Pub/Sub with Cloud Functions integration in GCP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Google Cloud Pub/Sub?

Google Cloud Pub/Sub is a messaging service that lets applications send and receive messages asynchronously. It helps different parts of a system talk to each other without waiting.

beginner

How does a Cloud Function get triggered by a Pub/Sub message?

A Cloud Function subscribes to a Pub/Sub topic. When a message is published to that topic, the function automatically runs and receives the message data.

beginner

What is the role of a Pub/Sub topic in Cloud Functions integration?

A Pub/Sub topic acts like a mailbox where messages are sent. Cloud Functions listen to this mailbox and react when new messages arrive.

intermediate

Why use Pub/Sub with Cloud Functions instead of calling functions directly?

Using Pub/Sub decouples parts of your system. It allows messages to queue up if the function is busy, making the system more reliable and scalable.

intermediate

What is the format of the message data received by a Cloud Function from Pub/Sub?

The message data is base64-encoded. The function needs to decode it to get the original message content.

What triggers a Cloud Function when using Pub/Sub integration?

A A new message published to a Pub/Sub topic

B A user clicking a button in the UI

C A scheduled timer event

D Manual invocation from the console

Cloud Functions are triggered automatically when a new message is published to the subscribed Pub/Sub topic.

How does Pub/Sub help improve system reliability with Cloud Functions?

A By queuing messages if the function is busy

B By running functions faster

C By storing data permanently

D By encrypting messages automatically

Pub/Sub queues messages so they are not lost if the Cloud Function is busy or temporarily unavailable.

What must you do to the message data in a Cloud Function triggered by Pub/Sub?

A Decode it from base64

B Encrypt it

C Convert it to JSON automatically

D Ignore it

Pub/Sub message data is base64-encoded, so the function must decode it to use the original content.

Which of the following is NOT a benefit of using Pub/Sub with Cloud Functions?

A Decoupling system components

B Automatic scaling of functions

C Direct synchronous calls between services

D Handling bursts of messages smoothly

Pub/Sub enables asynchronous communication, not direct synchronous calls.

What is a Pub/Sub topic?

A A channel where messages are sent and received

B A database for storing messages

C A Cloud Function configuration

D A user account in GCP

A Pub/Sub topic is like a channel or mailbox where messages are published and from which subscribers receive messages.

Explain how Cloud Functions and Pub/Sub work together to process messages.

Think about how messages flow from sender to function.

Describe the benefits of using Pub/Sub with Cloud Functions in a cloud application.

Consider system design and handling load.

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