Cloud Functions with Pub/Sub triggers in GCP - Mini Project: Build & Apply

Cloud Functions with Pub/Sub triggers

📖 Scenario: You are building a simple cloud function that reacts to messages published on a Pub/Sub topic. This function will process incoming messages and log their content.

🎯 Goal: Create a Google Cloud Function triggered by a Pub/Sub topic. The function should receive messages, decode them, and log the message data.

📋 What You'll Learn

Create a Pub/Sub topic named my-topic

Create a Cloud Function named process_pubsub_message

Configure the Cloud Function to trigger on messages published to my-topic

The function should decode the base64 message data and log it

💡 Why This Matters

🌍 Real World

Cloud Functions triggered by Pub/Sub are commonly used to process events asynchronously, such as processing user uploads, notifications, or IoT device messages.

💼 Career

Understanding how to connect Pub/Sub with Cloud Functions is essential for cloud developers and engineers working on event-driven architectures in Google Cloud.

1

Create the Pub/Sub topic

Create a Pub/Sub topic named my-topic using the gcloud CLI command.

GCP

# Use the gcloud command to create the topic
# Your code here

Hint

Use the gcloud pubsub topics create command followed by the topic name.

2

Write the Cloud Function code

Write a Python function named process_pubsub_message that accepts event and context parameters. Inside the function, decode the base64 message data from event['data'] and assign it to a variable called message.

GCP

def process_pubsub_message(event, context):
    # Decode the Pub/Sub message data
    # Your code here

Hint

Use the base64 module to decode event['data'] and then decode bytes to a UTF-8 string.

3

Add logging of the message

Inside the process_pubsub_message function, import the logging module and add a line to log the decoded message using logging.info().

GCP

gcloud pubsub topics create my-topic

def process_pubsub_message(event, context):
    import base64
    import logging
    message = base64.b64decode(event['data']).decode('utf-8')
    # Log the message
    # Your code here

Hint

Use logging.info() to log the message with a descriptive text.

4

Deploy the Cloud Function with Pub/Sub trigger

Deploy the Cloud Function named process_pubsub_message using the gcloud CLI. Set the runtime to python39, the trigger to the Pub/Sub topic my-topic, and the entry point to process_pubsub_message.

GCP

gcloud pubsub topics create my-topic

def process_pubsub_message(event, context):
    import base64
    import logging
    message = base64.b64decode(event['data']).decode('utf-8')
    logging.info(f'Received message: {message}')

# Deploy the function using gcloud
# Your code here

Hint

Use gcloud functions deploy with the correct flags for runtime, trigger-topic, and entry-point.

Practice

1. What happens when a Google Cloud Function is triggered by a Pub/Sub message?

easy

A. The function runs automatically when a new message arrives on the Pub/Sub topic.

B. The function must be manually started after the message is published.

C. The function only runs if the message contains a specific keyword.

D. The function runs only once a day regardless of messages.

Solution

1. Step 1: Understand Pub/Sub trigger behavior

   Cloud Functions connected to Pub/Sub topics run automatically when a new message is published to that topic.

2. Step 2: Confirm automatic execution

   There is no need for manual start or keyword filtering by default; the function triggers on every message.

3. Final Answer:

   The function runs automatically when a new message arrives on the Pub/Sub topic. -> Option A

4. Quick Check:

   Pub/Sub triggers auto-run functions on new messages [OK]

Hint: Pub/Sub triggers run functions automatically on new messages [OK]

Common Mistakes:

• Thinking the function needs manual start
• Assuming filtering happens automatically
• Believing the function runs on a schedule

2. Which of the following is the correct way to declare a Cloud Function triggered by Pub/Sub in Node.js?

easy

A. exports.myFunction = (event, context) => { console.log(event.data); };

B. function myFunction(pubsubMessage) { console.log(pubsubMessage.data); }

C. exports.myFunction = (req, res) => { res.send('Hello'); };

D. function myFunction(event) { return event.data; }

Solution

1. Step 1: Identify correct Cloud Function signature for Pub/Sub

   In Node.js, a Pub/Sub triggered function uses (event, context) parameters, where event.data contains the message.

2. Step 2: Check options for correct syntax

   exports.myFunction = (event, context) => { console.log(event.data); }; uses exports with (event, context) and accesses event.data correctly. Others either use wrong parameters or HTTP function style.

3. Final Answer:

   exports.myFunction = (event, context) => { console.log(event.data); }; -> Option A

4. Quick Check:

   Pub/Sub functions use (event, context) signature [OK]

Hint: Pub/Sub functions use (event, context) parameters in Node.js [OK]

Common Mistakes:

• Using HTTP function signature (req, res)
• Missing exports keyword
• Using wrong parameter names

3. Given this Python Cloud Function triggered by Pub/Sub:

def hello_pubsub(event, context):
    import base64
    message = base64.b64decode(event['data']).decode('utf-8')
    print(f'Received message: {message}')

What will be printed if the Pub/Sub message data is base64 encoding of 'Hello World'?

medium

A. Received message: event['data']

B. Received message: b'Hello World'

C. Received message: Hello World

D. Error: base64 decoding failed

Solution

1. Step 1: Decode base64 message data

   The function decodes event['data'] from base64 to bytes, then decodes bytes to UTF-8 string.

2. Step 2: Print the decoded message

   The decoded string is 'Hello World', so the print statement outputs 'Received message: Hello World'.

3. Final Answer:

   Received message: Hello World -> Option C

4. Quick Check:

   Base64 decode then UTF-8 decode gives original message [OK]

Hint: Base64 decode Pub/Sub data to get original message text [OK]

Common Mistakes:

• Printing raw base64 string instead of decoded
• Forgetting to decode bytes to string
• Assuming event['data'] is plain text

4. You deployed a Cloud Function triggered by Pub/Sub, but it never runs when messages are published. What is the most likely cause?

medium

A. Cloud Functions do not support Pub/Sub triggers.

B. The function code has a syntax error.

C. The Pub/Sub topic is empty and cannot trigger functions.

D. The function is not subscribed to the correct Pub/Sub topic.

Solution

1. Step 1: Check Pub/Sub subscription linkage

   If the function is not linked to the correct Pub/Sub topic, it will not trigger on published messages.

2. Step 2: Eliminate other causes

   Syntax errors cause deployment failure, empty topics can still trigger if messages arrive, and Cloud Functions do support Pub/Sub triggers.

3. Final Answer:

   The function is not subscribed to the correct Pub/Sub topic. -> Option D

4. Quick Check:

   Correct topic subscription is required for triggers [OK]

Hint: Verify function subscribes to the right Pub/Sub topic [OK]

Common Mistakes:

• Assuming empty topic blocks triggers
• Ignoring subscription configuration
• Thinking Cloud Functions lack Pub/Sub support

5. You want to ensure your Cloud Function triggered by Pub/Sub processes messages exactly once, even if Pub/Sub retries delivery. What is the best approach?

hard

A. Disable retries in Pub/Sub settings to avoid duplicate triggers.

B. Implement idempotent processing in the function using unique message IDs.

C. Use a global variable in the function to track processed messages.

D. Rely on Pub/Sub to guarantee exactly-once delivery automatically.

Solution

1. Step 1: Understand Pub/Sub delivery guarantees

   Pub/Sub guarantees at-least-once delivery, so duplicates can occur if retries happen.

2. Step 2: Implement idempotency in function

   To handle duplicates, the function should process messages idempotently, often by checking unique message IDs to avoid repeated work.

3. Step 3: Evaluate other options

   Disabling retries risks message loss, global variables don't persist across executions, and Pub/Sub does not guarantee exactly-once delivery by itself.

4. Final Answer:

   Implement idempotent processing in the function using unique message IDs. -> Option B

5. Quick Check:

   Idempotent code handles retries safely [OK]

Hint: Make function idempotent using message IDs to avoid duplicates [OK]

Common Mistakes:

• Expecting Pub/Sub to do exactly-once delivery
• Disabling retries risking lost messages
• Using non-persistent variables for tracking