1import { Kafka } from 'kafkajs';
2 
3const kafka = new Kafka({
4  clientId: 'order-service',
Multiple brokers = high availability. One broker down = others continue
5  brokers: ['kafka-1:9092', 'kafka-2:9092', 'kafka-3:9092'],
6});
7 
8// Producer: publish events
9const producer = kafka.producer();
10await producer.connect();
11 
12await producer.send({
13  topic: 'order-events',
14  messages: [{
Partition key: all events for a user go to the same partition = ordered per user
15    key: order.userId,              // Partition key — same user's orders → same partition (ordered)
16    value: JSON.stringify({
17      eventType: 'ORDER_PLACED',
18      orderId: order.id,
19      userId: order.userId,
20      totalCents: order.totalCents,
21      timestamp: new Date().toISOString(),
22    }),
23  }],
24});
25 
26// Consumer: each service has its own consumer group
Consumer group ID: all instances of the same service share one group. Each event processed once per group
27const consumer = kafka.consumer({ groupId: 'inventory-service' });
28await consumer.connect();
29await consumer.subscribe({ topic: 'order-events', fromBeginning: false });
30 
31await consumer.run({
32  eachMessage: async ({ message }) => {
Design for idempotency: network issues can cause the same message to be delivered twice
33    const event = JSON.parse(message.value!.toString());
34    if (event.eventType === 'ORDER_PLACED') {
35      await inventoryService.reserveItems(event.orderId);
36      // Kafka auto-commits offset on success — won't reprocess
37    }
38  },
39});
40 
41// Multiple services consume the SAME topic independently:
42// inventory-service (groupId: 'inventory-service')  → gets all events
43// email-service     (groupId: 'email-service')       → gets all events
44// fraud-service     (groupId: 'fraud-service')       → gets all events