Background Functions

You use background functions when you want to have your Cloud Function invoked indirectly in response to an event, such as a message on a Pub/Sub topic, a change in a Cloud Storage bucket, or a Firebase event.

For information on how to retry background functions, see Retrying Background Functions.

Sample usage

The examples below show how to process events from Pub/Sub and Cloud Storage. For more information about handling events from different sources, see Calling Cloud Functions.

Pub/Sub example

This example shows a Cloud Function triggered by Pub/Sub events. Every time a message is published to a Pub/Sub topic, the function is invoked, and a greeting using data derived from the message is written to the log.

Node.js

functions/helloworld/index.js 
/**
 * Background Cloud Function to be triggered by Pub/Sub.
 * This function is exported by index.js, and executed when
 * the trigger topic receives a message.
 *
 * @param {object} data The event payload.
 * @param {object} context The event metadata.
 */
exports.helloPubSub = (data, context) => {
  const pubSubMessage = data;
  const name = pubSubMessage.data
    ? Buffer.from(pubSubMessage.data, 'base64').toString()
    : 'World';

  console.log(`Hello, ${name}!`);
};

Python

functions/helloworld/main.py 
def hello_pubsub(event, context):
    """Background Cloud Function to be triggered by Pub/Sub.
    Args:
         event (dict):  The dictionary with data specific to this type of
         event. The `data` field contains the PubsubMessage message. The
         `attributes` field will contain custom attributes if there are any.
         context (google.cloud.functions.Context): The Cloud Functions event
         metadata. The `event_id` field contains the Pub/Sub message ID. The
         `timestamp` field contains the publish time.
    """
    import base64

    print("""This Function was triggered by messageId {} published at {}
    """.format(context.event_id, context.timestamp))

    if 'data' in event:
        name = base64.b64decode(event['data']).decode('utf-8')
    else:
        name = 'World'
    print('Hello {}!'.format(name))

Go

functions/helloworld/hello_pubsub.go 

// Package helloworld provides a set of Cloud Functions samples.
package helloworld

import (
	"context"
	"log"
)

// PubSubMessage is the payload of a Pub/Sub event.
type PubSubMessage struct {
	Data []byte `json:"data"`
}

// HelloPubSub consumes a Pub/Sub message.
func HelloPubSub(ctx context.Context, m PubSubMessage) error {
	name := string(m.Data)
	if name == "" {
		name = "World"
	}
	log.Printf("Hello, %s!", name)
	return nil
}

For more information about deploying Cloud Functions triggered by Pub/Sub events, see Pub/Sub Triggers and Pub/Sub Tutorial.

Cloud Storage example

This example shows a Cloud Function triggered by Cloud Storage events. Every time an object is created in a Cloud Storage bucket, the function is invoked, and a message about the change is written to the log.

Node.js

functions/helloworld/index.js 
/**
 * Background Cloud Function to be triggered by Cloud Storage.
 *
 * @param {object} data The event payload.
 * @param {object} context The event metadata.
 */
exports.helloGCS = (data, context) => {
  const file = data;
  if (file.resourceState === 'not_exists') {
    console.log(`File ${file.name} deleted.`);
  } else if (file.metageneration === '1') {
    // metageneration attribute is updated on metadata changes.
    // on create value is 1
    console.log(`File ${file.name} uploaded.`);
  } else {
    console.log(`File ${file.name} metadata updated.`);
  }
};

Python

functions/helloworld/main.py 
def hello_gcs(event, context):
    """Background Cloud Function to be triggered by Cloud Storage.
    Args:
         event (dict): The dictionary with data specific to this type of event.
         context (google.cloud.functions.Context): The Cloud Functions
         event metadata.
    """
    print("File: {}.".format(event['objectId']))

Go

functions/helloworld/hello_cloud_storage.go 

// Package helloworld provides a set of Cloud Functions samples.
package helloworld

import (
	"context"
	"log"
	"time"
)

// GCSEvent is the payload of a GCS event.
type GCSEvent struct {
	Kind                    string                 `json:"kind"`
	ID                      string                 `json:"id"`
	SelfLink                string                 `json:"selfLink"`
	Name                    string                 `json:"name"`
	Bucket                  string                 `json:"bucket"`
	Generation              string                 `json:"generation"`
	Metageneration          string                 `json:"metageneration"`
	ContentType             string                 `json:"contentType"`
	TimeCreated             time.Time              `json:"timeCreated"`
	Updated                 time.Time              `json:"updated"`
	TemporaryHold           bool                   `json:"temporaryHold"`
	EventBasedHold          bool                   `json:"eventBasedHold"`
	RetentionExpirationTime time.Time              `json:"retentionExpirationTime"`
	StorageClass            string                 `json:"storageClass"`
	TimeStorageClassUpdated time.Time              `json:"timeStorageClassUpdated"`
	Size                    string                 `json:"size"`
	MD5Hash                 string                 `json:"md5Hash"`
	MediaLink               string                 `json:"mediaLink"`
	ContentEncoding         string                 `json:"contentEncoding"`
	ContentDisposition      string                 `json:"contentDisposition"`
	CacheControl            string                 `json:"cacheControl"`
	Metadata                map[string]interface{} `json:"metadata"`
	CRC32C                  string                 `json:"crc32c"`
	ComponentCount          int                    `json:"componentCount"`
	Etag                    string                 `json:"etag"`
	CustomerEncryption      struct {
		EncryptionAlgorithm string `json:"encryptionAlgorithm"`
		KeySha256           string `json:"keySha256"`
	}
	KMSKeyName    string `json:"kmsKeyName"`
	ResourceState string `json:"resourceState"`
}

// HelloGCS consumes a GCS event.
func HelloGCS(ctx context.Context, e GCSEvent) error {
	if e.ResourceState == "not_exists" {
		log.Printf("File %v deleted.", e.Name)
		return nil
	}
	if e.Metageneration == "1" {
		// The metageneration attribute is updated on metadata changes.
		// The on create value is 1.
		log.Printf("File %v created.", e.Name)
		return nil
	}
	log.Printf("File %v metadata updated.", e.Name)
	return nil
}

For more information about deploying Cloud Functions triggered by Cloud Storage events, see Cloud Storage Triggers and Cloud Storage Tutorial.

Function parameters

Background functions are passed arguments holding data associated with the event that triggered the function's execution. The parameters of background functions are described below:

Node.js 8+

In the Node.js runtimes version 8 and above, your function is passed the arguments (data, context, callback):

Property Description Type
data The data object for the event. Its type depends on the event. Object
context The context object for the event. Object
context.eventId A unique ID for the event. For example: "70172329041928". String
context.timestamp The date/time this event was created. For example: "2018-04-09T07:56:12.975Z". String (ISO 8601)
context.eventType The type of the event. For example: "google.pubsub.topic.publish". String
context.resource The resource that emitted the event. Object
callback

A callback to signal completion of the function's execution. Follows the "errback" convention, which interprets the first argument as an error:

callback();                    // Success
callback(null, 'Success!');    // Success
callback(1);                   // Error
callback(new Error('Failed')); // Error
 Function

Node.js 6 (Deprecated)

In the Node.js 6 runtime, your function is passed the arguments (event, callback):

Property Description Type
event An object representing the event that triggered the function. Object
event.data The data object for the event. Its type depends on the event. Object
event.context The context object for the event. Object
event.context.eventId A unique ID for the event. For example: "70172329041928". String
event.context.timestamp The date/time this event was created. For example: "2018-04-09T07:56:12.975Z". String (ISO 8601)
event.context.eventType The type of the event. For example: "google.pubsub.topic.publish". String
event.context.resource The resource that emitted the event. String
callback

A callback to signal completion of the function's execution. Follows the "errback" convention, which interprets the first argument as an error:

callback();                    // Success
callback(null, 'Success!');    // Success
callback(1);                   // Error
callback(new Error('Failed')); // Error
 Function

Python

In the Python runtime, your function is passed the arguments (data, context):

Property Description Type
data A dictionary containing the data for the event. Its format depends on the event. Cloud Storage Object or PubsubMessage
context The context object for the event. Context
context.event_id A unique ID for the event. For example: "70172329041928". String
context.timestamp The date/time this event was created. For example: "2018-04-09T07:56:12.975Z". String (ISO 8601)
context.event_type The type of the event. For example: "google.pubsub.topic.publish". String
context.resource The resource that emitted the event. String

Go

In the Go runtime, your function is passed the arguments (ctx, Event):

Property Description Type
ctx A context.Context value which carries metadata about the event. You can retrieve the metadata using the cloud.google.com/go/functions/metadata package. context.Context
Event

A struct, whose type you define, which the event payload will be unmarshaled into using json.Unmarshal() . The event payload depends on the trigger for which the function was registered.

The struct definition that you supply in your code must correspond to the structure of the event type. The structure for each event is documented on the corresponding event's trigger page.

Note that your struct is not required to define every field contained in the payload. You might only want to use certain fields in your function, in which case you only need to define those fields in your struct. You can also rename a field (for example, when the JSON field name contains an underscore) using JSON tags on that field.

User-defined struct

The event data depends on the trigger for which the function was registered, for example, Pub/Sub or Cloud Storage. In the case of direct-triggered functions, triggered using the gcloud functions call command, the event data contains the message you sent directly.

Terminating background functions

You must signal when background functions have completed. Otherwise, your function can continue to run and be forcibly terminated by the system. You can signal function completion in each runtime as described below:

Node.js 8+

In the Node.js runtimes version 8 and above, signal function completion by either:

  • Invoking the callback argument,
  • Returning a Promise,
  • Wrapping your function using the async keyword (which causes your function to implicitly return a Promise), or
  • Returning a value.

If invoking the callback argument or synchronously returning a value, ensure that all asynchronous processes have completed first. If returning a Promise, Cloud Functions ensures that the Promise is settled before terminating.

The following example function returns a Promise:

functions/node8/index.js 
const fetch = require('node-fetch');

/**
 * Background Cloud Function that returns a Promise. Note that we don't pass
 * a "callback" argument to the function.
 *
 * @param {object} data The Cloud Functions event data.
 * @returns {Promise}
 */
exports.helloPromise = data => {
  return fetch(data.endpoint);
};

Node.js 6 (Deprecated)

In the Node.js 6 runtime, signal function completion by either:

  • Invoking the callback argument,
  • Returning a Promise, or
  • Returning a value.

If invoking the callback argument or synchronously returning a value, ensure that all asynchronous processes have completed first. If returning a Promise, Cloud Functions ensures that the Promise is settled before terminating.

The following example function returns a Promise:

functions/background/index.js 
const requestPromiseNative = require('request-promise-native');

/**
 * Background Cloud Function that returns a Promise. Note that we don't pass
 * a "callback" argument to the function.
 *
 * @param {object} event The Cloud Functions event.
 * @param {object} event.data The event data.
 * @returns {Promise}
 */
exports.helloPromise = event => {
  return requestPromiseNative({
    uri: event.data.endpoint,
  });
};

Python

In the Python runtime, signal function completion by returning a value:

functions/helloworld/main.py 
def hello_background(event, context):
    """Background Cloud Function.
    Args:
         event (dict): The dictionary with data specific to the given event.
         context (google.cloud.functions.Context): The Cloud Functions event
         metadata.
    """
    if event and 'name' in event:
        name = event['name']
    else:
        name = 'World'
    return 'Hello {}!'.format(name)

Go

In the Go runtime, signal function completion by returning a value:

functions/helloworld/hello_pubsub.go 

// Package helloworld provides a set of Cloud Functions samples.
package helloworld

import (
	"context"
	"log"
)

// PubSubMessage is the payload of a Pub/Sub event.
type PubSubMessage struct {
	Data []byte `json:"data"`
}

// HelloPubSub consumes a Pub/Sub message.
func HelloPubSub(ctx context.Context, m PubSubMessage) error {
	name := string(m.Data)
	if name == "" {
		name = "World"
	}
	log.Printf("Hello, %s!", name)
	return nil
}

Next steps