# KafkaReplyPartitionAssigner

**Kind:** Class

**Source:** [`packages/microservices/helpers/kafka-reply-partition-assigner.ts`](https://github.com/nestjs/nest/blob/master/packages/microservices/helpers/kafka-reply-partition-assigner.ts#L14)

**Part of:** [Microservices](subsystem-packages-microservices)

`KafkaReplyPartitionAssigner` is a Kafka consumer-group partition assigner used by NestJS Kafka clients for reply topics. It creates stable reply-partition assignments based on client identity, helping ensure request/reply responses are consumed by the client instance that initiated the request.

## Methods

| Method | Signature | Returns |
|---|---|---|
| `assign` | `assign(group: { members: GroupMember[]; topics: string[]; })` | `Promise<GroupMemberAssignment[]>` |
| `protocol` | `protocol(subscription: { topics: string[]; userData: Buffer; })` | `GroupState` |
| `getPreviousAssignment` | `getPreviousAssignment()` | `void` |
| `decodeMember` | `decodeMember(member: GroupMember)` | `void` |

## Properties

| Property | Type |
|---|---|
| `name` | `any` |
| `version` | `any` |

## Diagram

```mermaid
graph LR
  Client[Kafka client instance] --> Assigner[KafkaReplyPartitionAssigner]
  Assigner --> Protocol[protocol()]
  Assigner --> Decode[decodeMember()]
  Decode --> Previous[getPreviousAssignment()]
  Previous --> Assign[assign()]
  Assign --> Group[Kafka consumer group assignments]
  Group --> Replies[Reply topic partitions]
```

## Usage

```ts
import { Kafka } from 'kafkajs';
import { KafkaReplyPartitionAssigner } from '@nestjs/microservices/helpers/kafka-reply-partition-assigner';

const kafka = new Kafka({
  clientId: 'orders-client',
  brokers: ['localhost:9092'],
});

const replyAssigner = new KafkaReplyPartitionAssigner('orders-client');

const consumer = kafka.consumer({
  groupId: 'orders-client-replies',
  partitionAssigners: [replyAssigner],
});

await consumer.connect();
await consumer.subscribe({
  topic: 'orders.reply',
  fromBeginning: false,
});

await consumer.run({
  eachMessage: async ({ message }) => {
    console.log('Received reply:', message.value?.toString());
  },
});
```

## AI Coding Instructions

- Configure this assigner on Kafka consumers responsible for request/reply response topics, not ordinary event-consumer groups.
- Use a stable, unique client ID for each logical Kafka client; changing it can alter reply partition ownership.
- Preserve assignment metadata compatibility when modifying `protocol()` or `decodeMember()`, since group members exchange this data during rebalances.
- Keep `assign()` deterministic: members and partitions should be consistently ordered to avoid unnecessary Kafka group rebalances.
- Prefer integrating through the NestJS Kafka client configuration rather than manually invoking `assign()` or `getPreviousAssignment()`.

## Relationships

- IMPORTS → `loadPackage`
- IMPORTS → `isUndefined`