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Using WebSockets

WebSockets are long-lived TCP connections that enable bi-directional, real-time communication between client and server. Both Cloudflare Durable Objects and Workers can act as WebSocket endpoints – either as a client or as a server. Because WebSocket sessions are long-lived, applications commonly use Durable Objects to accept either the client or server connection. While there are other use cases for using Workers exclusively with WebSockets, for example proxying WebSocket messages, WebSockets are most useful when combined with Durable Objects.

Because Durable Objects provide a single-point-of-coordination between Cloudflare Workers, a single Durable Object instance can be used in parallel with WebSockets to coordinate between multiple clients, such as participants in a chat room or a multiplayer game. Refer to Cloudflare Edge Chat Demo ↗ for an example of using Durable Objects with WebSockets.

Both Cloudflare Durable Objects and Workers can use the Web Standard WebSocket API to build applications, but a major differentiator of Cloudflare Durable Objects relative to other platforms is the ability to Hibernate WebSocket connections to save costs.

This guide covers:

  1. Building a WebSocket server using Web Standard APIs
  2. Using WebSocket Hibernation APIs.

WebSocket Standard API

WebSocket connections are established by making an HTTP GET request with the Upgrade: websocket header. A Cloudflare Worker is commonly used to validate the request, proxy the request to the Durable Object to accept the server side connection, and return the client side connection in the response.

// Worker
export default {
  async fetch(request, env, ctx) {
    if (request.method === "GET" && 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(null, {
          status: 426,
          statusText: "Durable Object expected Upgrade: websocket",
          headers: {
            "Content-Type": "text/plain",
          },
        });
      }


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


      // The Durable Object's fetch handler will accept the server side connection and return
      // the client
      return stub.fetch(request);
    }


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

Each WebSocket server in this example is represented by a Durable Object. This WebSocket server creates a single WebSocket connection and responds to all messages over that connection with the total number of accepted WebSocket connections. In the Durable Object's fetch handler we create client and server connections and add event listeners for relevant event types.

import { DurableObject } from "cloudflare:workers";


// 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,
    });
  }
}

To execute this code, configure your wrangler.toml file to include a Durable Object binding and migration based on the namespace and class name chosen previously.

wrangler.toml
name = "websocket-server"


[[durable_objects.bindings]]
name = "WEBSOCKET_SERVER"
class_name = "WebSocketServer"


[[migrations]]
tag = "v1"
new_classes = ["WebSocketServer"]

A full example can be found in Build a WebSocket server.

WebSocket Hibernation API

In addition to Workers WebSocket API, Cloudflare Durable Objects can use the WebSocket Hibernation API which extends the Web Standard WebSocket API to reduce costs. Specifically, billable Duration (GB-s) charges are not incurred during periods of inactivity. Note that other events, for example alarms, can prevent a Durable Object from being inactive and therefore prevent this cost saving.

The WebSocket consists of Cloudflare-specific extensions to the Web Standard WebSocket API. These extensions are either present on the DurableObjectState interface, or as handler methods on the Durable Object class.

The Worker used in the WebSocket Standard API example does not require any code changes to make use of the WebSocket Hibernation API. The changes to the Durable Object are described in the code sample below. In summary, DurableObjectState::acceptWebSocket is called to accept the server side of the WebSocket connection, and handler methods are defined on the Durable Object class for relevant event types rather than adding event listeners.

If an event occurs for a hibernated Durable Object's corresponding handler method, it will return to memory. This will call the Durable Object's constructor, so it is best to minimize work in the constructor when using WebSocket hibernation.

import { DurableObject } from "cloudflare:workers";


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


    // Calling `acceptWebSocket()` informs 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.
    // Unlike `ws.accept()`, `state.acceptWebSocket(ws)` informs the Workers Runtime that the WebSocket
    // is "hibernatable", so the runtime does not need to pin this Durable Object to memory while
    // the connection is open. During periods of inactivity, the Durable Object can be evicted
    // from memory, but the WebSocket connection will remain open. If at some later point the
    // WebSocket receives a message, the runtime will recreate the Durable Object
    // (run the `constructor`) and deliver the message to the appropriate handler.
    this.ctx.acceptWebSocket(server);


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


  async webSocketMessage(ws, message) {
    // Upon receiving a message from the client, reply with the same message,
    // but will prefix the message with "[Durable Object]: " and return the
    // total number of connections.
    ws.send(
      `[Durable Object] message: ${message}, connections: ${this.ctx.getWebSockets().length}`,
    );
  }


  async webSocketClose(ws, code, reason, wasClean) {
    // If the client closes the connection, the runtime will invoke the webSocketClose() handler.
    ws.close(code, "Durable Object is closing WebSocket");
  }
}

Similar to the WebSocket Standard API example, to execute this code, configure your wrangler.toml file to include a Durable Object binding and migration based on the namespace and class name chosen previously.

wrangler.toml
name = "websocket-hibernation-server"


[[durable_objects.bindings]]
name = "WEBSOCKET_HIBERNATION_SERVER"
class_name = "WebSocketHibernationServer"


[[migrations]]
tag = "v1"
new_classes = ["WebSocketHibernationServer"]

A full example can be found in Build a WebSocket server with WebSocket Hibernation.

Extended methods

serializeAttachment

  • serializeAttachment(value any ): void

    • Keeps a copy of value associated with the WebSocket to survive hibernation. The value can be any type supported by the structured clone algorithm ↗, which is true of most types. If the value needs to be durable please use Durable Object Storage.

    • If you modify value after calling this method, those changes will not be retained unless you call this method again. The serialized size of value is limited to 2,048 bytes, otherwise this method will throw an error. If you need larger values to survive hibernation, use the Storage API and pass the corresponding key to this method so it can be retrieved later.

deserializeAttachment

  • deserializeAttachment(): any

    • Retrieves the most recent value passed to serializeAttachment(), or null if none exists.
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