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Build a WebSocket server

Build a WebSocket server using Durable Objects and Workers.

This example shows how to build a WebSocket server using and Workers. The example exposes an endpoint to create a new WebSocket connection. This WebSocket connection echos any message while including the total number of WebSocket connections currently established. For more information, refer to Use Durable Objects with WebSockets.

import { DurableObject } from "cloudflare:workers";


// Worker
export default {
  async fetch(request, env, ctx) {
    if (request.url.endsWith("/websocket")) {
      // Expect to receive a WebSocket Upgrade request.
      // If there is one, accept the request and return a WebSocket Response.
      const upgradeHeader = request.headers.get("Upgrade");
      if (!upgradeHeader || upgradeHeader !== "websocket") {
        return new Response("Durable Object expected Upgrade: websocket", {
          status: 426,
        });
      }


      // This example will refer to the same Durable Object,
      // since the name "foo" is hardcoded.
      let id = env.WEBSOCKET_SERVER.idFromName("foo");
      let stub = env.WEBSOCKET_SERVER.get(id);


      return stub.fetch(request);
    }


    return new Response(null, {
      status: 400,
      statusText: "Bad Request",
      headers: {
        "Content-Type": "text/plain",
      },
    });
  },
};


// Durable Object
export class WebSocketServer extends DurableObject {
  currentlyConnectedWebSockets;


  constructor(ctx, env) {
    // This is reset whenever the constructor runs because
    // regular WebSockets do not survive Durable Object resets.
    //
    // WebSockets accepted via the Hibernation API can survive
    // a certain type of eviction, but we will not cover that here.
    super(ctx, env);
    this.currentlyConnectedWebSockets = 0;
  }


  async fetch(request) {
    // Creates two ends of a WebSocket connection.
    const webSocketPair = new WebSocketPair();
    const [client, server] = Object.values(webSocketPair);


    // Calling `accept()` tells the runtime that this WebSocket is to begin terminating
    // request within the Durable Object. It has the effect of "accepting" the connection,
    // and allowing the WebSocket to send and receive messages.
    server.accept();
    this.currentlyConnectedWebSockets += 1;


    // Upon receiving a message from the client, the server replies with the same message,
    // and the total number of connections with the "[Durable Object]: " prefix
    server.addEventListener("message", (event) => {
      server.send(
        `[Durable Object] currentlyConnectedWebSockets: ${this.currentlyConnectedWebSockets}`,
      );
    });


    // If the client closes the connection, the runtime will close the connection too.
    server.addEventListener("close", (cls) => {
      this.currentlyConnectedWebSockets -= 1;
      server.close(cls.code, "Durable Object is closing WebSocket");
    });


    return new Response(null, {
      status: 101,
      webSocket: client,
    });
  }
}

Finally, configure your Wrangler file to include a Durable Object binding and migration based on the and class name chosen previously.

{
  "name": "websocket-server",
  "durable_objects": {
    "bindings": [
      {
        "name": "WEBSOCKET_SERVER",
        "class_name": "WebSocketServer"
      }
    ]
  },
  "migrations": [
    {
      "tag": "v1",
      "new_classes": [
        "WebSocketServer"
      ]
    }
  ]
}