Cloud Functions with Pub/Sub triggers in GCP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the trigger type for a Cloud Function.

GCP

gcloud functions deploy myFunction --gen2 --trigger-[1] my-topic --runtime python312

Drag options to blanks, or click blank then click option'

Ahttp

Beventarc

Cpubsub

Dstorage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'http' instead of 'pubsub' for Pub/Sub triggers.

Confusing storage triggers with Pub/Sub triggers.

The --trigger-pubsub flag sets the function to trigger on Pub/Sub messages.

2fill in blank

medium

Complete the code to specify the Pub/Sub topic name for the trigger.

GCP

gcloud functions deploy myFunction --gen2 --trigger-pubsub [1] --runtime python312

Drag options to blanks, or click blank then click option'

Aprojects/my-project/topics/my-topic

Bmy-topic

Cprojects/my-project/my-topic

Dtopics/my-topic

Attempts:

3 left

💡 Hint

Common Mistakes

Using only the topic name without the full path.

Omitting the 'topics' part in the path.

The full resource name projects/my-project/topics/my-topic specifies the Pub/Sub topic.

3fill in blank

hard

Fix the error in the function code to correctly decode the Pub/Sub message data.

GCP

def my_function(event, context):
    import base64
    message = base64.b64decode(event['message']['[1]']).decode('utf-8')
    print(f"Received message: {message}")

Drag options to blanks, or click blank then click option'

Amessage

Battributes

Cpayload

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'message' or 'payload' instead of 'data'.

Trying to decode a non-existent key.

The Pub/Sub message data is in the event['message']['data'] field of the event.

4fill in blank

hard

Fill both blanks to create a Cloud Function that logs the message ID and publish time.

GCP

def my_function(event, context):
    message_id = event['message']['[1]']
    publish_time = event['message']['[2]']
    print(f"Message ID: {message_id}, Published at: {publish_time}")

Drag options to blanks, or click blank then click option'

Aevent_id

Btimestamp

CpublishTime

DmessageId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'event_id' instead of 'messageId'.

Using 'timestamp' instead of 'publishTime'.

The message ID is accessed with event['message']['messageId'] and the publish time with event['message']['publishTime'].

5fill in blank

hard

Fill all three blanks to deploy a Cloud Function with a Pub/Sub trigger, specifying the runtime and entry point.

GCP

gcloud functions deploy [1] --gen2 --runtime [2] --trigger-pubsub [3] --entry-point my_function

Drag options to blanks, or click blank then click option'

AmyFunction

Bpython312

Cprojects/my-project/topics/my-topic

Dnodejs20

Attempts:

3 left

💡 Hint

Common Mistakes

Using a runtime that does not match the language.

Omitting the full topic path.

The function name is myFunction, the runtime is python312, and the Pub/Sub topic is specified with the full path.

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