Pub/Sub with Cloud Functions integration in GCP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens when a Cloud Function subscribed to a Pub/Sub topic throws an error?

You have a Cloud Function triggered by a Pub/Sub topic. The function processes messages but sometimes throws an error during execution.

What is the behavior of Pub/Sub and the Cloud Function when an error occurs?

A The message is acknowledged and removed from the subscription, so it is not retried.

B The message is not acknowledged and is retried until the function succeeds or the message expires.

C The message is sent to a dead-letter topic immediately without retries.

D The Cloud Function automatically scales down to zero and stops receiving messages.

💡 Hint

Think about how Pub/Sub handles message acknowledgment and retries.

When a Cloud Function triggered by Pub/Sub throws an error, the message is not acknowledged. Pub/Sub retries delivering the message until the function succeeds or the message expires based on the subscription's ack deadline and retention policy.

❓ Configuration

intermediate

Which IAM role must be granted to a Cloud Function to allow it to be triggered by a Pub/Sub topic?

You want to deploy a Cloud Function that triggers when messages are published to a specific Pub/Sub topic.

Which IAM role should you assign to the Cloud Function's service account to allow this trigger?

A roles/pubsub.publisher

B roles/pubsub.subscriber

C roles/cloudfunctions.invoker

D roles/pubsub.viewer

💡 Hint

Consider which role allows receiving messages from Pub/Sub.

The Cloud Function needs the 'roles/pubsub.subscriber' role to pull messages from the Pub/Sub subscription and be triggered properly.

❓ Architecture

advanced

How to design a reliable system to process Pub/Sub messages with Cloud Functions ensuring no message loss?

You need to build a system where Cloud Functions process Pub/Sub messages reliably without losing any messages, even if the function fails temporarily.

Which design choice best ensures message durability and processing reliability?

A Use a Pub/Sub topic with a subscription that has a dead-letter topic configured and Cloud Functions with retry enabled.

B Use a Pub/Sub topic with no dead-letter topic and disable retries on Cloud Functions to avoid duplicate processing.

C Use Cloud Storage to store messages and trigger Cloud Functions on file creation instead of Pub/Sub.

D Use a Pub/Sub topic with a push subscription directly calling the Cloud Function URL without retries.

💡 Hint

Think about how to handle failed messages and retries safely.

Configuring a dead-letter topic allows failed messages to be captured for later inspection. Enabling retries on Cloud Functions ensures messages are retried until successful processing, preventing message loss.

❓ security

advanced

Which practice improves security when integrating Pub/Sub with Cloud Functions?

You want to secure your Cloud Function triggered by Pub/Sub so that only authorized messages can invoke it.

Which practice best enhances security in this integration?

A Use a push subscription with an authentication token and verify it in the Cloud Function code.

B Use a pull subscription and grant the Cloud Function's service account the subscriber role only.

C Make the Cloud Function public and rely on Pub/Sub topic permissions to restrict access.

D Disable authentication on the Cloud Function to allow all Pub/Sub messages to trigger it.

💡 Hint

Consider how to authenticate messages sent to the Cloud Function.

Using a push subscription with an authentication token ensures that only Pub/Sub can invoke the Cloud Function. Verifying the token in the function code adds an extra security layer.

✅ Best Practice

expert

What is the best way to handle high-throughput Pub/Sub message processing with Cloud Functions to avoid cold starts and throttling?

Your system receives a high volume of Pub/Sub messages. You use Cloud Functions to process them, but you notice cold starts and throttling affecting performance.

Which approach best addresses these issues?

A Use Cloud Run with Pub/Sub push subscription for better scaling and control over concurrency.

B Increase the memory allocation of Cloud Functions to reduce cold start time.

C Use multiple Cloud Functions with different Pub/Sub topics to distribute load evenly.

D Disable retries on Cloud Functions to reduce duplicate invocations and throttling.

💡 Hint

Think about alternatives to Cloud Functions that handle scaling better.

Cloud Run provides better control over concurrency and scales smoothly for high-throughput workloads, reducing cold starts and throttling compared to Cloud Functions.

Practice

1.

What is the main role of Google Cloud Pub/Sub when integrated with Cloud Functions?

easy

A. To send messages that trigger Cloud Functions automatically

B. To store large files for Cloud Functions to process

C. To provide a user interface for Cloud Functions

D. To manage user authentication for Cloud Functions

Solution

1. Step 1: Understand Pub/Sub's purpose

   Pub/Sub is designed to send messages between parts of an app, acting as a messenger.

2. Step 2: Connect Pub/Sub with Cloud Functions

   Cloud Functions run code automatically when they receive messages from Pub/Sub.

3. Final Answer:

   To send messages that trigger Cloud Functions automatically -> Option A

4. Quick Check:

   Pub/Sub triggers Cloud Functions [OK]

Hint: Pub/Sub sends messages; Cloud Functions react to them [OK]

Common Mistakes:

• Thinking Pub/Sub stores files
• Confusing Pub/Sub with UI or authentication
• Assuming Cloud Functions manage user login

2.

Which of the following is the correct way to specify a Pub/Sub trigger in a Cloud Function deployment command?

gcloud functions deploy myFunction --trigger-?

easy

A. --trigger-http my-topic

B. --trigger-bucket my-topic

C. --trigger-topic my-topic

D. --trigger-event my-topic

Solution

1. Step 1: Identify the trigger type for Pub/Sub

   Pub/Sub triggers require specifying the topic name with --trigger-topic.

2. Step 2: Match the correct flag

   --trigger-topic is the correct flag; others relate to storage or HTTP triggers.

3. Final Answer:

   --trigger-topic my-topic -> Option C

4. Quick Check:

   Pub/Sub trigger uses --trigger-topic [OK]

Hint: Pub/Sub triggers use --trigger-topic flag [OK]

Common Mistakes:

• Using --trigger-bucket which is for Cloud Storage
• Using --trigger-http which is for HTTP triggers
• Confusing event triggers with topic triggers

3.

Given this Cloud Function code triggered by Pub/Sub messages, what will be logged when a message with data "Hello" is published?

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

medium

A. Received message: event['data']

B. Received message: Hello

C. Received message: b'Hello'

D. Error: 'data' key missing

Solution

1. Step 1: Decode the Pub/Sub message data

   The message data is base64 encoded; decoding it gives the original string "Hello".

2. Step 2: Print the decoded message

   The print statement outputs "Received message: Hello" exactly.

3. Final Answer:

   Received message: Hello -> Option B

4. Quick Check:

   Base64 decode then print = Received message: Hello [OK]

Hint: Decode base64 data to get original message [OK]

Common Mistakes:

• Printing raw base64 bytes instead of decoded string
• Using event['data'] directly without decoding
• Assuming 'data' key is missing

4.

What is the likely cause if a Cloud Function triggered by Pub/Sub never runs after publishing messages?

medium

A. The Cloud Function is not subscribed to the correct Pub/Sub topic

B. The Cloud Function code has a syntax error

C. The Pub/Sub topic does not exist

D. The Cloud Function has too much memory allocated

Solution

1. Step 1: Check the Cloud Function trigger configuration

   If the function is not subscribed to the right topic, it won't receive messages.

2. Step 2: Rule out other causes

   Syntax errors cause deployment failure, not silent no-run; topic existence is needed but usually checked; memory size doesn't stop triggers.

3. Final Answer:

   The Cloud Function is not subscribed to the correct Pub/Sub topic -> Option A

4. Quick Check:

   Wrong topic subscription = no trigger [OK]

Hint: Check topic subscription if function never runs [OK]

Common Mistakes:

• Assuming syntax error causes silent no-run
• Ignoring topic existence or permissions
• Blaming memory size for trigger issues

5.

You want to build a system where multiple Cloud Functions react independently to the same Pub/Sub messages. What is the best way to set this up?

hard

A. Publish messages multiple times to different topics

B. Create multiple Pub/Sub topics, each with one Cloud Function subscription

C. Use one Cloud Function that calls other functions internally

D. Create one Pub/Sub topic and multiple subscriptions, each triggering a different Cloud Function

Solution

1. Step 1: Understand Pub/Sub fan-out pattern

   One topic can have multiple subscriptions; each subscription can trigger a different Cloud Function independently.

2. Step 2: Compare options

   Multiple topics add complexity; one function calling others is indirect; publishing multiple times is inefficient.

3. Final Answer:

   Create one Pub/Sub topic and multiple subscriptions, each triggering a different Cloud Function -> Option D

4. Quick Check:

   One topic, many subscriptions = multiple triggers [OK]

Hint: Use multiple subscriptions on one topic for parallel triggers [OK]

Common Mistakes:

• Creating many topics unnecessarily
• Using one function to call others instead of triggers
• Publishing duplicate messages to multiple topics