System Design: The Pub-Sub Abstraction

Publish-subscribe lets one publication feed independently managed subscriptions. The design problem is to support that fan-out without making a slow subscriber stop every other consumer.

What is a pub-sub system?

Publish-subscribe (pub-sub) sends messages through a named topic or similar routing abstraction. Publishers do not need to address each subscriber directly. Delivery time, retention, filtering, and retry guarantees depend on the subscription contract; subscribers need not receive a publication simultaneously.

As a conceptual example, map a social-media author to a publisher, a post to a message, and followers to subscribers. An actual feed may add ranking, filtering, offline storage, and authorization, so this mapping describes fan-out rather than a complete social platform.

Motivation