Push vs Pull Subscription in GCP Pub/Sub: Key Differences and Usage

In Google Cloud Pub/Sub, a push subscription sends messages automatically to a specified endpoint, while a pull subscription requires the client to request messages. Push is good for real-time delivery, pull offers more control over message processing.

Quick Comparison

Here is a quick side-by-side comparison of push and pull subscriptions in Google Cloud Pub/Sub.

Factor	Push Subscription	Pull Subscription
Message Delivery	Automatically sends messages to an HTTP endpoint	Client explicitly requests messages from Pub/Sub
Client Control	Less control, messages pushed as they arrive	More control, client controls when and how many messages to pull
Use Case	Real-time event processing, webhooks	Batch processing, manual message handling
Acknowledgement	Client acknowledges via HTTP response	Client acknowledges via API call
Scaling	Endpoint must handle incoming traffic	Client controls polling rate and concurrency
Complexity	Simpler client, needs reliable endpoint	More complex client, handles polling and retries

Key Differences

Push subscriptions automatically send messages to a configured HTTP(S) endpoint as soon as they arrive. This means the server hosting the endpoint must be ready to receive and process messages in real time. The client acknowledges messages by returning a success HTTP status code. This approach is great for event-driven architectures where immediate processing is needed.

In contrast, pull subscriptions require the client to actively request messages from Pub/Sub. The client controls when to pull messages and how many to pull at once, allowing for flexible batch processing and retry logic. The client acknowledges messages explicitly through API calls after processing. This method suits workloads that need more control over message flow or cannot maintain a constantly available endpoint.

Overall, push is simpler for real-time delivery but depends on endpoint availability, while pull offers more control and reliability at the cost of client complexity.

Code Comparison

Example of pulling messages from a Pub/Sub subscription using Python.

python

from google.cloud import pubsub_v1
import time

subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path('your-project-id', 'your-subscription')

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

streaming_pull_future = subscriber.subscribe(subscription_path, callback=callback)
print('Listening for messages on {}'.format(subscription_path))

try:
    streaming_pull_future.result()
except KeyboardInterrupt:
    streaming_pull_future.cancel()

Output

Received message: Hello, Pub/Sub!
Received message: Another message

Push Subscription Equivalent

Example of a simple HTTP server in Python that receives push messages from Pub/Sub.

python

from flask import Flask, request, abort

app = Flask(__name__)

@app.route('/pubsub/push', methods=['POST'])
def pubsub_push():
    envelope = request.get_json()
    if not envelope:
        abort(400, 'No JSON payload received')
    if 'message' not in envelope:
        abort(400, 'No message field in JSON')
    pubsub_message = envelope['message']
    data = pubsub_message.get('data')
    if data:
        print(f'Received push message: {data}')
    return '', 200

if __name__ == '__main__':
    app.run(port=8080)

Output

Received push message: SGVsbG8sIFB1Yi9TdWIh

When to Use Which

Choose push subscriptions when you want messages delivered immediately to a web service or endpoint that can handle incoming HTTP requests reliably. This is ideal for real-time event processing and webhook-style integrations.

Choose pull subscriptions when you need more control over message processing, such as batching, retry logic, or when your client cannot maintain a constantly available endpoint. Pull is better for offline processing or complex workflows.

Key Takeaways

Push subscriptions send messages automatically to an HTTP endpoint for real-time delivery.

Pull subscriptions require clients to request messages, offering more control over processing.

Use push for simple, immediate event handling with reliable endpoints.

Use pull when you need flexible message control or batch processing.

Acknowledgement methods differ: HTTP response for push, API call for pull.