset.StandardCharsets;
import java.util.Base64;
import java.util.logging.Logger;

public class RetryPubSub implements CloudEventsFunction {
  private static final Logger logger = Logger.getLogger(RetryPubSub.class.getName());

  // Use Gson (https://github.com/google/gson) to parse JSON content.
  private static final Gson gson = new Gson();

  @Override
  public void accept(CloudEvent event) throws Exception {
    if (event.getData() == null) {
      logger.warning("No data found in event!");
      return;
    }

    // Extract Cloud Event data and convert to PubSubBody
    String cloudEventData = new String(event.getData().toBytes(), StandardCharsets.UTF_8);
    PubSubBody body = gson.fromJson(cloudEventData, PubSubBody.class);

    String encodedData = body.getMessage().getData();
    String decodedData =
        new String(Base64.getDecoder().decode(encodedData), StandardCharsets.UTF_8);

    // Retrieve and decode PubSubMessage data into a JsonElement.
    // Function is expecting a user-supplied JSON message which determines whether
    // to retry or not.
    JsonElement jsonPubSubMessageElement = gson.fromJson(decodedData, JsonElement.class);

    boolean retry = false;
    // Get the value of the "retry" JSON parameter, if one exists
    if (jsonPubSubMessageElement != null && jsonPubSubMessageElement.isJsonObject()) {
      JsonObject jsonPubSubMessageObject = jsonPubSubMessageElement.getAsJsonObject();

      if (jsonPubSubMessageObject.has("retry")
          && jsonPubSubMessageObject.get("retry").getAsBoolean()) {
        retry = true;
      }
    }

    // Retry if appropriate
    if (retry) {
      // Throwing an exception causes the execution to be retried
      throw new RuntimeException("Retrying...");
    } else {
      logger.info("Not retrying...");
    }
  }
}

C#

using CloudNative.CloudEvents;
using Google.Cloud.Functions.Framework;
using Google.Events.Protobuf.Cloud.PubSub.V1;
using Microsoft.Extensions.Logging;
using System;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;

namespace Retry;

public class Function : ICloudEventFunction<MessagePublishedData>
{
    private readonly ILogger _logger;

    public Function(ILogger<Function> logger) =>
        _logger = logger;

    public Task HandleAsync(CloudEvent cloudEvent, MessagePublishedData data, CancellationToken cancellationToken)
    {
        bool retry = false;
        string text = data.Message?.TextData;

        // Get the value of the "retry" JSON parameter, if one exists.
        if (!string.IsNullOrEmpty(text))
        {
            JsonElement element = JsonSerializer.Deserialize<JsonElement>(data.Message.TextData);

            retry = element.TryGetProperty("retry", out var property) &&
                property.ValueKind == JsonValueKind.True;
        }

        // Throwing an exception causes the execution to be retried.
        if (retry)
        {
            throw new InvalidOperationException("Retrying...");
        }
        else
        {
            _logger.LogInformation("Not retrying...");
        }
        return Task.CompletedTask;
    }
}

Ruby

require "functions_framework"

FunctionsFramework.cloud_event "retry_or_not" do |event|
  try_again = event.data["retry"]

  begin
    # Simulate a failure
    raise "I failed!"
  rescue RuntimeError => e
    logger.warn "Caught an error: #{e}"
    if try_again
      # Raise an exception to return a 500 and trigger a retry.
      logger.info "Trying again..."
      raise ex
    else
      # Return normally to end processing of this event.
      logger.info "Giving up."
    end
  end
end

PHP


use Google\CloudFunctions\CloudEvent;

function tipsRetry(CloudEvent $event): void
{
    $cloudEventData = $event->getData();
    $pubSubData = $cloudEventData['message']['data'];

    $json = json_decode(base64_decode($pubSubData), true);

    // Determine whether to retry the invocation based on a parameter
    $tryAgain = $json['some_parameter'];

    if ($tryAgain) {
        /**
         * Functions with automatic retries enabled should throw exceptions to
         * indicate intermittent failures that a retry might fix. In this
         * case, a thrown exception will cause the original function
         * invocation to be re-sent.
         */
        throw new Exception('Intermittent failure occurred; retrying...');
    }

    /**
     * If a function with retries enabled encounters a non-retriable
     * failure, it should return *without* throwing an exception.
     */
    $log = fopen(getenv('LOGGER_OUTPUT') ?: 'php://stderr', 'wb');
    fwrite($log, 'Not retrying' . PHP_EOL);
}

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.

Configure the retry policy

Depending on the needs of your Cloud Run function, you may want to configure the retry policy directly. This would allow you to set up any combination of the following:

Shorten the retry window from 7 days to as little as 10 minutes.
Change the minimum and maximum backoff time for the exponential backoff retry strategy.
Change the retry strategy to retry immediately.
Configure a dead-letter topic.
Set a maximum and minimum number of delivery attempts.

To configure the retry policy:

Write an HTTP function.
Use the Pub/Sub API to create a Pub/Sub subscription, specifying the URL of the function as the target.

See Pub/Sub documentation on handling failures for a more information on configuring Pub/Sub directly.

Enable event-driven function retries

Why event-driven functions fail to complete

Semantics of retry

Event driven functions created with Cloud Run

Event driven functions created with Cloud Functions v2 API

Enable or disable retries

Configure retries from the gcloud command-line tool

Configure retries from the console

Retry window

Best practices

Use retry to handle transient errors

Set an end condition to avoid infinite retry loops

Node.js

Python

Go

Java

C#

Ruby

PHP

Distinguish between functions that can be retried and fatal errors

Node.js

Python

Go

Java

C#

Ruby

PHP

Make retryable event-driven functions idempotent

Configure the retry policy

Next steps

Configure retries from the `gcloud` command-line tool