GCP Pub Sub Messaging

Pub/Sub is a managed message broker. Publishers send messages to topics; subscribers receive from subscriptions. Messages are decoupled—publisher doesn't know who's listening. Used for event streaming (user signed up, product viewed, payment processed). At-least-once delivery: if subscriber crashes before acknowledging, message is redelivered. Ordering: within a partition, messages are ordered. Filtering: receive only messages matching criteria.