Event-Driven Architecture

Overview

Patterns for decoupling services via events instead of synchronous calls: message queues, pub/sub, event sourcing, CQRS, sagas, and streaming. Scope of THIS skill = the architecture and its distributed-systems traps (delivery semantics, ordering, outbox, schema evolution). For job-queue mechanics — worker pools, scheduling/cron, retry/backoff internals, generic DLQ plumbing — see loom-background-jobs; cross-reference rather than duplicate.

The one law that governs everything below: networked delivery is at-least-once; therefore every consumer must be idempotent. Read the Delivery Semantics section first — most EDA bugs are a violation of it.

Code samples are TypeScript for concreteness; the patterns are language-agnostic. Assume crypto.randomUUID(), a broker client, and a datastore are in scope.

Delivery Semantics — "Exactly-once" Is (Mostly) a Lie

Two-Generals: across an unreliable network you cannot guarantee a message is delivered exactly once. You get to pick a failure mode: