Event-Driven Architecture

Patterns for services that communicate via asynchronous events. Broker-agnostic at the conceptual level; defaults inline per use case (NATS for lightweight, Kafka for log-based replay, RabbitMQ for command queues). Protobuf schemas for events (protobuf-architect) so contracts get the same buf breaking discipline as gRPC. Outbox pattern is mandatory for any write that emits an event. Schemas, SQL, and tooling shapes in RECIPES.md; pinned brokers and client libs in STACK.md.

1. Event vs message vs command — the three shapes

• Events are immutable past-tense facts. OrderPlaced was placed; nothing changes that. Subscribers react however they want.
• Commands have one intended handler. Multiple handlers reacting to a command is almost always wrong — it's an event in disguise. Rename it.
• Naming: events are <Noun><PastVerb> (OrderPlaced, ShipmentDispatched); commands are <Verb><Noun> (PlaceOrder, SendShipment).
• Choose the shape per use case, not per technology. Both Kafka and RabbitMQ can carry either; the discipline is in the schema and contract.

2. Schema — Protobuf

Per protobuf-architect: events are .proto messages, code-generated, validated by protovalidate, and protected from breaking changes by buf breaking in CI.

Envelope contract (every event):

• event_id — UUID v7, sortable + unique. Subscribers dedupe on it.
• occurred_at — RFC 3339 timestamp. Replay tools sort by this.
• aggregate_id — the entity the event is about. Drives partitioning.
• schema_version — integer; bump on additive changes inside a topic version.

Payload discipline:

• Minimal. IDs and the few facts subscribers need — not the full aggregate state. Subscribers fetch via sql-architect repositories. Big payloads make schema evolution and replay expensive.
• Field numbers reserved on delete per protobuf-architect §3. Never reuse.
• One file per resource's events — orders/v1/events.proto holds every event the orders context emits.

Canonical schema in RECIPES §1.

3. Topic / subject naming

Hierarchical, snake_case, versioned. Pick a convention and enforce it.

<org>.<context>.<resource>.<version>.<event_name>

• Matches the Buf-style package path from protobuf-architect.
• Version is part of the topic name, not just the schema. New major version → new topic. Run in parallel until consumers migrate.
• Lowercase + dots (NATS, Pub/Sub) or lowercase + underscores (Kafka). Pick one for your broker and stick to it.
• Document the catalog somewhere queryable — Confluent Schema Registry, BSR, or a simple events.md in the repo.

Examples in RECIPES §5.

4. Outbox pattern — mandatory for "DB write + event emit"

The dual-write problem: an HTTP handler writes a row and publishes an event. If the DB commits but the broker rejects, the event is lost — silent inconsistency. If the broker accepts but the DB rolls back, subscribers process a phantom event.

Outbox fixes this with a single transactional write.

1. Handler BEGIN TX → INSERT INTO aggregate → INSERT INTO outbox → COMMIT.
2. Separate publisher reads unpublished rows, sends them to the broker, marks them published.

• Outbox is a regular table in the same DB as the aggregate. The write is atomic with the business write.
• Publisher is separate — a goroutine, a sidecar, a cron, or CDC (Debezium reading Postgres WAL). CDC is the most robust; goroutine is fine for small services.
• At-least-once delivery — the same event can be republished if the publisher crashes between PUBLISH and UPDATE. Consumers must be idempotent (§6).
• Outbox table grows — partition or purge published rows older than 7–30 days.
• Why mandatory: there is no working pattern that avoids both the lost-event and phantom-event failure modes without the outbox. Anything else (publish-before-commit, publish-after-commit) is broken under failure.

Schema + publisher shapes in RECIPES §2.

5. Ordering and partitioning

Event ordering is the single hardest part of event-driven systems. Order is per-key, not global.

• Order is preserved within a partition / subject, but not across partitions. Kafka partitions by message key; NATS via subject hierarchy; RabbitMQ via consistent-hash exchanges.
• Partition key is the aggregate ID. All events for order_id=abc land on the same partition, processed in order by one consumer. Different orders process in parallel.
• Globally-ordered events are a smell. If you "need" global order, you actually need a single consumer (and you've lost scaling), or you're modeling the domain wrong.
• Consumers process one partition at a time per instance. Concurrent processing within a partition breaks ordering. Most client libs handle this; verify your config.

6. Idempotency — consumer must dedupe

Brokers deliver at least once. Consumers see the same event more than once under network failure, restart, or rebalance.

• Dedupe by `event_id`. Each consumer keeps a small store (Redis with TTL, or a processed_events table) of recently-seen IDs. Reject duplicates.
• Idempotent side effects — design the handler so a duplicate is harmless: INSERT ... ON CONFLICT DO NOTHING, UPDATE ... WHERE version = ? (with optimistic concurrency).
• TTL on dedupe store — events older than the broker's retention can't be replayed anyway.
• Exactly-once illusion: idempotent consumer + at-least-once delivery = "effectively exactly-once" from the business perspective. Don't chase true exactly-once at the protocol level — far more expensive than just making consumers idempotent.

Concrete handler + dedupe table in RECIPES §3.

7. Dead-letter queues (DLQs)

Some events can't be processed — schema mismatch, downstream service down too long, business invariant violation. Don't let them block the partition.

• Every consumer has a DLQ. A topic / subject named <original>.dlq receives messages the consumer gave up on.
• Retry policy first, DLQ second. N attempts with exponential backoff (typical: 3 attempts), then DLQ.
• DLQs are monitored. Per observability-architect: a Prometheus counter <svc>_dlq_messages_total with an alert on any non-zero value. A DLQ that quietly fills is a silent outage.
• DLQ tooling — operator scripts to inspect, replay, or discard. Both audited.

Retry policy + tool CLI shape in RECIPES §4.

8. Backpressure

When the consumer can't keep up with the producer, the system needs to slow down — gracefully.

• Prefetch / consumer concurrency limits. Don't let one consumer instance buffer 10,000 in-flight messages.
• Lag-based autoscaling. Watch consumer-group lag; scale out the consumer pool when lag grows.
• Reject upstream when persistently overloaded — return 503 Service Unavailable with Retry-After per rest-api-architect §3. Better than building a backlog you can't drain.
• No unbounded queues in memory. A handler that reads from one topic and writes to another needs bounded size + a timeout.

Per-broker tuning knobs in RECIPES §6.

9. Schema evolution

Per protobuf-architect §4 — additive changes stay in the version; breaking changes go to a new vN.

• Additive: new optional field, new enum value, new event type on a new topic. Safe.
• Breaking: field removal, type change, semantic change. Bump to <topic>.v2; both run side-by-side until consumers migrate.
• `buf breaking` in CI catches accidental breakage in the proto files. Per protobuf-architect §8.
• Consumer compatibility tests — for each known consumer version, replay a sample event and assert it deserializes cleanly. Catches semantic-level breakage that's still wire-compatible.

10. Saga / orchestration / choreography

For multi-step workflows that span services. Two opposing patterns — comparison table in RECIPES § 6.

• Start with choreography — events flowing service-to-service, each subscriber reacts. Simpler.
• Promote to orchestration (Temporal, Camunda, AWS Step Functions) when the flow is genuinely complex — 5+ services, branching, retries, compensation.
• Compensation actions for partial failures: OrderCancelled undoes PaymentCaptured via RefundIssued. Domain-level, not technical rollback.
• Correlation ID propagated through every event in a saga — ties the chain together (observability-architect §5).

11. Broker selection — when each fits

The pattern in §1–10 works on any modern broker. Full strengths/best-for table in RECIPES § 7. Defaults:

• Lightweight: NATS JetStream — single binary, conf-driven, easy to operate.
• Log-based: Kafka — when replay, retention, and analytics consumers matter.
• Command queues / legacy fit: RabbitMQ — when integration with non-Kafka non-NATS systems forces the choice.
• Cloud-managed (SNS+SQS, Pub/Sub, Service Bus) — when ops cost matters more than feature parity.

Pick once per system; switching mid-flight is expensive.

12. Cross-skill ties

• protobuf-architect — event schemas + buf breaking discipline.
• grpc-architect — synchronous counterpart; when a call should be RPC vs event.
• sql-architect — outbox lives in your domain DB; same transactional discipline.
• ddd-architect §6 — domain events as the natural source of integration events.
• observability-architect — consumer lag, DLQ depth, processing latency are first-class metrics.
• grafana-architect — per-consumer-group lag dashboards; DLQ alerts.
• rest-api-architect §8 — the HTTP Idempotency-Key discipline is the same idea consumers need internally.