How do you implement a microservice communication pattern in Node.js?

Question

Accepted Answer

**Microservice communication** — services talk via HTTP, gRPC, or messages.

**HTTP (synchronous):**
```javascript
// Service A calls Service B
async function getOrderWithUser(orderId) {
    const order = await fetch(`http://order-service/orders/${orderId}`);
    const user = await fetch(`http://user-service/users/${order.userId}`);
    return { ...order, user };
}
```

**Message queue (asynchronous):**
```javascript
// Service A publishes event
await channel.publish("orders", "order.created", {
    orderId: 123,
    userId: 456,
});

// Service B consumes event
await channel.consume("orders", async (msg) => {
    const event = JSON.parse(msg.content);
    if (msg.fields.routingKey === "order.created") {
        await sendConfirmation(event.userId);
    }
    channel.ack(msg);
});
```

**Patterns:**
- **Sync** — HTTP/gRPC (simple, tight coupling).
- **Async** — message queue (decoupled, eventual consistency).
- **Event sourcing** — append-only event log.
- **Saga** — distributed transactions.

Node.js: How do you implement a microservice communication pattern in Node.js?