Google Cloud's [Pub/Sub](https://cloud.google.com/pubsub/docs/) 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

```javascript
const {onMessagePublished} = require("firebase-functions/pubsub");
const logger = require("firebase-functions/logger");
```

### Python

```python
from firebase_functions import pubsub_fn
```

## 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

```javascript
exports.hellopubsub = onMessagePublished("topic-name", (event) => {
```

### Python

```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."""
```

## 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

```javascript
  // 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);
  }
```

### Python

```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"]
```

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

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

### Python

```python
# Decode the PubSub message body.
message_body = base64.b64decode(event.data.message.data)
```

## 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

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

### Python

```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"]
```