event-driven-architect

Event sourcing in TypeScript stores state changes as immutable domain events rather than current state. Define an aggregate that applies events to mutate state, persist uncommitted events to an event store, and rebuild the aggregate by replaying events in order.

An in-memory event bus uses Node.js EventEmitter for pub/sub within a single process, while a Redis event bus publishes events across processes and services via Redis channels. Redis also supports streams for event replay and persistence.

CQRS separates command handling, which appends events through aggregates, from queries that read denormalized read models. Projectors subscribe to events and update the read model, keeping writes consistent and reads fast.

Event sourcing adds complexity and is unnecessary for simple CRUD applications without audit or replay requirements. It fits best when you need complete history, temporal queries, or decoupled reactions to state changes.

Wrap handlers in try/catch so one failing subscriber does not break others, and route failures to a dead letter queue for later inspection. Handlers should also be idempotent to tolerate duplicate event delivery.