# MqttRecordSerializer

**Kind:** Class

**Source:** [`packages/microservices/serializers/mqtt-record.serializer.ts`](https://github.com/nestjs/nest/blob/master/packages/microservices/serializers/mqtt-record.serializer.ts#L5)

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

`MqttRecordSerializer` converts an `MqttRecord` into a JSON string suitable for MQTT transport. It preserves MQTT publish options such as `qos`, `retain`, and `dup`, while serializing object payloads into the record's `data` field.

**Implements:** `Serializer`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `serialize` | `serialize(packet: ReadPacket)` | `string` |

## Diagram

```mermaid
graph LR
  A[MqttRecord] --> B[MqttRecordSerializer.serialize]
  B --> C[Serialize object data when needed]
  C --> D[Merge MQTT publish options]
  D --> E[JSON string payload]
  E --> F[MQTT transport]
```

## Usage

```ts
import { MqttRecord } from '@nestjs/microservices';
import { MqttRecordSerializer } from '@nestjs/microservices/serializers/mqtt-record.serializer';

const record = new MqttRecord(
  { deviceId: 'sensor-42', temperature: 21.5 },
  {
    qos: 1,
    retain: true,
  },
);

const serializer = new MqttRecordSerializer();
const payload = serializer.serialize(record);

console.log(payload);
// {"data":"{\"deviceId\":\"sensor-42\",\"temperature\":21.5}","qos":1,"retain":true}
```

## AI Coding Instructions

- Pass an `MqttRecord` containing both the message payload (`data`) and MQTT-specific publish options.
- Object payloads are JSON-stringified before being embedded in the final serialized message; avoid passing circular object structures.
- Keep MQTT options compatible with the underlying MQTT client, including fields such as `qos`, `retain`, and `dup`.
- Use this serializer at the MQTT transport boundary; application handlers should typically work with deserialized message data instead of the serialized string.

## Relationships

- IMPORTS → `isObject`