Pub/Sub triggers

Google Cloud's Pub/Sub is a globally distributed message bus that automatically scales as you need it. You can trigger a function whenever a new Pub/Sub message is sent to a specific topic.

Import the required modules

To get started, import the modules required for handling Pub/Sub events:

Node.js

const {onMessagePublished} = require("firebase-functions/pubsub"); const logger = require("firebase-functions/logger");
index.js

Python

from firebase_functions import pubsub_fn
main.py

Trigger the function

You must specify the Pub/Sub topic name that you want to trigger your function, and set the event within the event handler:

Node.js

exports.hellopubsub = onMessagePublished("topic-name", (event) => {
index.js

Python

@pubsub_fn.on_message_published(topic="topic-name") def hellopubsub(event: pubsub_fn.CloudEvent[pubsub_fn.MessagePublishedData]) -> None:  """Log a message using data published to a Pub/Sub topic."""
main.py

Access the pub/sub message payload

The payload for the Pub/Sub message is accessible from the message object returned to your function. For messages with JSON in the Pub/Sub message body, the Firebase SDK for Cloud Functions has a helper property to decode the message. For example, here is a message published with a simple JSON payload:

gcloud pubsub topics publish topic-name --message '{"name":"Xenia"}'

You can access a JSON data payload like this via the json property:

Node.js

 // Get the `name` attribute of the PubSub message JSON body.  let name = null;  try {  name = event.data.message.json.name;  } catch (e) {  logger.error("PubSub message was not JSON", e);  }
index.js

Python

# Get the `name` attribute of the PubSub message JSON body. try: data = event.data.message.json except ValueError: print("PubSub message was not JSON") return if data is None: return if "name" not in data: print("No 'name' key") return name = data["name"]
main.py

Other, non-JSON payloads are contained in the Pub/Sub message as base64 encoded strings in the message object. To read a message like the following, you must decode the base64 encoded string as shown:

gcloud pubsub topics publish topic-name --message 'MyMessage'

Node.js

// Decode the PubSub Message body. const message = event.data.message; const messageBody = message.data ?  Buffer.from(message.data, "base64").toString() :  null;
index.js

Python

# Decode the PubSub message body. message_body = base64.b64decode(event.data.message.data)
main.py

Access message attributes

Pub/Sub message can be sent with data attributes set in the publish command. For example, you could publish a message with a name attribute:

gcloud pubsub topics publish topic-name --attribute name=Xenia

You can read such attributes from the corresponding property of the message object:

Node.js

// Get the `name` attribute of the message. const name = event.data.message.attributes.name;
index.js

Python

# Get the `name` attribute of the message. if "name" not in event.data.message.attributes: print("No 'name' attribute") return name = event.data.message.attributes["name"]
main.py