Cloud Functions with Pub/Sub triggers in GCP - Commands & Configuration

Introduction

Sometimes you want your code to run automatically when a message is sent to a topic. Cloud Functions with Pub/Sub triggers let you do this by running your code whenever a new message arrives on a Pub/Sub topic.

When you want to process data as soon as it is published to a messaging topic without manual intervention.

When you need to run small pieces of code in response to events from other services or applications.

When you want to decouple your application components so they communicate through messages.

When you want to automatically trigger workflows based on messages sent by other systems.

When you want to scale your processing automatically based on the number of messages.

Config File - function-source/main.py

function-source/main.py

def hello_pubsub(event, context):
    import base64
    if 'data' in event:
        message = base64.b64decode(event['data']).decode('utf-8')
        print(f'Received message: {message}')
    else:
        print('No data in message')

This Python file contains the Cloud Function code. The hello_pubsub function runs when a message arrives. It decodes the message data from base64 and prints it. If no data is present, it prints a different message.

Commands

Create a Pub/Sub topic named 'example-topic' where messages will be sent to trigger the function.

Terminal

gcloud pubsub topics create example-topic

Expected Output

Created topic [projects/your-project-id/topics/example-topic].

Deploy the Cloud Function named 'helloPubSubFunction' using Python 3.10. It triggers on messages published to 'example-topic'. The function entry point is 'hello_pubsub'.

Terminal

gcloud functions deploy helloPubSubFunction --runtime python310 --trigger-topic example-topic --entry-point hello_pubsub --region us-central1 --quiet

Expected Output

Deploying function (may take a while)...



Deploying function (helloPubSubFunction)...done.



availableMemoryMb: 256

entryPoint: hello_pubsub

name: projects/your-project-id/locations/us-central1/functions/helloPubSubFunction

runtime: python310

status: ACTIVE

trigger:

  eventType: google.pubsub.topic.publish

  resource: projects/your-project-id/topics/example-topic

→

--runtime - Specifies the language runtime version.

→

--trigger-topic - Sets the Pub/Sub topic that triggers the function.

→

--entry-point - Defines the function name in the source code.

Send a message 'Hello, Cloud Functions!' to the 'example-topic' to trigger the Cloud Function.

Terminal

gcloud pubsub topics publish example-topic --message "Hello, Cloud Functions!"

Expected Output

messageIds:

- "1234567890123456"

→

--message - Specifies the message content to publish.

Check the logs of the Cloud Function to see the output from the triggered function.

Terminal

gcloud functions logs read helloPubSubFunction --region us-central1 --limit 5

Expected Output

2024-06-01 12:00:00 helloPubSubFunction: Received message: Hello, Cloud Functions!

2024-06-01 11:59:00 helloPubSubFunction: Function execution started

2024-06-01 11:59:01 helloPubSubFunction: Function execution finished

→

--limit - Limits the number of log entries shown.

Key Concept

If you remember nothing else from this pattern, remember: Cloud Functions run your code automatically whenever a message is sent to a Pub/Sub topic you choose.

Common Mistakes

Deploying the function without specifying the correct trigger topic.

The function will not run because it is not connected to any Pub/Sub topic.

Always use the --trigger-topic flag with the exact topic name when deploying the function.

Publishing messages to a different topic than the one the function listens to.

The function will not be triggered because it listens only to its configured topic.

Publish messages only to the topic that triggers your function.

Not decoding the base64 message data inside the function.

The function will print unreadable data instead of the actual message content.

Decode the message data from base64 before using it.

Summary

Create a Pub/Sub topic to send messages.

Deploy a Cloud Function triggered by that topic with your code.

Publish messages to the topic to automatically run your function.

Check function logs to see the output and confirm it ran.