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Build Live Cursors with Next.js, RPC and Durable Objects

Last reviewed: about 2 months ago

Developer Spotlight community contribution

Written by: Ivan Buendia

Profile: LinkedIn

In this tutorial, you will learn how to build a real-time Next.js ↗ app that displays the live cursor location of each connected user using Durable Objects, the Workers' built-in RPC (Remote Procedure Call) system, and the OpenNext ↗ Cloudflare adapter.

The application works like this:

  • An ID is generated for each user that navigates to the application, which is used for identifying the WebSocket connection in the Durable Object.
  • Once the WebSocket connection is established, the application sends a message to the WebSocket Durable Object to determine the current number of connected users.
  • A user can close all active WebSocket connections via a Next.js server action that uses an RPC method.
  • It handles WebSocket and mouse movement events to update the location of other users' cursors in the UI and to send updates about the user's own cursor, as well as join and leave WebSocket events.
Animated gif of real-time Next.js app for visualizing live cursors

1. Create a Next.js Workers Project

  1. Run the following command to create your Next.js Worker named next-rpc:

    Terminal window
    npm create cloudflare@latest -- next-rpc --framework=next --experimental

    For setup, select the following options:

    • For What would you like to start with?, choose Framework Starter.
    • For Which development framework do you want to use?, choose Next.js.
    • Complete the framework's own CLI wizard.
    • For Do you want to use git for version control?, choose Yes.
    • For Do you want to deploy your application?, choose No (we will be making some changes before deploying).

  2. Change into your new directory:

    Terminal window
    cd next-rpc

  3. Install nanoid ↗ so that string IDs can be generated for clients:

    npm i nanoid

  4. Install perfect-cursors ↗ to interpolate cursor positions:

    npm i perfect-cursors

  5. Define workspaces for each Worker:

    Update your package.json file.

    package.json
    {
      "name": "next-rpc",
      "version": "0.1.0",
      "private": true,
      "scripts": {
        "dev": "next dev",
        "build": "next build",
        "start": "next start",
        "lint": "next lint",
        "deploy": "cloudflare && wrangler deploy",
        "preview": "cloudflare && wrangler dev",
        "cf-typegen": "wrangler types --env-interface CloudflareEnv env.d.ts"
      },
      "workspaces": [
        ".",
        "worker"
      ],
      // ...
    }

2. Create a Durable Object Worker

This Worker will manage the Durable Object and also have internal APIs that will be made available to the Next.js Worker using a WorkerEntrypoint class.

  1. Create another Worker named worker inside the Next.js directory:

    Terminal window
    npm create cloudflare@latest -- worker

    For setup, select the following options:

    • For What would you like to start with?, choose Hello World example.
    • For Which template would you like to use?, choose Hello World Worker using Durable Objects.
    • For Which language do you want to use?, choose TypeScript.
    • For Do you want to use git for version control?, choose Yes.
    • For Do you want to deploy your application?, choose No (we will be making some changes before deploying).

3. Build Durable Object Worker functionality

  1. In your worker/wrangler.toml file, update the Durable Object binding:

    worker/wrangler.toml
    # ... Other wrangler configuration settings
    

    [[durable_objects.bindings]]
    name = "CURSOR_SESSIONS"
    class_name = "CursorSessions"
    

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

  2. Initialize the main methods for the Durable Object and define types for WebSocket messages and cursor sessions in your worker/src/index.ts to support type-safe interaction:

    • WsMessage. Specifies the structure of WebSocket messages handled by the Durable Object.
    • Session. Represents the connected user's ID and current cursor coordinates.
    worker/src/index.ts
    import { DurableObject } from 'cloudflare:workers';
    

    export type WsMessage =
      | { type: "message"; data: string }
      | { type: "quit"; id: string }
      | { type: "join"; id: string }
      | { type: "move"; id: string; x: number; y: number }
      | { type: "get-cursors" }
      | { type: "get-cursors-response"; sessions: Session[] };
    

    export type Session = { id: string; x: number; y: number };
    

    export class CursorSessions extends DurableObject<Env> {
      constructor(ctx: DurableObjectState, env: Env) {}
      broadcast(message: WsMessage, self?: string) {}
      async webSocketMessage(ws: WebSocket, message: string) {}
      async webSocketClose(ws: WebSocket, code: number) {}
      closeSessions() {}
      async fetch(request: Request) {
        return new Response("Hello");
      }
    }
    

    export default {
      async fetch(request, env, ctx) {
        return new Response("Ok");
      },
    } satisfies ExportedHandler<Env>;

    Now update worker-configuration.d.ts by running:

    Terminal window
    cd worker && npm run cf-typegen

  3. Update the Durable Object to manage WebSockets:

    worker/src/index.ts
    // Rest of the code
    

    export class CursorSessions extends DurableObject<Env> {
      sessions: Map<WebSocket, Session>;
    

      constructor(ctx: DurableObjectState, env: Env) {
        super(ctx, env);
        this.sessions = new Map();
        this.ctx.getWebSockets().forEach((ws) => {
          const meta = ws.deserializeAttachment();
          this.sessions.set(ws, { ...meta });
        });
      }
    

      broadcast(message: WsMessage, self?: string) {
        this.ctx.getWebSockets().forEach((ws) => {
          const { id } = ws.deserializeAttachment();
          if (id !== self) ws.send(JSON.stringify(message));
        });
      }
    

      async webSocketMessage(ws: WebSocket, message: string) {
        if (typeof message !== "string") return;
        const parsedMsg: WsMessage = JSON.parse(message);
        const session = this.sessions.get(ws);
        if (!session) return;
    

        switch (parsedMsg.type) {
          case "move":
            session.x = parsedMsg.x;
            session.y = parsedMsg.y;
            ws.serializeAttachment(session);
            this.broadcast(parsedMsg, session.id);
            break;
    

          case "get-cursors":
            const sessions: Session[] = [];
            this.sessions.forEach((session) => {
              sessions.push(session);
            });
            const wsMessage: WsMessage = { type: "get-cursors-response", sessions };
            ws.send(JSON.stringify(wsMessage));
            break;
    

          case "message":
            this.broadcast(parsedMsg);
            break;
    

          default:
            break;
        }
      }
    

      async webSocketClose(ws: WebSocket, code: number) {
        const id = this.sessions.get(ws)?.id;
        id && this.broadcast({ type: 'quit', id });
        this.sessions.delete(ws);
        ws.close();
      }
    

      closeSessions() {
        this.ctx.getWebSockets().forEach((ws) => ws.close());
      }
    

      async fetch(request: Request) {
        const url = new URL(request.url);
        const webSocketPair = new WebSocketPair();
        const [client, server] = Object.values(webSocketPair);
        this.ctx.acceptWebSocket(server);
        const id = url.searchParams.get("id");
        if (!id) {
          return new Response("Missing id", { status: 400 });
        }
    

        // Set Id and Default Position
        const sessionInitialData: Session = { id, x: -1, y: -1 };
        server.serializeAttachment(sessionInitialData);
        this.sessions.set(server, sessionInitialData);
        this.broadcast({ type: "join", id }, id);
    

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

    export default {
      async fetch(request, env, ctx) {
        if (request.url.match("/ws")) {
          const upgradeHeader = request.headers.get("Upgrade");
          if (!upgradeHeader || upgradeHeader !== "websocket") {
            return new Response("Durable Object expected Upgrade: websocket", {
              status: 426,
            });
          }
          const id = env.CURSOR_SESSIONS.idFromName("globalRoom");
          const stub = env.CURSOR_SESSIONS.get(id);
          return stub.fetch(request);
        }
        return new Response(null, {
          status: 400,
          statusText: "Bad Request",
          headers: {
            "Content-Type": "text/plain",
          },
        });
      },
    } satisfies ExportedHandler<Env>;
    • The main fetch handler routes requests with a /ws URL to the CursorSessions Durable Object where a WebSocket connection is established.
    • The CursorSessions class manages WebSocket connections, session states, and broadcasts messages to other connected clients.
      • When a new WebSocket connection is established, the Durable Object broadcasts a join message to all connected clients; similarly, a quit message is broadcast when a client disconnects.
      • It tracks each WebSocket client's last cursor position under the move message, which is broadcasted to all active clients.
      • When a get-cursors message is received, it sends the number of currently active clients to the specific client that requested it.

  4. Extend the WorkerEntrypoint class for RPC:

    worker/src/index.ts
    import { DurableObject } from 'cloudflare:workers';
    import { DurableObject, WorkerEntrypoint } from 'cloudflare:workers';
    // ... rest of the code
    

    export class SessionsRPC extends WorkerEntrypoint<Env> {
      async closeSessions() {
        const id = this.env.CURSOR_SESSIONS.idFromName("globalRoom");
        const stub = this.env.CURSOR_SESSIONS.get(id);
        // Invoking Durable Object RPC method. Same `wrangler dev` session.
        await stub.closeSessions();
      }
    }
    

    export default {
      async fetch(request, env, ctx) {
        if (request.url.match("/ws")) {
    // ...

  5. Leave the Durable Object Worker running. It's used for RPC and serves as a local WebSocket server:

    npm run dev

  6. Use the resulting address from the previous step to set the Worker host as a public environment variable in your Next.js project:

    next-rpc/.env.local
    NEXT_PUBLIC_WS_HOST=localhost:8787

4. Build Next.js Worker functionality

  1. In your Next.js wrangler.toml file, declare the external Durable Object binding and the Service binding to SessionsRPC:

    next-rpc/wrangler.toml
    # ... rest of the configuration
    compatibility_flags = ["nodejs_compat"]
    

    # Minification helps to keep the Worker bundle size down and improve start up time.
    minify = true
    

    # Use the new Workers + Assets to host the static frontend files
    assets = { directory = ".worker-next/assets", binding = "ASSETS" }
    

    [[durable_objects.bindings]]
    name = "CURSOR_SESSIONS"
    class_name = "CursorSessions"
    script_name = "worker"
    

    [[services]]
    binding = "RPC_SERVICE"
    service = "worker"
    entrypoint = "SessionsRPC"

  2. Update your env.d.ts file for type-safety:

    next-rpc/env.d.ts
    interface CloudflareEnv {
      CURSOR_SESSIONS: DurableObjectNamespace<
        import("./worker/src/index").CursorSessions
      >;
      RPC_SERVICE: Service<import("./worker/src/index").SessionsRPC>;
      ASSETS: Fetcher;
    }

  3. Include Next.js server side logic:

    • Add a server action to close all active WebSocket connections.
    • Use the RPC method closeSessions from the RPC_SERVICE Service binding instead of invoking the Durable Object RPC method because of the limitation mentioned in the note above.
    • The server component generates unique IDs using nanoid to identify the WebSocket connection within the Durable Object.
    • Set the dynamic ↗ value to force-dynamic to ensure unique ID generation and avoid static rendering
    src/app/page.tsx
    import { getCloudflareContext } from "@opennextjs/cloudflare";
    import { Cursors } from "./cursor";
    import { nanoid } from "nanoid";
    

    export const dynamic = "force-dynamic";
    

    async function closeSessions() {
      "use server";
      const cf = await getCloudflareContext();
      await cf.env.RPC_SERVICE.closeSessions();
    

      // Note: Not supported in `wrangler dev`
      // const id = cf.env.CURSOR_SESSIONS.idFromName("globalRoom");
      // const stub = cf.env.CURSOR_SESSIONS.get(id);
      // await stub.closeSessions();
    }
    

    export default function Home() {
      const id = `ws_${nanoid(50)}`;
      return (
        <main className="flex min-h-screen flex-col items-center p-24 justify-center">
          <div className="border border-dashed w-full">
            <p className="pt-2 px-2">Server Actions</p>
            <div className="p-2">
              <form action={closeSessions}>
                <button className="border px-2 py-1">Close WebSockets</button>
              </form>
            </div>
          </div>
          <div className="border border-dashed w-full mt-2.5">
            <p className="py-2 px-2">Live Cursors</p>
            <div className="px-2 space-y-2">
              <Cursors id={id}></Cursors>
            </div>
          </div>
        </main>
      );
    }

  4. Create a client component to manage WebSocket and mouse movement events:

    src/app/cursor.tsx
    src/app/cursor.tsx
    "use client";
    import { useCallback, useEffect, useLayoutEffect, useReducer, useRef, useState } from "react";
    import type { Session, WsMessage } from "../../worker/src/index";
    import { PerfectCursor } from "perfect-cursors";
    

    const INTERVAL = 55;
    

    export function Cursors(props: { id: string }) {
      const wsRef = useRef<WebSocket | null>(null);
      const [cursors, setCursors] = useState<Map<string, Session>>(new Map());
      const lastSentTimestamp = useRef(0);
      const [messageState, dispatchMessage] = useReducer(messageReducer, {
        in: "",
        out: "",
      });
      const [highlightedIn, highlightIn] = useHighlight();
      const [highlightedOut, highlightOut] = useHighlight();
    

      function startWebSocket() {
        const wsProtocol = window.location.protocol === "https:" ? "wss" : "ws";
        const ws = new WebSocket(
          `${wsProtocol}://${process.env.NEXT_PUBLIC_WS_HOST}/ws?id=${props.id}`,
        );
        ws.onopen = () => {
          highlightOut();
          dispatchMessage({ type: "out", message: "get-cursors" });
          const message: WsMessage = { type: "get-cursors" };
          ws.send(JSON.stringify(message));
        };
        ws.onmessage = (message) => {
          const messageData: WsMessage = JSON.parse(message.data);
          highlightIn();
          dispatchMessage({ type: "in", message: messageData.type });
          switch (messageData.type) {
            case "quit":
              setCursors((prev) => {
                const updated = new Map(prev);
                updated.delete(messageData.id);
                return updated;
              });
              break;
            case "join":
              setCursors((prev) => {
                const updated = new Map(prev);
                if (!updated.has(messageData.id)) {
                  updated.set(messageData.id, { id: messageData.id, x: -1, y: -1 });
                }
                return updated;
              });
              break;
            case "move":
              setCursors((prev) => {
                const updated = new Map(prev);
                const session = updated.get(messageData.id);
                if (session) {
                  session.x = messageData.x;
                  session.y = messageData.y;
                } else {
                  updated.set(messageData.id, messageData);
                }
                return updated;
              });
              break;
            case "get-cursors-response":
              setCursors(
                new Map(
                  messageData.sessions.map((session) => [session.id, session]),
                ),
              );
              break;
            default:
              break;
          }
        };
        ws.onclose = () => setCursors(new Map());
        return ws;
      }
    

      useEffect(() => {
        const abortController = new AbortController();
        document.addEventListener(
          "mousemove",
          (ev) => {
            const x = ev.pageX / window.innerWidth,
              y = ev.pageY / window.innerHeight;
            const now = Date.now();
            if (
              now - lastSentTimestamp.current > INTERVAL &&
              wsRef.current?.readyState === WebSocket.OPEN
            ) {
              const message: WsMessage = { type: "move", id: props.id, x, y };
              wsRef.current.send(JSON.stringify(message));
              lastSentTimestamp.current = now;
              highlightOut();
              dispatchMessage({ type: "out", message: "move" });
            }
          },
          {
            signal: abortController.signal,
          },
        );
        return () => abortController.abort();
        // eslint-disable-next-line react-hooks/exhaustive-deps
      }, []);
    

      useEffect(() => {
        wsRef.current = startWebSocket();
        return () => wsRef.current?.close();
        // eslint-disable-next-line react-hooks/exhaustive-deps
      }, [props.id]);
    

      function sendMessage() {
        highlightOut();
        dispatchMessage({ type: "out", message: "message" });
        wsRef.current?.send(
          JSON.stringify({ type: "message", data: "Ping" } satisfies WsMessage),
        );
      }
    

      const otherCursors = Array.from(cursors.values()).filter(
        ({ id, x, y }) => id !== props.id && x !== -1 && y !== -1,
      );
    

      return (
        <>
          <div className="flex border">
            <div className="px-2 py-1 border-r">WebSocket Connections</div>
            <div className="px-2 py-1"> {cursors.size} </div>
          </div>
          <div className="flex border">
            <div className="px-2 py-1 border-r">Messages</div>
            <div className="flex flex-1">
              <div className="px-2 py-1 border-r">↓</div>
              <div
                className="w-full px-2 py-1 [word-break:break-word] transition-colors duration-500"
                style={{
                  backgroundColor: highlightedIn ? "#ef4444" : "transparent",
                }}
              >
                {messageState.in}
              </div>
            </div>
            <div className="flex flex-1">
              <div className="px-2 py-1 border-x">↑</div>
              <div
                className="w-full px-2 py-1 [word-break:break-word] transition-colors duration-500"
                style={{
                  backgroundColor: highlightedOut ? "#60a5fa" : "transparent",
                }}
              >
                {messageState.out}
              </div>
            </div>
          </div>
          <div className="flex gap-2">
            <button onClick={sendMessage} className="border px-2 py-1">
              ws message
            </button>
            <button
              className="border px-2 py-1 disabled:opacity-80"
              onClick={() => {
                wsRef.current?.close();
                wsRef.current = startWebSocket();
              }}
            >
              ws reconnect
            </button>
            <button
              className="border px-2 py-1"
              onClick={() => wsRef.current?.close()}
            >
              ws close
            </button>
          </div>
          <div>
            {otherCursors.map((session) => (
              <SvgCursor
                key={session.id}
                point={[
                  session.x * window.innerWidth,
                  session.y * window.innerHeight,
                ]}
              />
            ))}
          </div>
        </>
      );
    }
    

    function SvgCursor({ point }: { point: number[] }) {
      const refSvg = useRef<SVGSVGElement>(null);
      const animateCursor = useCallback((point: number[]) => {
        refSvg.current?.style.setProperty(
          "transform",
          `translate(${point[0]}px, ${point[1]}px)`,
        );
      }, []);
      const onPointMove = usePerfectCursor(animateCursor);
      useLayoutEffect(() => onPointMove(point), [onPointMove, point]);
      const [randomColor] = useState(
        `#${Math.floor(Math.random() * 16777215)
          .toString(16)
          .padStart(6, "0")}`,
      );
      return (
        <svg
          ref={refSvg}
          height="32"
          width="32"
          viewBox="0 0 32 32"
          xmlns="http://www.w3.org/2000/svg"
          className={"absolute -top-[12px] -left-[12px] pointer-events-none"}
        >
          <defs>
            <filter id="shadow" x="-40%" y="-40%" width="180%" height="180%">
              <feDropShadow dx="1" dy="1" stdDeviation="1.2" floodOpacity="0.5" />
            </filter>
          </defs>
          <g fill="none" transform="rotate(0 16 16)" filter="url(#shadow)">
            <path
              d="M12 24.4219V8.4069L23.591 20.0259H16.81l-.411.124z"
              fill="white"
            />
            <path
              d="M21.0845 25.0962L17.4795 26.6312L12.7975 15.5422L16.4835 13.9892z"
              fill="white"
            />
            <path
              d="M19.751 24.4155L17.907 25.1895L14.807 17.8155L16.648 17.04z"
              fill={randomColor}
            />
            <path
              d="M13 10.814V22.002L15.969 19.136l.428-.139h4.768z"
              fill={randomColor}
            />
          </g>
        </svg>
      );
    }
    

    function usePerfectCursor(cb: (point: number[]) => void, point?: number[]) {
      const [pc] = useState(() => new PerfectCursor(cb));
    

      useLayoutEffect(() => {
        if (point) pc.addPoint(point);
        return () => pc.dispose();
        // eslint-disable-next-line react-hooks/exhaustive-deps
      }, [pc]);
    

      useLayoutEffect(() => {
        PerfectCursor.MAX_INTERVAL = 58;
      }, []);
    

      const onPointChange = useCallback(
        (point: number[]) => pc.addPoint(point),
        [pc],
      );
    

      return onPointChange;
    }
    

    type MessageState = { in: string; out: string };
    type MessageAction = { type: "in" | "out"; message: string };
    function messageReducer(state: MessageState, action: MessageAction) {
      switch (action.type) {
        case "in":
          return { ...state, in: action.message };
        case "out":
          return { ...state, out: action.message };
        default:
          return state;
      }
    }
    

    function useHighlight(duration = 250) {
      const timestampRef = useRef(0);
      const [highlighted, setHighlighted] = useState(false);
      function highlight() {
        timestampRef.current = Date.now();
        setHighlighted(true);
        setTimeout(() => {
          const now = Date.now();
          if (now - timestampRef.current >= duration) {
            setHighlighted(false);
          }
        }, duration);
      }
      return [highlighted, highlight] as const;
    }

    The generated ID is used here and passed as a parameter to the WebSocket server:

    const ws = new WebSocket(
      `${wsProtocol}://${process.env.NEXT_PUBLIC_WS_HOST}/ws?id=${props.id}`,
    );

    The component starts the WebSocket connection and handles 4 types of WebSocket messages, which trigger updates to React's state:

    • join. Received when a new WebSocket connection is established.
    • quit. Received when a WebSocket connection is closed.
    • move. Received when a user's cursor moves.
    • get-cursors-response. Received when a client sends a get-cursors message, which is triggered once the WebSocket connection is open.

    It sends the user's cursor coordinates to the WebSocket server during the mousemove ↗ event, which then broadcasts them to all active WebSocket clients.

    Although there are multiple strategies you can use together for real-time cursor synchronization (e.g., batching, interpolation, etc.), in this tutorial throttling, spline interpolation and position normalization are used:

    document.addEventListener(
      "mousemove",
      (ev) => {
        const x = ev.pageX / window.innerWidth,
          y = ev.pageY / window.innerHeight;
        const now = Date.now();
        if (
            now - lastSentTimestamp.current > INTERVAL &&
            wsRef.current?.readyState === WebSocket.OPEN
        ) {
          const message: WsMessage = { type: "move", id: props.id, x, y };
          wsRef.current.send(JSON.stringify(message));
          lastSentTimestamp.current = now;
          // ...
        }
      }
    );

    Each animated cursor is controlled by a PerfectCursor instance, which animates its position along a spline curve defined by the cursor's latest positions:

    // SvgCursor react component
    const refSvg = useRef<SVGSVGElement>(null);
    const animateCursor = useCallback((point: number[]) => {
      refSvg.current?.style.setProperty(
        "transform",
        `translate(${point[0]}px, ${point[1]}px)`,
      );
    }, []);
    const onPointMove = usePerfectCursor(animateCursor);
    // A `point` is added to the path whenever its vule updates;
    useLayoutEffect(() => onPointMove(point), [onPointMove, point]);
    // ...

  5. Run Next.js development server:

    npm run dev

  6. Open the App in the browser.

5. Deploy the project

  1. Change into your Durable Object Worker directory:

    Terminal window
    cd worker

    Deploy the Worker:

    npm run deploy

    Copy only the host from the generated Worker URL, excluding the protocol, and set NEXT_PUBLIC_WS_HOST in .env.local to this value (e.g., worker-unique-identifier.workers.dev).

    next-rpc/.env.local
    NEXT_PUBLIC_WS_HOST=localhost:8787
    NEXT_PUBLIC_WS_HOST=worker-unique-identifier.workers.dev

  2. Change into your root directory and deploy your Next.js app:

    npm run deploy

Summary

In this tutorial, you learned how to integrate Next.js with Durable Objects to build a real-time application to visualize cursors. You also learned how to use Workers' built-in RPC system alongside Next.js server actions. The complete code for this tutorial is available on GitHub ↗.

You can check other Cloudflare tutorials or related resources:

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