Pull vs push subscriptions in GCP - Performance Comparison

Time Complexity: Pull vs push subscriptions

O(n)

Understanding Time Complexity

When using messaging services, it is important to understand how the number of messages affects processing time.

We want to know how the time to receive messages grows as more messages are sent.

Scenario Under Consideration

Analyze the time complexity of receiving messages using pull and push subscriptions.

// Pull subscription example
for (int i = 0; i < n; i++) {
  message = subscriber.pull();
  process(message);
}

// Push subscription example
// Messages are sent automatically to an endpoint as they arrive

This shows pulling messages one by one versus receiving messages pushed automatically.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: Pulling messages via API calls or receiving pushed messages via HTTP requests.
• How many times: Pull requires one API call per message; push receives messages as they arrive without repeated calls.

How Execution Grows With Input

Pull subscription time grows with the number of messages because each message requires a separate request.

Push subscription time grows more smoothly since messages arrive automatically without repeated requests.

Input Size (n)	Approx. API Calls/Operations
10	10 pull calls or 10 push deliveries
100	100 pull calls or 100 push deliveries
1000	1000 pull calls or 1000 push deliveries

Pattern observation: Pull requires one call per message, so calls grow linearly with messages; push delivers messages automatically, so no extra calls are needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to receive messages grows directly in proportion to the number of messages.

Common Mistake

[X] Wrong: "Pull subscriptions are faster because you control when to get messages."

[OK] Correct: Pull requires a separate request for each message, which adds overhead and grows with message count, making it slower as messages increase.

Interview Connect

Understanding how message delivery scales helps you design systems that handle load efficiently and choose the right subscription type.

Self-Check

"What if we batch pull requests to get multiple messages at once? How would the time complexity change?"

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