Messaging & Streaming
Pub/Sub - global, serverless messaging for decoupling services. A publisher sends messages to a topic; subscribers pull or get pushed from a subscription. Asynchronous, at-least-once delivery, autoscaling. Pick when you see "decouple", "buffer", "fan-out", "ingest events", or "async between microservices".
Pub/Sub is a pipe, not a processor or a database. The classic pipeline is Pub/Sub → Dataflow → BigQuery: Pub/Sub ingests and buffers, then hands off. Don't pick Pub/Sub to "transform data" - that's Dataflow, covered under Data Processing.
Messages are retained for 7 days by default, configurable up to a maximum of 31 days, for unacknowledged/replay. Once acknowledged (and past retention) they're gone. Pub/Sub is a pipe, not a database - use it to move events, not to store them long-term.