--- name: socketio-a67b8a5a454c description: >- Socket.IO is a Node.js library for bidirectional, event-based real-time communication between browsers (or other clients) and a server, with WebSocket transport, HTTP long-polling fallback, automatic reconnection, rooms and namespaces. Use when a user asks to build a chat, live notifications, a real-time dashboard, presence indicators, collaborative editing or pub/sub messaging, add WebSocket communication, authenticate socket connections, or scale Socket.IO across several servers with the Redis adapter. license: Apache-2.0 compatibility: 'Node.js 18+ recommended for the server (socket.io 4.x supports older), any modern browser or Node.js for the client' metadata: author: terminal-skills version: 1.1.0 category: development repository: https://github.com/socketio/socket.io tags: "socketio, websocket, realtime, chat, events" --- # Socket.IO ## Overview Socket.IO adds its own protocol on top of WebSocket and HTTP long-polling: named events, acknowledgements, rooms, namespaces, automatic reconnection and broadcasting. A Socket.IO client can only talk to a Socket.IO server (and the reverse); it is not a plain WebSocket endpoint. The current release line is 4.x (`socket.io` and `socket.io-client` 4.8). This skill covers server setup with Express, a typed browser client, JWT authentication in a middleware, rooms with acknowledgements, presence and scaling with the Redis adapter. ## Instructions ### Step 1: Install ```bash npm install socket.io express jsonwebtoken # server npm install socket.io-client # browser / Node client npm install @socket.io/redis-adapter redis # only for several server instances ``` ### Step 2: Server with authentication and rooms The auth middleware runs before `connection`. The client sends the token in the handshake `auth` object; do not put tokens in the URL query. This server was run end to end with socket.io 4.8.4: a bad token is rejected with `Invalid token`, and a message sent to a room reaches the other member. ```typescript // server.ts import { createServer } from 'node:http' import { Server } from 'socket.io' import express from 'express' import jwt from 'jsonwebtoken' const secret = process.env.JWT_SECRET if (!secret) throw new Error('JWT_SECRET is required') const app = express() const httpServer = createServer(app) const io = new Server(httpServer, { cors: { origin: ['http://localhost:3000'] }, // list the real frontend origins connectionStateRecovery: { maxDisconnectionDuration: 2 * 60 * 1000 }, }) io.use((socket, next) => { const token = socket.handshake.auth.token if (typeof token !== 'string') return next(new Error('Authentication required')) try { const claims = jwt.verify(token, secret, { algorithms: ['HS256'] }) as { sub: string; name: string } socket.data.userId = claims.sub socket.data.userName = claims.name next() } catch { next(new Error('Invalid token')) } }) io.on('connection', (socket) => { socket.join(`user:${socket.data.userId}`) // one room per user: reach all of a user's tabs socket.on('join-room', async (roomId: string, ack: (r: unknown) => void) => { await socket.join(roomId) socket.to(roomId).emit('user-joined', { userId: socket.data.userId, roomId }) const members = (await io.in(roomId).fetchSockets()).map((s) => s.data.userName) ack({ ok: true, members }) }) socket.on('room-message', ({ roomId, text }: { roomId: string; text: string }) => { if (!socket.rooms.has(roomId)) return // only members may post io.to(roomId).emit('message', { roomId, text, from: socket.data.userName, at: Date.now() }) }) socket.on('disconnect', (reason) => console.log(`${socket.id} left: ${reason}`)) }) httpServer.listen(3001, () => console.log('Socket.IO on :3001')) ``` `connectionStateRecovery` lets a client that dropped briefly resume with its rooms and missed events (`socket.recovered` is true on the client). It is not supported by the Redis adapter. ### Step 3: Browser client (React) Create the socket inside the effect so React StrictMode's double mount cleans up, and keep it in state so components re-render when it exists. ```typescript // useSocket.ts import { useEffect, useState } from 'react' import { io, Socket } from 'socket.io-client' export function useSocket(url: string, token: string) { const [socket, setSocket] = useState(null) const [isConnected, setIsConnected] = useState(false) useEffect(() => { const s = io(url, { auth: { token } }) s.on('connect', () => setIsConnected(true)) s.on('disconnect', () => setIsConnected(false)) s.on('connect_error', (err) => console.error('socket rejected:', err.message)) setSocket(s) return () => { s.disconnect() } }, [url, token]) return { socket, isConnected } } ``` Join a room and wait for the server's reply with an acknowledgement: ```typescript const reply = await socket.emitWithAck('join-room', 'general') // { ok: true, members: ['Maria'] } socket.on('message', (m) => console.log(m.from, m.text)) socket.emit('room-message', { roomId: 'general', text: 'Standup at 10:00' }) ``` Reconnection is on by default. After a reconnect without state recovery (`socket.recovered === false`) the server has forgotten the rooms: re-join them in the `connect` handler. ### Step 4: Presence Count connections per user instead of keeping a user-to-socket map: users have several tabs, and a map is also wrong once there is more than one server. ```typescript io.on('connection', async (socket) => { const { userId, userName } = socket.data const sockets = await io.in(`user:${userId}`).fetchSockets() if (sockets.length === 1) io.emit('presence-update', { userId, userName, status: 'online' }) socket.on('disconnect', async () => { const left = await io.in(`user:${userId}`).fetchSockets() if (left.length === 0) io.emit('presence-update', { userId, userName, status: 'offline' }) }) }) ``` `fetchSockets()` works across servers when the Redis adapter is installed. For a large user base keep presence in Redis or the database instead. ### Step 5: Scale with the Redis adapter ```typescript // server-scaled.ts import { createClient } from 'redis' import { createAdapter } from '@socket.io/redis-adapter' const pubClient = createClient({ url: process.env.REDIS_URL }) // e.g. redis://10.0.0.5:6379 const subClient = pubClient.duplicate() await Promise.all([pubClient.connect(), subClient.connect()]) const io = new Server(httpServer, { adapter: createAdapter(pubClient, subClient) }) ``` This part is from the official docs and was not run (no Redis in the test environment). For new deployments the docs recommend the sharded adapter (`@socket.io/redis-adapter` `createShardedAdapter`, Redis 7+). The docs warn that the `redis` package may not restore subscriptions after a reconnect; `ioredis` is the alternative. Redis must be on a private network: adapter messages are not signed or encrypted, and anyone who can publish to the channels can inject events. With several servers and long-polling enabled, the load balancer must use sticky sessions; otherwise restrict clients to `transports: ['websocket']`. ## Examples ### Example 1: Group chat with rooms Request: "Build a group chat where users join rooms, see who is in each room and messages appear instantly." The agent sets up the server from Step 2 and the hook from Step 3, shows `members` from the `join-room` acknowledgement in a member list, and renders `message` events in a list. Result: two browsers logged in as different users, both in `general`, see `Maria: Standup at 10:00` at once; a third user who has not joined sees nothing. ### Example 2: Live notification badge Request: "When someone mentions a user in a comment, update their notification bell without a page refresh." On the server, after saving the comment: `io.to('user:42').emit('notification', { id, text })`. The `user:` room from Step 2 reaches every open tab of that user, on any server when the Redis adapter is used. The client does `socket.on('notification', n => setUnread(c => c + 1))`. ## Guidelines - Always authenticate in `io.use` and check room membership before relaying messages; otherwise anyone can connect and read or post to any room. - Set `cors.origin` to the real frontend origins. It only matters when the page and server are on different origins. - Send small payloads (ids, short text) and let the client fetch details; large events slow every connected client. - Use acknowledgements (`emitWithAck`) when the sender needs to know the server handled the event. Add a timeout with `socket.timeout(5000).emitWithAck(...)` so the promise rejects instead of waiting forever. - Server-side state in a variable (user maps, counters) is per process; move it to Redis or a database before you run two instances. - For a plain WebSocket endpoint (no Socket.IO protocol) use the `ws` package instead. --- Source attribution: terminal-skills Original project: https://github.com/TerminalSkills/skills/blob/8a9852d489f274538be9bf1c9121aeb403750b1c/skills/socketio/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. "Licensor" shall mean the copyright owner or entity authorized by the copyright owner that is granting the License. "Legal Entity" shall mean the union of the acting entity and all other entities that control, are controlled by, or are under common control with that entity. For the purposes of this definition, "control" means (i) the power, direct or indirect, to cause the direction or management of such entity, whether by contract or otherwise, or (ii) ownership of fifty percent (50%) or more of the outstanding shares, or (iii) beneficial ownership of such entity. "You" (or "Your") shall mean an individual or Legal Entity exercising permissions granted by this License. "Source" form shall mean the preferred form for making modifications, including but not limited to software source code, documentation source, and configuration files. "Object" form shall mean any form resulting from mechanical transformation or translation of a Source form, including but not limited to compiled object code, generated documentation, and conversions to other media types. "Work" shall mean the work of authorship, whether in Source or Object form, made available under the License, as indicated by a copyright notice that is included in or attached to the work (an example is provided in the Appendix below). "Derivative Works" shall mean any work, whether in Source or Object form, that is based on (or derived from) the Work and for which the editorial revisions, annotations, elaborations, or other modifications represent, as a whole, an original work of authorship. For the purposes of this License, Derivative Works shall not include works that remain separable from, or merely link (or bind by name) to the interfaces of, the Work and Derivative Works thereof. "Contribution" shall mean any work of authorship, including the original version of the Work and any modifications or additions to that Work or Derivative Works thereof, that is intentionally submitted to the Licensor for inclusion in the Work by the copyright owner or by an individual or Legal Entity authorized to submit on behalf of the copyright owner. For the purposes of this definition, "submitted" means any form of electronic, verbal, or written communication sent to the Licensor or its representatives, including but not limited to communication on electronic mailing lists, source code control systems, and issue tracking systems that are managed by, or on behalf of, the Licensor for the purpose of discussing and improving the Work, but excluding communication that is conspicuously marked or otherwise designated in writing by the copyright owner as "Not a Contribution." "Contributor" shall mean Licensor and any individual or Legal Entity on behalf of whom a Contribution has been received by the Licensor and subsequently incorporated within the Work. 2. Grant of Copyright License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare Derivative Works of, publicly display, publicly perform, sublicense, and distribute the Work and such Derivative Works in Source or Object form. 3. Grant of Patent License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted. If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the Work or a Contribution incorporated within the Work constitutes direct or contributory patent infringement, then any patent licenses granted to You under this License for that Work shall terminate as of the date such litigation is filed. 4. Redistribution. You may reproduce and distribute copies of the Work or Derivative Works thereof in any medium, with or without modifications, and in Source or Object form, provided that You meet the following conditions: (a) You must give any other recipients of the Work or Derivative Works a copy of this License; and (b) You must cause any modified files to carry prominent notices stating that You changed the files; and (c) You must retain, in the Source form of any Derivative Works that You distribute, all copyright, patent, trademark, and attribution notices from the Source form of the Work, excluding those notices that do not pertain to any part of the Derivative Works; and (d) If the Work includes a "NOTICE" text file as part of its distribution, then any Derivative Works that You distribute must include a readable copy of the attribution notices contained within such NOTICE file, excluding any notices that do not pertain to any part of the Derivative Works, in at least one of the following places: within a NOTICE text file distributed as part of the Derivative Works; within the Source form or documentation, if provided along with the Derivative Works; or, within a display generated by the Derivative Works, if and wherever such third-party notices normally appear. The contents of the NOTICE file are for informational purposes only and do not modify the License. You may add Your own attribution notices within Derivative Works that You distribute, alongside or as an addendum to the NOTICE text from the Work, provided that such additional attribution notices cannot be construed as modifying the License. You may add Your own copyright statement to Your modifications and may provide additional or different license terms and conditions for use, reproduction, or distribution of Your modifications, or for any such Derivative Works as a whole, provided Your use, reproduction, and distribution of the Work otherwise complies with the conditions stated in this License. 5. Submission of Contributions. Unless You explicitly state otherwise, any Contribution intentionally submitted for inclusion in the Work by You to the Licensor shall be under the terms and conditions of this License, without any additional terms or conditions. Notwithstanding the above, nothing herein shall supersede or modify the terms of any separate license agreement you may have executed with Licensor regarding such Contributions. 6. Trademarks. This License does not grant permission to use the trade names, trademarks, service marks, or product names of the Licensor, except as required for reasonable and customary use in describing the origin of the Work and reproducing the content of the NOTICE file. 7. Disclaimer of Warranty. Unless required by applicable law or agreed to in writing, Licensor provides the Work (and each Contributor provides its Contributions) on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. You are solely responsible for determining the appropriateness of using or redistributing the Work and assume any risks associated with Your exercise of permissions under this License. 8. Limitation of Liability. In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall any Contributor be liable to You for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising as a result of this License or out of the use or inability to use the Work (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if such Contributor has been advised of the possibility of such damages. 9. Accepting Warranty or Additional Liability. While redistributing the Work or Derivative Works thereof, You may choose to offer, and charge a fee for, acceptance of support, warranty, indemnity, or other liability obligations and/or rights consistent with this License. However, in accepting such obligations, You may act only on Your own behalf and on Your sole responsibility, not on behalf of any other Contributor, and only if You agree to indemnify, defend, and hold each Contributor harmless for any liability incurred by, or claims asserted against, such Contributor by reason of your accepting any such warranty or additional liability. END OF TERMS AND CONDITIONS Copyright 2025 Terminal Skills Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: terminal-skills Original project: https://github.com/TerminalSkills/skills/blob/8a9852d489f274538be9bf1c9121aeb403750b1c/skills/socketio/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Product Management, Design Roles: Frontend Developer, Backend Developer, Full-Stack Developer, Product Manager, UX Designer Specialties: Backend Tools: Socket.Io, Redis Country applicability: Country not specified Classification basis: Source category, role and specialization labels Instruction SHA-256: a507f0451aaf75e6baf2c8e6a4230be0ddbb59e4a70d06959783063e79c612dc