Bun WebSocket Server

Bun has built-in WebSocket support integrated with Bun.serve().

Quick Start

const server = Bun.serve({
  fetch(req, server) {
    // Upgrade to WebSocket
    if (server.upgrade(req)) {
      return; // Upgraded successfully
    }
    return new Response("Not a WebSocket request", { status: 400 });
  },
  websocket: {
    open(ws) {
      console.log("Client connected");
    },
    message(ws, message) {
      console.log("Received:", message);
      ws.send(`Echo: ${message}`);
    },
    close(ws) {
      console.log("Client disconnected");
    },
  },
});

console.log(`WebSocket server running on ws://localhost:${server.port}`);

WebSocket Handlers

Bun.serve({
  fetch(req, server) {
    server.upgrade(req);
  },
  websocket: {
    // Client connected
    open(ws) {
      console.log("New connection");
    },

    // Message received
    message(ws, message) {
      // message is string | Buffer
      if (typeof message === "string") {
        console.log("Text:", message);
      } else {
        console.log("Binary:", message);
      }
    },

    // Connection closed
    close(ws, code, reason) {
      console.log(`Closed: ${code} - ${reason}`);
    },

    // Drain event (buffer flushed)
    drain(ws) {
      console.log("Buffer drained");
    },

    // Ping received
    ping(ws, data) {
      // Pong sent automatically
    },

    // Pong received
    pong(ws, data) {
      console.log("Pong received");
    },
  },
});

Sending Messages

websocket: {
  message(ws, message) {
    // Send text
    ws.send("Hello");

    // Send JSON
    ws.send(JSON.stringify({ type: "greeting", data: "Hello" }));

    // Send binary
    ws.send(new Uint8Array([1, 2, 3]));
    ws.send(Buffer.from("binary data"));

    // Send with compression
    ws.send("compressed message", true);

    // Check if buffer is full
    const bufferedAmount = ws.send("data");
    if (bufferedAmount > 1024 * 1024) {
      console.log("Buffer getting full");
    }
  },
}

Attaching Data to Connections

interface UserData {
  id: string;
  name: string;
  joinedAt: Date;
}

Bun.serve<UserData>({
  fetch(req, server) {
    const url = new URL(req.url);
    const userId = url.searchParams.get("userId");

    // Attach data during upgrade
    server.upgrade(req, {
      data: {
        id: userId,
        name: "User " + userId,
        joinedAt: new Date(),
      },
    });
  },
  websocket: {
    open(ws) {
      // Access attached data
      console.log(`${ws.data.name} connected`);
    },
    message(ws, message) {
      console.log(`${ws.data.name}: ${message}`);
    },
  },
});

Pub/Sub (Topics)

Bun.serve({
  fetch(req, server) {
    const url = new URL(req.url);
    const room = url.searchParams.get("room") || "general";

    server.upgrade(req, {
      data: { room },
    });
  },
  websocket: {
    open(ws) {
      // Subscribe to a topic
      ws.subscribe(ws.data.room);

      // Publish to topic (excludes sender)
      ws.publish(ws.data.room, `User joined ${ws.data.room}`);
    },
    message(ws, message) {
      // Broadcast to all in room (excludes sender)
      ws.publish(ws.data.room, message);
    },
    close(ws) {
      // Unsubscribe (automatic on close)
      ws.unsubscribe(ws.data.room);
      ws.publish(ws.data.room, "User left");
    },
  },
});

Broadcasting to All Clients

Bun.serve({
  fetch(req, server) {
    server.upgrade(req);
  },
  websocket: {
    open(ws) {
      // Subscribe to global topic
      ws.subscribe("global");
    },
    message(ws, message) {
      // Broadcast to ALL clients including sender
      server.publish("global", message);
    },
  },
});

Server-Level Publish

const server = Bun.serve({
  fetch(req, server) {
    const url = new URL(req.url);

    // HTTP endpoint to publish
    if (url.pathname === "/broadcast") {
      const message = url.searchParams.get("msg");
      server.publish("global", message);
      return new Response("Broadcasted");
    }

    server.upgrade(req);
  },
  websocket: {
    open(ws) {
      ws.subscribe("global");
    },
  },
});

// Can also publish from outside fetch
setInterval(() => {
  server.publish("global", `Server time: ${new Date().toISOString()}`);
}, 5000);

WebSocket Options

Bun.serve({
  websocket: {
    // Max message size (default 16MB)
    maxPayloadLength: 1024 * 1024, // 1MB

    // Idle timeout in seconds (default 120)
    idleTimeout: 60,

    // Backpressure limit
    backpressureLimit: 1024 * 1024,

    // Enable compression
    perMessageDeflate: true,
    // Or with options
    perMessageDeflate: {
      compress: "shared",
      decompress: "shared",
    },

    // Send/receive pings
    sendPings: true,

    // Handlers
    open(ws) {},
    message(ws, message) {},
    close(ws) {},
  },
});

Client-Side Connection

// Browser
const ws = new WebSocket("ws://localhost:3000");

ws.onopen = () => {
  ws.send("Hello Server!");
};

ws.onmessage = (event) => {
  console.log("Received:", event.data);
};

ws.onclose = () => {
  console.log("Disconnected");
};

Authentication

Bun.serve({
  fetch(req, server) {
    // Verify auth before upgrade
    const token = req.headers.get("Authorization");

    if (!verifyToken(token)) {
      return new Response("Unauthorized", { status: 401 });
    }

    const user = decodeToken(token);
    server.upgrade(req, {
      data: { userId: user.id },
    });
  },
  websocket: {
    open(ws) {
      console.log(`Authenticated user ${ws.data.userId} connected`);
    },
  },
});

Common Errors

Error	Cause	Fix
Upgrade failed	Invalid request	Check upgrade headers
Connection closed	Client disconnect	Handle in close handler
Message too large	Exceeds maxPayloadLength	Increase limit or chunk data
Backpressure	Slow client	Check buffer, wait for drain

Common Patterns

Chat Room

Bun.serve({
  fetch(req, server) {
    const url = new URL(req.url);
    const username = url.searchParams.get("user") || "Anonymous";

    server.upgrade(req, {
      data: { username },
    });
  },
  websocket: {
    open(ws) {
      ws.subscribe("chat");
      ws.publish("chat", `${ws.data.username} joined`);
    },
    message(ws, message) {
      ws.publish("chat", `${ws.data.username}: ${message}`);
    },
    close(ws) {
      ws.publish("chat", `${ws.data.username} left`);
    },
  },
});

When to Load References

Load references/compression.md when:

• perMessageDeflate configuration
• Compression tuning
• Binary message handling

Load references/scaling.md when:

• Multiple server instances
• Redis pub/sub integration
• Horizontal scaling