Building Google Cloud Platform Solutions

Duplicate messages

Cloud Pub/Sub guarantees at-least-once delivery of each message submitted to a topic. It does not, however, guarantee exactly-once delivery. Though typically rare, duplicate messages absolutely will occur, and so any service that consumes messages should be designed to handle duplicate messages. For example, suppose a customer loyalty service subscribes to a customer-orders topic and increases a customer's reward points whenever that customer places an order. The subscriber will eventually receive duplicate order-created messages for a customer order, and should be designed not to double-credit that user.

One common method for handling duplicate messages is to ensure any operations resulting from processing a message are ...