Google Cloud Pub/Sub is a globally
distributed message
bus that automatically scales as you need it. You can create a
function that handles Google Cloud Pub/Sub events
by using functions.pubsub
.
Trigger a pub/sub function
You can trigger a function whenever a new Pub/Sub message is sent to a specific
topic. You must specify the Pub/Sub topic name that
you want to trigger your function, and set the event within the
onPublish()
event handler:
exports.helloPubSub = functions.pubsub.topic('topic-name').onPublish((message) => { // ... });
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:
// Get the `name` attribute of the PubSub message JSON body. let name = null; try { name = message.json.name; } catch (e) { console.error('PubSub message was not JSON', e); }
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'
// Decode the PubSub Message body. const messageBody = message.data ? Buffer.from(message.data, 'base64').toString() : null;
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
Message.attributes
:
// Get the `name` attribute of the message. const name = message.attributes.name;
You might notice that some basic data such as the message ID or the
message publish time are not available in Message.attributes
. To work around
this, you can access the triggering event's
context.eventId
and
context.timestamp
,
with which you can determine when any message was published
and differentiate one message from another. For example:
exports.myFunction = functions.pubsub.topic('topic1').onPublish((message, context) => {
console.log('The function was triggered at ', context.timestamp);
console.log('The unique ID for the event is', context.eventId);
});