Architecture & System Design › Events & Integration · also in Queues & Async Processing

Event

A record that something happened.

Also known as: event, domain event, integration event

In integration, an Event is a immutable record that something happened: OrderPlaced {orderId, items, at} — past tense, broadcast to whoever cares, carrying the facts (not instructions). Producers emit without knowing consumers; consumers react independently. Time, ordering and schema discipline make events trustworthy; without them, event-driven systems decay into rumor networks.

OrderPlaced → inventory reserves, email sends, analytics records (each independent)

Good events are self-contained (consumers act without querying back), versioned (schemas evolve compatibly), ordered where causality matters (per-entity sequencing), and retained (replayable for new consumers and recovery).

The classic mistakes:

• Commands disguised as events. ProcessPayment NOW (imperative, one recipient) dressed as an event couples producer intent to consumer action. Events state facts; commands request actions (see event vs command).
• Anemic events (ids only). OrderChanged {id: 42} forces every consumer to query back — reintroducing coupling and thundering reads. Carry the facts consumers need.
• No schema versioning. Unversioned JSON evolving silently breaks consumers on every producer deploy. Version, compatibly evolve, register schemas.
• Assuming global order. Cross-entity ordering doesn’t exist in distributed logs; per-key order plus causal design covers real needs. Don’t promise what physics forbids.
• No retention/replay. Ephemeral events strand new consumers (no history) and break recovery. Retain windows that cover onboarding and repair.
• God events. Kitchen-sink payloads coupling every consumer to everything. Right-size: domain-meaningful facts, lean fields.
• Events as the only integration. Queries still need synchronous answers sometimes; events complement request/response, they don’t replace it.

How to design them: past-tense facts, self-contained, versioned, retained, ordered per entity. Events are the nervous system — keep the signals clean and every organ works.