# TcpClientOptions

**Kind:** Interface

**Source:** [`packages/microservices/interfaces/client-metadata.interface.ts`](https://github.com/nestjs/nest/blob/master/packages/microservices/interfaces/client-metadata.interface.ts#L37)

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

`TcpClientOptions` configures a microservice client that communicates over the TCP transport. It defines the target host and port, optional serialization behavior, TLS settings, socket implementation, and the maximum inbound buffer size.

## Properties

| Property | Type |
|---|---|
| `transport` | `Transport.TCP` |
| `options` | `{ host?: string; port?: number; serializer?: Serializer; deserializer?: Deserializer; tlsOptions?: ConnectionOptions; socketClass?: Type<TcpSocket>; maxBufferSize?: number; }` |

## Diagram

```mermaid
graph LR
  Client[TCP Microservice Client] --> Config[TcpClientOptions]
  Config --> Transport[transport: Transport.TCP]
  Config --> Connection[options]
  Connection --> Host[host]
  Connection --> Port[port]
  Connection --> Serialization[serializer / deserializer]
  Connection --> TLS[tlsOptions]
  Connection --> Socket[socketClass]
  Connection --> Buffer[maxBufferSize]
  Client --> Server[TCP Server]
```

## Usage

```ts
import { ClientProxyFactory, Transport } from '@nestjs/microservices';
import type { TcpClientOptions } from '@nestjs/microservices';

const tcpOptions: TcpClientOptions = {
  transport: Transport.TCP,
  options: {
    host: '127.0.0.1',
    port: 3001,
    maxBufferSize: 1024 * 1024,
  },
};

const client = ClientProxyFactory.create(tcpOptions);

client.send({ cmd: 'get_user' }, { id: '123' }).subscribe(response => {
  console.log(response);
});
```

## AI Coding Instructions

- Always set `transport` to `Transport.TCP`; this interface is only valid for TCP client configurations.
- Provide `host` and `port` values that match the TCP microservice server configuration.
- Use matching `serializer` and `deserializer` implementations on both client and server when customizing message encoding.
- Configure `tlsOptions` only when connecting to a TLS-enabled TCP server, using compatible Node.js `ConnectionOptions`.
- Set `maxBufferSize` appropriately for expected payload sizes to prevent oversized TCP message buffering.

## How it works

`TcpClientOptions` is the public TypeScript configuration interface for a TCP microservice client. Its `transport` field is required and must be `Transport.TCP`; it is one member of the `ClientOptions` union. [client-metadata.interface.ts:17-24](packages/microservices/interfaces/client-metadata.interface.ts#L17-L24) [client-metadata.interface.ts:34-38](packages/microservices/interfaces/client-metadata.interface.ts#L34-L38)

- `options` is optional. When present, it may contain `host`, `port`, `serializer`, `deserializer`, `tlsOptions`, `socketClass`, and `maxBufferSize`. [client-metadata.interface.ts:37-51](packages/microservices/interfaces/client-metadata.interface.ts#L37-L51)
- `ClientProxyFactory.create()` defaults a missing `options` object to `{}` and, for TCP/default transport selection, constructs `ClientTCP` with those options. [client-proxy-factory.ts:42-49](packages/microservices/client/client-proxy-factory.ts#L42-L49) [client-proxy-factory.ts:70-73](packages/microservices/client/client-proxy-factory.ts#L70-L73)
- `host` and `port` select the connection endpoint. If absent, `ClientTCP` uses `localhost` and `3000`, respectively. [client-tcp.ts:30-33](packages/microservices/client/client-tcp.ts#L30-L33) [constants.ts:3-4](packages/microservices/constants.ts#L3-L4)
- `serializer` must implement `serialize(value, options?)`; `deserializer` must implement `deserialize(value, options?)`, which may return a value or promise. If either is absent or falsy, the client initializes an identity serializer or incoming-response deserializer, respectively. [serializer.interface.ts:10-12](packages/microservices/interfaces/serializer.interface.ts#L10-L12) [deserializer.interface.ts:10-15](packages/microservices/interfaces/deserializer.interface.ts#L10-L15) [client-proxy.ts:208-231](packages/microservices/client/client-proxy.ts#L208-L231)
- `tlsOptions` has Node TLS `ConnectionOptions` type. When it is set, the client creates its underlying socket with `tls.connect`, merging these options while overriding its `port` and `host`; otherwise it creates a `net.Socket` and explicitly connects it during `connect()`. [client-metadata.interface.ts:42-45](packages/microservices/interfaces/client-metadata.interface.ts#L42-L45) [client-tcp.ts:65-68](packages/microservices/client/client-tcp.ts#L65-L68) [client-tcp.ts:99-121](packages/microservices/client/client-tcp.ts#L99-L121)
- `socketClass` is a `TcpSocket` constructor. It defaults to `JsonSocket`. [client-metadata.interface.ts:44-45](packages/microservices/interfaces/client-metadata.interface.ts#L44-L45) [client-tcp.ts:32-36](packages/microservices/client/client-tcp.ts#L32-L36)
- `maxBufferSize` is passed only when `socketClass` is exactly the built-in `JsonSocket`; custom socket classes receive no such constructor option. [client-tcp.ts:113-121](packages/microservices/client/client-tcp.ts#L113-L121) For `JsonSocket`, an omitted value defaults to `(512 * 1024 * 1024) / 4` characters. [json-socket.ts:7-11](packages/microservices/helpers/json-socket.ts#L7-L11) [json-socket.ts:21-24](packages/microservices/helpers/json-socket.ts#L21-L24)
- With `JsonSocket`, received data accumulates in a string buffer. If its length exceeds `maxBufferSize`, the buffer is cleared and `MaxPacketLengthExceededException` is thrown; the base socket catches this, emits an error, and ends the socket. [json-socket.ts:30-43](packages/microservices/helpers/json-socket.ts#L30-L43) [tcp-socket.ts:54-60](packages/microservices/helpers/tcp-socket.ts#L54-L60)
- The interface itself declares no runtime validation for option values. The visible TCP constructor reads the fields and initializes serializer/deserializer without checks. [client-tcp.ts:30-40](packages/microservices/client/client-tcp.ts#L30-L40)