Message Ordering & Deduplication

By default, Google Cloud Pub/Sub delivers messages in a best-effort order and guarantees at-least-once delivery, which means messages may arrive out of order and duplicates are possible. For many use cases this is acceptable, but some applications require strict ordering or exactly-once processing. This lesson covers Pub/Sub's ordering and deduplication features and how to use them effectively.

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Message Ordering

The Problem

Without ordering, Pub/Sub may deliver messages out of the order they were published. This happens because Pub/Sub distributes messages across multiple servers for scalability. Consider a scenario where you publish three status updates for an order:

1. Order created
2. Payment processed
3. Order shipped

Without ordering guarantees, a subscriber might receive "Order shipped" before "Payment processed", leading to incorrect state transitions.

Ordering Keys

Pub/Sub supports ordered delivery using ordering keys. When you publish messages with the same ordering key, Pub/Sub guarantees that the subscriber receives them in the order they were published.