order-service/       -> owns orders table, publishes OrderCreated events
inventory-service/   -> owns inventory table, subscribes to OrderCreated
payment-service/     -> owns payments table, handles payment processing
notification-service -> stateless, subscribes to events, sends emails/SMS

Event-Driven Communication

interface DomainEvent {
  eventId: string;
  eventType: string;
  aggregateId: string;
  timestamp: string;
  version: number;
  payload: Record<string, unknown>;
}

const orderCreatedEvent: DomainEvent = {
  eventId: crypto.randomUUID(),
  eventType: "order.created",
  aggregateId: orderId,
  timestamp: new Date().toISOString(),
  version: 1,
  payload: { customerId, items, totalAmount },
};

await broker.publish("orders", orderCreatedEvent);

async function handleOrderCreated(event: DomainEvent) {
  const { items } = event.payload as OrderPayload;

  for (const item of items) {
    await db.inventory.update({
      where: { productId: item.productId },
      data: { quantity: { decrement: item.quantity } },
    });
  }

  await markEventProcessed(event.eventId);
}

Use idempotency keys (eventId) to handle duplicate deliveries safely.

Saga Pattern (Orchestration)

class OrderSaga {
  private steps: SagaStep[] = [
    {
      name: "reserveInventory",
      execute: (ctx) => inventoryService.reserve(ctx.items),
      compensate: (ctx) => inventoryService.release(ctx.items),
    },
    {
      name: "processPayment",
      execute: (ctx) => paymentService.charge(ctx.customerId, ctx.amount),
      compensate: (ctx) => paymentService.refund(ctx.paymentId),
    },
    {
      name: "confirmOrder",
      execute: (ctx) => orderService.confirm(ctx.orderId),
      compensate: (ctx) => orderService.cancel(ctx.orderId),
    },
  ];

  async run(context: SagaContext): Promise<void> {
    const completed: SagaStep[] = [];

    for (const step of this.steps) {
      try {
        const result = await step.execute(context);
        Object.assign(context, result);
        completed.push(step);
      } catch (error) {
        for (const s of completed.reverse()) {
          await s.compensate(context);
        }
        throw new SagaFailedError(step.name, error);
      }
    }
  }
}

API Gateway Pattern

# Kong or similar gateway config
services:
  - name: orders
    url: http://order-service:3000
    routes:
      - paths: ["/api/v1/orders"]
        methods: [GET, POST]
    plugins:
      - name: rate-limiting
        config:
          minute: 100
      - name: jwt
      - name: correlation-id

  - name: users
    url: http://user-service:3000
    routes:
      - paths: ["/api/v1/users"]
    plugins:
      - name: rate-limiting
        config:
          minute: 200

Health Check Pattern

app.get("/health", async (req, res) => {
  const checks = {
    database: await checkDatabase(),
    cache: await checkRedis(),
    broker: await checkMessageBroker(),
  };

  const healthy = Object.values(checks).every(c => c.status === "up");

  res.status(healthy ? 200 : 503).json({
    status: healthy ? "healthy" : "degraded",
    checks,
    version: process.env.APP_VERSION,
    uptime: process.uptime(),
  });
});

Anti-Patterns

Sharing a database between services (tight coupling)
Synchronous HTTP chains across multiple services (cascading failures)
Building a distributed monolith (services cannot deploy independently)
Missing circuit breakers on inter-service calls
Not implementing idempotency for event handlers
Using distributed transactions instead of sagas

Checklist

Each service owns its own data store
Services communicate via events for async workflows
Saga pattern used for multi-service transactions with compensation
Circuit breakers protect against cascading failures
API gateway handles routing, rate limiting, and authentication
Health check endpoints report dependency status
Event handlers are idempotent (safe to process duplicates)
Services can be deployed and scaled independently