Topics and subscriptions in GCP - Mini Project: Build & Apply

Create and Configure Pub/Sub Topics and Subscriptions in GCP

📖 Scenario: You are setting up a messaging system in Google Cloud Platform (GCP) using Pub/Sub. This system will allow different parts of your application to communicate by sending and receiving messages asynchronously.First, you will create a topic where messages can be published. Then, you will create a subscription to that topic so that messages can be received and processed.

🎯 Goal: Build a Pub/Sub topic named news-updates and a subscription named news-subscriber that listens to this topic.

📋 What You'll Learn

Create a Pub/Sub topic called news-updates.

Create a subscription called news-subscriber for the news-updates topic.

Set the subscription's acknowledgment deadline to 20 seconds.

Use the Google Cloud SDK gcloud commands in the correct order.

💡 Why This Matters

🌍 Real World

Messaging systems in cloud applications often use Pub/Sub to decouple components and handle asynchronous communication.

💼 Career

Cloud engineers and developers use Pub/Sub topics and subscriptions to build scalable, event-driven architectures.

1

Create the Pub/Sub topic

Use the gcloud pubsub topics create command to create a topic named news-updates.

GCP

# Your command here to create the topic news-updates

Hint

Remember the command format: gcloud pubsub topics create [TOPIC_NAME].

2

Create the subscription variable

Create a variable called SUBSCRIPTION_NAME and set it to news-subscriber.

GCP

gcloud pubsub topics create news-updates
# Define SUBSCRIPTION_NAME variable here

Hint

Use the syntax VARIABLE_NAME=value to create a variable in shell.

3

Create the subscription with acknowledgment deadline

Use the gcloud pubsub subscriptions create command to create a subscription named news-subscriber for the topic news-updates. Set the acknowledgment deadline to 20 seconds using the --ack-deadline flag.

GCP

gcloud pubsub topics create news-updates
SUBSCRIPTION_NAME=news-subscriber
# Create subscription with ack deadline 20 seconds

Hint

Use --ack-deadline=20 to set the acknowledgment deadline to 20 seconds.

4

Verify the subscription configuration

Use the gcloud pubsub subscriptions describe command with the subscription name news-subscriber to verify the subscription exists and has the correct acknowledgment deadline.

GCP

gcloud pubsub topics create news-updates
SUBSCRIPTION_NAME=news-subscriber
gcloud pubsub subscriptions create $SUBSCRIPTION_NAME --topic=news-updates --ack-deadline=20
# Describe the subscription to verify

Hint

This command shows the subscription details including the acknowledgment deadline.

Practice

1. What is the main role of a topic in Google Cloud Pub/Sub?

easy

A. To authenticate users

B. To receive messages from subscribers

C. To store messages permanently

D. To send messages to subscribers

Solution

1. Step 1: Understand the role of a topic

   A topic is where messages are sent or published by a publisher.

2. Step 2: Differentiate topic from subscription

   Subscriptions receive messages from topics, not the other way around.

3. Final Answer:

   To send messages to subscribers -> Option D

4. Quick Check:

   Topic = message sender [OK]

Hint: Topics send; subscriptions receive messages [OK]

Common Mistakes:

• Confusing topic as receiver instead of sender
• Thinking topics store messages permanently
• Mixing roles of topic and subscription

2. Which of the following is the correct way to create a subscription to a topic using gcloud CLI?

easy

A. gcloud pubsub subscriptions create my-sub --topic=my-topic

B. gcloud pubsub topics create my-sub --subscription=my-topic

C. gcloud pubsub subscriptions create my-topic --topic=my-sub

D. gcloud pubsub topics create my-topic --subscription=my-sub

Solution

1. Step 1: Identify correct command syntax

   The command to create a subscription uses subscriptions create with the subscription name and --topic flag for the topic.

2. Step 2: Verify gcloud pubsub subscriptions create my-sub --topic=my-topic matches syntax

   gcloud pubsub subscriptions create my-sub --topic=my-topic correctly uses gcloud pubsub subscriptions create my-sub --topic=my-topic.

3. Final Answer:

   gcloud pubsub subscriptions create my-sub --topic=my-topic -> Option A

4. Quick Check:

   Subscriptions create with --topic flag [OK]

Hint: Subscriptions created with 'subscriptions create' and --topic [OK]

Common Mistakes:

• Using topics create instead of subscriptions create
• Swapping topic and subscription names
• Using --subscription flag instead of --topic

3. Given the following code snippet, what will be printed if the subscription receives a message?

from google.cloud import pubsub_v1

subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path('my-project', 'my-sub')

def callback(message):
    print(f'Received: {message.data.decode()}')
    message.ack()

streaming_pull_future = subscriber.subscribe(subscription_path, callback=callback)
print('Listening for messages...')

medium

A. Only Listening for messages...

B. Only Received:

C. Listening for messages... followed by Received:

D. Error because callback is not called

Solution

1. Step 1: Understand code flow

   The code subscribes to a topic and sets a callback to print received messages and acknowledge them.

2. Step 2: Identify output behavior

   It prints 'Listening for messages...' immediately, then prints 'Received: ...' when a message arrives.

3. Final Answer:

   Listening for messages... followed by Received: <message content> -> Option C

4. Quick Check:

   Subscribe prints listening then message received [OK]

Hint: Callback prints message after initial listening print [OK]

Common Mistakes:

• Assuming callback runs before print
• Thinking no output until message arrives
• Ignoring message acknowledgment

4. You created a subscription but messages are not being received. Which of the following is the most likely cause?

medium

A. The subscription is not attached to the correct topic

B. The topic does not exist

C. The subscription name is too long

D. The messages are too large

Solution

1. Step 1: Check subscription-topic link

   If subscription is not linked to the right topic, it won't receive messages.

2. Step 2: Evaluate other options

   Topic existence is important but usually creation fails if topic missing; name length or message size usually cause errors, not silent no messages.

3. Final Answer:

   The subscription is not attached to the correct topic -> Option A

4. Quick Check:

   Subscription-topic mismatch blocks messages [OK]

Hint: Check subscription's topic link first if no messages [OK]

Common Mistakes:

• Ignoring subscription-topic association
• Assuming message size silently blocks delivery
• Thinking subscription name length causes no messages

5. You want to ensure that messages published to a topic are delivered to multiple independent services. Which setup should you use?

hard

A. Create multiple topics, one per service, and one subscription per topic

B. Create one topic and multiple subscriptions, one per service

C. Create one subscription and multiple topics linked to it

D. Create one topic and one subscription shared by all services

Solution

1. Step 1: Understand message fan-out

   One topic can send messages to many subscriptions, allowing multiple services to receive independently.

2. Step 2: Evaluate options

   Create one topic and multiple subscriptions, one per service uses one topic with multiple subscriptions, which is the recommended pattern for multiple independent consumers.

3. Final Answer:

   Create one topic and multiple subscriptions, one per service -> Option B

4. Quick Check:

   One topic, many subscriptions for multiple services [OK]

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

Common Mistakes:

• Creating multiple topics unnecessarily
• Using one subscription shared by all services
• Linking subscriptions to multiple topics