Topics and subscriptions in GCP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a topic in Google Cloud Pub/Sub?

A topic is a named resource to which messages are sent by publishers. It acts like a mailbox where messages are stored until subscribers receive them.

beginner

What is a subscription in Google Cloud Pub/Sub?

A subscription is a named resource representing the stream of messages from a single, specific topic, to be delivered to the subscriber.

intermediate

How do topics and subscriptions work together?

Publishers send messages to topics. Subscribers create subscriptions to those topics to receive messages. Each subscription gets its own copy of every message sent to the topic.

intermediate

What happens if a subscriber does not acknowledge a message in Pub/Sub?

The message remains unacknowledged and will be redelivered after a deadline, ensuring reliable delivery until acknowledged.

beginner

Can a topic have multiple subscriptions in Google Cloud Pub/Sub?

Yes, a topic can have multiple subscriptions. Each subscription independently receives all messages published to the topic.

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

A It deletes messages after delivery

B It delivers messages to subscribers

C It processes messages before delivery

D It stores messages sent by publishers

A topic stores messages sent by publishers until subscribers receive them.

What does a subscription represent in Pub/Sub?

A A stream of messages from a topic to a subscriber

B A message queue for a topic

C A publisher sending messages

D A storage bucket for messages

A subscription represents the stream of messages from a topic to a subscriber.

If a subscriber does not acknowledge a message, what happens?

A The message is redelivered after a deadline

B The message is lost

C The message is deleted immediately

D The subscription is deleted

Unacknowledged messages are redelivered after a deadline to ensure reliable delivery.

Can one topic have multiple subscriptions?

A No, only one subscription per topic

B No, subscriptions belong to different topics

C Yes, each subscription receives all messages

D Yes, but subscriptions share messages

A topic can have multiple subscriptions, each receiving all messages independently.

Who sends messages to a topic in Pub/Sub?

A Subscribers

B Publishers

C Subscriptions

D Cloud Storage

Publishers send messages to topics.

Explain how topics and subscriptions work together in Google Cloud Pub/Sub.

Think of topics as mailboxes and subscriptions as mailboxes' readers.

Describe what happens when a message is not acknowledged by a subscriber.

Consider how Pub/Sub guarantees message delivery.

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