What is Pull Subscription in Google Cloud Pub/Sub

A pull subscription in Google Cloud Pub/Sub is a way for a subscriber to request messages from a topic when ready, instead of receiving them automatically. The subscriber pulls messages by calling the service, allowing control over when and how many messages to process.

How It Works

Imagine you have a mailbox where letters arrive. With a pull subscription, you decide when to check the mailbox and take out the letters. The mail service waits until you ask before giving you any mail.

In Google Cloud Pub/Sub, a pull subscription means your application asks the service for messages from a topic. The service holds messages until your app pulls them. This lets your app control the pace of message processing and handle messages when it is ready.

This is different from a push subscription, where messages are sent automatically to your app's endpoint without asking.

Example

This example shows how to pull messages from a Pub/Sub subscription using Python. It connects to the subscription, requests messages, and acknowledges them after processing.

python

from google.cloud import pubsub_v1

project_id = "your-project-id"
subscription_id = "your-subscription-id"

subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path(project_id, subscription_id)

response = subscriber.pull(
    request={"subscription": subscription_path, "max_messages": 5}
)

for received_message in response.received_messages:
    print(f"Received message: {received_message.message.data.decode('utf-8')}")
    subscriber.acknowledge(
        request={"subscription": subscription_path, "ack_ids": [received_message.ack_id]}
    )

Output

Received message: Hello, world!
Received message: Another message
...

When to Use

Use pull subscriptions when your application needs control over when it receives messages. This is helpful if your app processes messages in batches or has variable processing speed.

Real-world cases include:

• Apps that poll for messages during specific times to reduce resource use.
• Systems that want to control message flow to avoid overload.
• Debugging or testing scenarios where manual message retrieval is preferred.

Key Points

• Pull subscriptions require the subscriber to request messages.
• They provide control over message processing timing and volume.
• Messages must be acknowledged to avoid redelivery.
• Useful for batch processing and flow control.

Key Takeaways

Pull subscriptions let your app request messages when ready, giving processing control.

You must acknowledge messages after pulling to prevent duplicates.

Ideal for batch processing or apps with variable message handling speed.

Pull subscriptions differ from push, where messages are sent automatically.

Use pull when you want to control resource use and message flow.