Pull vs push subscriptions in GCP - Quick Revision & Key Differences

Recall & Review

beginner

What is a pull subscription in Google Cloud Pub/Sub?

A pull subscription is where the subscriber application requests messages from the Pub/Sub service when it is ready to process them.

beginner

What is a push subscription in Google Cloud Pub/Sub?

A push subscription automatically sends messages to a subscriber's endpoint (like a web server) as soon as they arrive.

intermediate

Which subscription type requires the subscriber to manage message pulling timing?

Pull subscriptions require the subscriber to control when and how often it pulls messages from the service.

intermediate

What is a key advantage of push subscriptions?

Push subscriptions reduce latency because messages are sent immediately to the subscriber without waiting for a pull request.

intermediate

Name one challenge of using push subscriptions.

The subscriber endpoint must be publicly reachable and able to handle incoming HTTPS requests reliably.

In a pull subscription, who decides when to receive messages?

A The subscriber application

B The Pub/Sub service

C Google Cloud automatically

D The message publisher

In pull subscriptions, the subscriber application requests messages when it is ready.

Which subscription type sends messages automatically to a subscriber endpoint?

A Pull subscription

B Push subscription

C Both pull and push

D Neither pull nor push

Push subscriptions send messages automatically to the subscriber's endpoint.

What must a subscriber have to use push subscriptions?

A A publicly accessible HTTPS endpoint

B A private database

C A pull request scheduler

D A message queue

Push subscriptions require a publicly accessible HTTPS endpoint to receive messages.

Which subscription type can help reduce message delivery latency?

A Pull subscription

B Neither

C Push subscription

D Both equally

Push subscriptions send messages immediately, reducing latency.

If a subscriber wants to control when to process messages, which subscription should they use?

A Push subscription

B None

C Both push and pull

D Pull subscription

Pull subscriptions let the subscriber control message retrieval timing.

Explain the difference between pull and push subscriptions in Google Cloud Pub/Sub.

Think about who initiates message delivery.

What are the advantages and challenges of using push subscriptions?

Consider message delivery speed and subscriber setup.

Practice

1. What is the main difference between a pull subscription and a push subscription in Google Cloud Pub/Sub?

easy

A. Pull subscriptions send messages automatically; push subscriptions require your app to request messages.

B. Pull subscriptions are faster than push subscriptions in all cases.

C. Pull subscriptions only work with HTTP; push subscriptions only work with gRPC.

D. Pull subscriptions require your app to request messages; push subscriptions send messages automatically.

Solution

1. Step 1: Understand pull subscription behavior

   Pull subscriptions require the client application to ask for messages explicitly.

2. Step 2: Understand push subscription behavior

   Push subscriptions automatically send messages to the client via HTTP POST.

3. Final Answer:

   Pull subscriptions require your app to request messages; push subscriptions send messages automatically. -> Option D

4. Quick Check:

   Pull = request, Push = automatic [OK]

Hint: Pull means ask; push means receive automatically [OK]

Common Mistakes:

• Confusing which subscription type initiates message delivery
• Thinking push requires app to poll for messages
• Assuming pull subscriptions send messages automatically

2. Which of the following is the correct way to configure a push subscription endpoint URL in Google Cloud Pub/Sub?

easy

A. Set the subscription's pushConfig with a valid HTTPS URL where messages will be sent.

B. Set the subscription's pullConfig with the endpoint URL to pull messages.

C. Configure the subscription with a TCP socket address for push delivery.

D. Use the subscription's pushConfig with an FTP URL for message delivery.

Solution

1. Step 1: Identify correct pushConfig usage

   Push subscriptions require setting pushConfig with a valid HTTPS URL for message delivery.

2. Step 2: Eliminate incorrect options

   PullConfig does not exist; TCP and FTP are not supported protocols for push endpoints.

3. Final Answer:

   Set the subscription's pushConfig with a valid HTTPS URL where messages will be sent. -> Option A

4. Quick Check:

   PushConfig needs HTTPS URL [OK]

Hint: Push subscriptions need HTTPS URL in pushConfig [OK]

Common Mistakes:

• Using pullConfig instead of pushConfig
• Setting non-HTTPS URLs for push endpoints
• Confusing pull and push subscription settings

3. Consider this code snippet for a pull subscription client in Python using Google Cloud Pub/Sub:

from google.cloud import pubsub_v1
subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path('my-project', 'my-sub')
response = subscriber.pull(subscription=subscription_path, max_messages=1)
print(len(response.received_messages))

What will this code print if there is exactly one message available in the subscription?

medium

A. Error due to missing ack

B. 0

C. 1

D. None

Solution

1. Step 1: Understand pull method behavior

   The pull method requests messages; if one message is available, it returns it in received_messages.

2. Step 2: Check printed output

   len(response.received_messages) will be 1 because exactly one message is pulled.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   One message pulled means length is 1 [OK]

Hint: Pull returns messages list; length matches available messages [OK]

Common Mistakes:

• Assuming pull returns zero without ack
• Expecting error if ack is missing
• Confusing pull with push subscription behavior

4. You have a push subscription configured, but your app is not receiving any messages. Which of the following is the most likely cause?

medium

A. The push endpoint URL is incorrect or unreachable.

B. The subscription is set to pull instead of push.

C. The app forgot to call pull() method.

D. The topic has no messages published.

Solution

1. Step 1: Check push subscription delivery requirements

   Push subscriptions require a reachable, correct HTTPS endpoint to deliver messages.

2. Step 2: Analyze other options

   Pull vs push setting is fixed; pull() is not used in push; no messages means no delivery but question implies messages exist.

3. Final Answer:

   The push endpoint URL is incorrect or unreachable. -> Option A

4. Quick Check:

   Push needs correct reachable URL [OK]

Hint: Push needs reachable endpoint URL [OK]

Common Mistakes:

• Confusing pull and push subscription methods
• Assuming pull() is needed for push subscriptions
• Ignoring network or URL errors

5. You want to design a system that processes messages only when your app is ready and can control the rate of message processing. Which subscription type should you choose and why?

hard

A. Push subscription, because it retries sending messages until acknowledged.

B. Pull subscription, because your app controls when to request messages.

C. Push subscription, because messages are sent automatically and fast.

D. Pull subscription, because it automatically scales message delivery.

Solution

1. Step 1: Identify control needs

   The app wants to control when and how fast messages are processed.

2. Step 2: Match subscription type to control

   Pull subscriptions let the app ask for messages when ready, controlling rate and timing.

3. Final Answer:

   Pull subscription, because your app controls when to request messages. -> Option B

4. Quick Check:

   Pull = app controls message flow [OK]

Hint: Choose pull to control message processing timing [OK]

Common Mistakes:

• Choosing push for control over message rate
• Assuming push retries mean better control
• Thinking pull auto-scales delivery