SNS

Amazon SNS is AWS’s publish/subscribe service. A publisher sends a message to a topic, and SNS pushes a copy to every subscriber. The publisher knows nothing about who is listening, which is what lets a system add a new consumer without changing the service that produces the event.

Start with SNS topics for the mechanics: subscriber types, message filtering, standard versus FIFO topics, and the quotas that shape a design.

Two pages cover the patterns built on top. SNS and fan-out architecture explains why each subscriber should usually sit behind its own queue rather than receiving from the topic directly. SNS and the outbox pattern explains why publishing to SNS does not, by itself, make a database write and an event atomic, and when a service can safely do without an outbox.