Push vs Pull Subscriptions

Google Cloud Pub/Sub supports two primary delivery mechanisms for messages: pull and push subscriptions. Choosing the right delivery mode is a critical architectural decision that affects latency, scalability, cost, and operational complexity. This lesson covers both modes in depth, comparing their characteristics and providing guidance on when to use each.

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Pull Subscriptions

With a pull subscription, the subscriber actively requests messages from Pub/Sub. The subscriber calls the pull API (or uses streaming pull) and Pub/Sub returns available messages. The subscriber processes each message and sends an acknowledgement.

How Pull Works

1. The subscriber opens a connection to Pub/Sub
2. The subscriber calls Pull or StreamingPull to request messages
3. Pub/Sub returns available messages (up to the configured limit)
4. The subscriber processes each message
5. The subscriber sends an Acknowledge request for each successfully processed message
6. If the subscriber does not acknowledge within the deadline, Pub/Sub redelivers the message

Streaming Pull vs Unary Pull