What does it mean for SNS to be a "fan-out" service?

SNS (Simple Notification Service) being described as a "fan-out" service refers to its ability to deliver a single message to multiple subscribers. This is a key feature of SNS, allowing one message published to an SNS topic to be automatically distributed to all subscribers of that topic, which can include AWS Lambda functions, HTTP/S endpoints, email addresses, or other supported protocols.

The fan-out pattern is particularly useful in scenarios where you want to broadcast information to multiple endpoints simultaneously without needing to send the same message multiple times. It streamlines communication and ensures that all interested parties receive the same information promptly.

The concept behind this is to reduce overhead and simplify the process of messaging across different services and environments. By utilizing SNS, developers can set up a one-to-many message distribution easily, making it a powerful tool for building decoupled, scalable applications.