j4flmao Event-Driven Architecture (EDA)
1. Skill Context
Focus: Decoupling microservices, asynchronous communication, scalable systems, and message brokers (Kafka, RabbitMQ, AWS SQS/SNS). Triggers: architecture, event-driven, eda, async, pub-sub, messages, outbox-pattern.
2. Core Concepts
Traditional microservices communicate via synchronous REST/gRPC. If Service A calls Service B, and Service B is down, Service A fails (tight coupling). In Event-Driven Architecture, Service A simply emits an event ("Order Created") to an Event Broker. It does not care who listens to it, nor if they are online. Service B (Billing) and Service C (Inventory) consume the event independently.
Events vs. Commands
- Command: "Create an Order". Directed at a specific service. Expected to fail or succeed. (Synchronous or Queue).
- Event: "Order Was Created". A historical fact. Directed at no one in particular. Cannot be rejected because it already happened. (Pub/Sub).
3. The Dual-Write Problem & The Outbox Pattern
The Anti-pattern:
// Extremely dangerous! What if the DB commits, but Kafka crashes?
await db.execute("INSERT INTO orders (id) VALUES (1)");
await kafka.send("OrderCreated", { id: 1 });
The Solution (Transactional Outbox):
Instead of sending the event directly to Kafka, you write the event into an outbox table in the same database transaction as the order.
BEGIN TRANSACTION;
INSERT INTO orders (id) VALUES (1);
INSERT INTO outbox (event_type, payload) VALUES ('OrderCreated', '{"id":1}');
COMMIT;
A separate background process (e.g., Debezium / Change Data Capture) continuously reads the outbox table and pushes the messages to Kafka. This guarantees 100% at-least-once delivery.
4. Architectural Hazards
- Event Pinball: Service A fires Event X. Service B listens to X and fires Y. Service C listens to Y and fires Z. When a bug occurs, tracing the "pinball" across 10 services is impossible without strict Distributed Tracing (OpenTelemetry, W3C Trace Context).
- Eventual Consistency: The UI will say "Order Successful", but the user's invoice might not appear in the billing tab for 3 seconds. The Frontend must be designed to handle eventual consistency (Polling, WebSockets, or Optimistic UI).
5. References
references/cqrs-event-sourcing.md— Deep dive into CQRS and Event Sourcing.