TCP Sockets – API

Use Bun's native TCP API implement performance sensitive systems like database clients, game servers, or anything that needs to communicate over TCP (instead of HTTP). This is a low-level API intended for library authors and for advanced use cases.

Start a server

To start a TCP server with Bun.listen:

Bun.listen({
  hostname: "localhost",
  port: 8080,
  socket: {
    data(socket, data) {}, // message received from client
    open(socket) {}, // socket opened
    close(socket) {}, // socket closed
    drain(socket) {}, // socket ready for more data
    error(socket, error) {}, // error handler
  },
});

An API designed for speed

In Bun, a set of handlers are declared once per server instead of assigning callbacks to each socket, as with Node.js EventEmitters or the web-standard WebSocket API.

Bun.listen({
  hostname: "localhost",
  port: 8080,
  socket: {
    open(socket) {},
    data(socket, data) {},
    drain(socket) {},
    close(socket) {},
    error(socket, error) {},
  },
});

For performance-sensitive servers, assigning listeners to each socket can cause significant garbage collector pressure and increase memory usage. By contrast, Bun only allocates one handler function for each event and shares it among all sockets. This is a small optimization, but it adds up.

Contextual data can be attached to a socket in the open handler.

type SocketData = { sessionId: string };

Bun.listen<SocketData>({
  hostname: "localhost",
  port: 8080,
  socket: {
    data(socket, data) {
      socket.write(`${socket.data.sessionId}: ack`);
    },
    open(socket) {
      socket.data = { sessionId: "abcd" };
    },
  },
});

To enable TLS, pass a tls object containing keyFile and certFile properties.

Bun.listen({
  hostname: "localhost",
  port: 8080,
  socket: {
    data(socket, data) {},
  },
  tls: {
    certFile: "cert.pem",
    keyFile: "key.pem",
  },
});

The result of Bun.listen is a server that conforms to the TCPSocket instance.

const server = Bun.listen({
  /* config*/
});

// stop listening
// parameter determines whether active connections are closed
server.stop(true);

// let Bun process exit even if server is still listening
server.unref();

Create a connection

Use Bun.connect to connect to a TCP server. Specify the server to connect to with hostname and port. TCP clients can define the same set of handlers as Bun.listen, plus a couple client-specific handlers.

// The client
const socket = Bun.connect({
  hostname: "localhost",
  port: 8080,

  socket: {
    data(socket, data) {},
    open(socket) {},
    close(socket) {},
    drain(socket) {},
    error(socket, error) {},

    // client-specific handlers
    connectError(socket, error) {}, // connection failed
    end(socket) {}, // connection closed by server
    timeout(socket) {}, // connection timed out
  },
});

To require TLS, specify tls: true.

// The client
const socket = Bun.connect({
  // ... config
  tls: true,
});

Hot reloading

Both TCP servers and sockets can be hot reloaded with new handlers.

Server

Client

Server
const server = Bun.listen({ /* config */ })

// reloads handlers for all active server-side sockets
server.reload({
  socket:
    data(){
      // new 'data' handler
    }
  }
})

Client
const socket = Bun.connect({ /* config */ })
socket.reload({
  data(){
    // new 'data' handler
  }
})

Buffering

Currently, TCP sockets in Bun do not buffer data. For performance-sensitive code, it's important to consider buffering carefully. For example, this:

socket.write("h");
socket.write("e");
socket.write("l");
socket.write("l");
socket.write("o");

...performs significantly worse than this:

socket.write("hello");

To simplify this for now, consider using Bun's ArrayBufferSink with the {stream: true} option:

const sink = new ArrayBufferSink({ stream: true, highWaterMark: 1024 });

sink.write("h");
sink.write("e");
sink.write("l");
sink.write("l");
sink.write("o");

queueMicrotask(() => {
  var data = sink.flush();
  if (!socket.write(data)) {
    // put it back in the sink if the socket is full
    sink.write(data);
  }
});

Corking — Support for corking is planned, but in the meantime backpressure must be managed manually with the drain handler.