重试事件驱动的函数

本文档介绍如何为事件驱动的函数(后台函数CloudEvent 函数)启用重试。HTTP 函数无法自动重试。

重试的语义

对于事件来源发出的每个事件,Cloud Functions 保证至少执行一次事件驱动的函数。但是,默认情况下,如果函数调用因错误而终止,则该函数不会被再次调用,且相应事件将被丢弃。当您针对事件驱动的函数启用重试时,Cloud Functions 将重试失败的函数调用,直到该函数调用成功或重试期限到期(默认为 7 天)。

为什么事件驱动的函数会无法完成

在极少数情况下,函数可能会由于内部错误而提早退出,并且默认情况下函数可能会自动重试,也可能不会自动重试。

更常见的情况是,事件驱动的函数可能由于函数代码本身抛出错误而无法成功完成。可能导致发生这种情况的一些原因如下:

  • 函数包含错误,且运行时抛出异常。
  • 函数无法访问服务端点,或者在尝试访问端点时超时。
  • 函数本身有意抛出异常(例如,当某个参数验证失败时)。
  • 以 Node.js 编写的函数返回遭拒的 promise 或将非 null 值传递给回调函数。

在上述任何情况下,函数默认都会停止执行,而事件会遭舍弃。如果要在出错时重试函数,可通过设置“失败时重试”属性来更改默认的重试政策。 这会导致事件在多达数天的时间里不断重试,直到函数成功完成。

启用和停用重试功能

要启用或停用重试功能,您可以使用 gcloud 命令行工具或 Cloud Console。默认情况下,重试功能处于停用状态。

使用 gcloud 命令行工具

要通过 gcloud 命令行工具启用重试功能,请在部署您的函数时添加 --retry 标志:

gcloud functions deploy FUNCTION_NAME --retry FLAGS...

要停用重试功能,请在不使用 --retry 标志的情况下重新部署函数:

gcloud functions deploy FUNCTION_NAME FLAGS...

使用 Cloud Console

您可以在 Cloud Console 中启用或停用重试功能,步骤如下:

  1. 在 Cloud Platform Console 中转到 Cloud Functions 概览页面

  2. 点击创建函数。或者,点击现有函数以转至其详情页面,然后点击修改

  3. 填写函数的必填字段。

  4. 确保触发器字段设置为事件驱动的函数触发器类型,例如 Cloud Pub/Sub 或 Cloud Storage。

  5. 点击更多以展开高级设置。

  6. 选中或取消选中标有失败时重试的复选框。

最佳做法

本部分介绍有关如何利用重试的最佳做法。

利用重试来应对暂时性错误

由于函数会一直重试,直到成功执行为止,因此在启用重试之前应进行彻底的测试,以从代码中清除错误之类的永久性错误。重试最适合用于应对较有可能通过重试解决的间歇性/暂时性故障,比如不稳定的服务端点或超时。

设置结束条件以避免无限重试循环

在启用重试时,您应防止函数陷入连续循环,这是一项最佳做法。为此,您可以在函数开始处理之前添加明确定义的结束条件。请注意,此方法仅在您的函数成功启动并且能够评估结束条件时才有效。

一种简单而有效的方法是舍弃时间戳早于特定时间的事件。在发生永久性故障或持续时间长于预期的故障时,这样做有助于保证函数执行次数不会太多。

例如,以下代码段会舍弃超过 10 秒前发生的所有事件:

Node.js

/**
 * Background Cloud 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.
 */
exports.avoidInfiniteRetries = (event, callback) => {
  const eventAge = Date.now() - Date.parse(event.timestamp);
  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

def avoid_infinite_retries(data, context):
    """Background Cloud Function that only executes within a certain
    time period after the triggering event.

    Args:
        data (dict): The event payload.
        context (google.cloud.functions.Context): The event metadata.
    Returns:
        None; output is written to Stackdriver Logging
    """

    timestamp = context.timestamp

    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(context.event_id, event_age_ms))
        return 'Timeout'

    # Do what the function is supposed to do
    print('Processed {} (age {}ms)'.format(context.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"

	"cloud.google.com/go/functions/metadata"
)

// 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, m PubSubMessage) error {
	meta, err := metadata.FromContext(ctx)
	if err != nil {
		// Assume an error on the function invoker and try again.
		return fmt.Errorf("metadata.FromContext: %v", err)
	}

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

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

Java


import com.google.cloud.functions.BackgroundFunction;
import com.google.cloud.functions.Context;
import com.google.events.cloud.pubsub.v1.Message;
import com.google.gson.Gson;
import java.time.Duration;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.util.logging.Logger;

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

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

  /**
   * Background Cloud Function that only executes within
   * a certain time period after the triggering event
   */
  @Override
  public void accept(Message message, Context context) {
    ZonedDateTime utcNow = ZonedDateTime.now(ZoneOffset.UTC);
    ZonedDateTime timestamp = ZonedDateTime.parse(context.timestamp());

    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...');
    }
}

区分可重试错误和致命错误

如果您的函数已启用重试功能,则任何未处理的错误都将触发重试。 请确保您的代码能够捕获所有不应导致重试的错误。

Node.js

/**
 * Background Cloud Function that demonstrates
 * how to toggle retries using a promise
 *
 * @param {object} event The Cloud Functions event.
 * @param {object} event.data Data included with the event.
 * @param {object} event.data.retry User-supplied parameter that tells the function whether to retry.
 */
exports.retryPromise = event => {
  const tryAgain = !!event.data.retry;

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

/**
 * Background Cloud Function that demonstrates
 * how to toggle retries using a callback
 *
 * @param {object} event The Cloud Functions event.
 * @param {object} event.data Data included with the event.
 * @param {object} event.data.retry User-supplied parameter that tells the function whether to retry.
 * @param {function} callback The callback function.
 */
exports.retryCallback = (event, callback) => {
  const tryAgain = !!event.data.retry;
  const err = new Error('Error!');

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

Python

from google.cloud import error_reporting
error_client = error_reporting.Client()

def retry_or_not(data, context):
    """Background Cloud Function that demonstrates how to toggle retries.

    Args:
        data (dict): The event payload.
        context (google.cloud.functions.Context): The event metadata.
    Returns:
        None; output is written to Stackdriver Logging
    """

    # Retry based on a user-defined parameter
    try_again = data.data.get('retry') is not None

    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 contains tips for writing Cloud Functions in Go.
package tips

import (
	"context"
	"errors"
	"log"
)

// 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, m PubSubMessage) error {
	name := string(m.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.BackgroundFunction;
import com.google.cloud.functions.Context;
import com.google.events.cloud.pubsub.v1.Message;
import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import java.nio.charset.StandardCharsets;
import java.util.Base64;
import java.util.logging.Logger;

public class RetryPubSub implements BackgroundFunction<Message> {
  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(Message message, Context context) {
    String bodyJson = new String(
        Base64.getDecoder().decode(message.getData()), StandardCharsets.UTF_8);
    JsonElement bodyElement = gson.fromJson(bodyJson, JsonElement.class);

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

      if (body.has("retry") && body.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);
}

使可重试的事件驱动函数具有幂等性

可重试的事件驱动函数必须是幂等函数。下面是一些有关如何使此类函数具有幂等性的一般指导原则:

  • 许多外部 API(如 Stripe)允许提供幂等键作为参数。如果您在使用此类 API,应将事件 ID 作为幂等键。
  • 幂等性与“至少一次”机制非常契合,因为它能确保重试的安全性。编写可靠代码的一项常见最佳做法是结合使用幂等性和重试。
  • 确保代码具有内在的幂等性。例如:
    • 确保即使发生多次变更 (mutation),执行结果也不会改变。
    • 在事务中,先查询数据库状态再更改状态。
    • 确保所有副作用本身也具有幂等性。
  • 在函数之外强制执行事务检查(不依赖代码)。 例如,在某个位置留存状态信息,并记录已处理事件的 ID。
  • 处理重复的带外函数调用。例如,设置一个单独的清理程序,在发生重复函数调用后执行清理。

后续步骤