Pull vs push subscriptions in GCP - CLI Comparison

Introduction

When you want to receive messages from a topic, you can choose how to get them. Pull subscriptions let you ask for messages when you are ready. Push subscriptions send messages to your app automatically. This helps you control how your app handles incoming data.

When your app wants to control when it processes messages to avoid overload.

When you want messages sent directly to your web server without asking.

When you have a service that can handle incoming HTTP requests instantly.

When you want to pull messages in batches for efficient processing.

When you want to avoid managing a web endpoint and prefer to fetch messages manually.

Commands

Create a pull subscription named 'my-pull-sub' for the topic 'my-topic'. The ack-deadline is 20 seconds, meaning you have 20 seconds to acknowledge a message after pulling it.

Terminal

gcloud pubsub subscriptions create my-pull-sub --topic=my-topic --ack-deadline=20

Expected Output

Created subscription [projects/your-project/subscriptions/my-pull-sub].

→

--ack-deadline - Sets how long you have to acknowledge a pulled message.

Create a push subscription named 'my-push-sub' for the topic 'my-topic'. Messages will be sent automatically to the specified HTTPS endpoint.

Terminal

gcloud pubsub subscriptions create my-push-sub --topic=my-topic --push-endpoint=https://example.com/push-handler

Expected Output

Created subscription [projects/your-project/subscriptions/my-push-sub].

→

--push-endpoint - Specifies the URL where messages will be pushed.

Pull up to 5 messages from the 'my-pull-sub' subscription and automatically acknowledge them so they are removed from the queue.

Terminal

gcloud pubsub subscriptions pull my-pull-sub --limit=5 --auto-ack

Expected Output

Received message: "Hello world 1"

Received message: "Hello world 2"

Received message: "Hello world 3"

Received message: "Hello world 4"

Received message: "Hello world 5"

→

--limit - Limits the number of messages pulled.

→

--auto-ack - Automatically acknowledges messages after pulling.

Show details about the push subscription 'my-push-sub', including its push endpoint and configuration.

Terminal

gcloud pubsub subscriptions describe my-push-sub

Expected Output

name: projects/your-project/subscriptions/my-push-sub

pushConfig:

  pushEndpoint: https://example.com/push-handler

ackDeadlineSeconds: 10

topic: projects/your-project/topics/my-topic

Key Concept

If you remember nothing else from this pattern, remember: pull subscriptions let your app ask for messages when ready, while push subscriptions send messages automatically to your app's endpoint.

Common Mistakes

Creating a push subscription without a valid HTTPS endpoint.

Messages cannot be delivered if the endpoint is missing or not reachable, causing message loss or retries.

Always provide a reachable HTTPS URL that can accept POST requests for push subscriptions.

Not acknowledging pulled messages in pull subscriptions.

Messages remain unacknowledged and will be redelivered, causing duplicate processing.

Use --auto-ack flag or explicitly acknowledge messages after processing.

Setting too short ack-deadline for pull subscriptions.

Your app may not finish processing before the deadline, causing message redelivery.

Set ack-deadline long enough for your processing time, or extend it dynamically.

Summary

Use 'gcloud pubsub subscriptions create' with --push-endpoint to make a push subscription that sends messages automatically.

Use 'gcloud pubsub subscriptions create' without --push-endpoint to make a pull subscription that waits for your app to ask for messages.

Pull messages with 'gcloud pubsub subscriptions pull' and acknowledge them to remove from the queue.

Check subscription details with 'gcloud pubsub subscriptions describe' to verify configuration.

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