Cluster adapter

How it works

The Cluster adapter allows using Socket.IO within a Node.js cluster.

Every packet sent to multiple clients (e.g. io.to("room1").emit() or socket.broadcast.emit()) is also sent to other workers via the IPC channel.

The source code of this adapter can be found here: https://github.com/socketio/socket.io/tree/main/packages/socket.io-cluster-adapter

Supported features

Feature	socket.io version	Support
Socket management	4.0.0	✅ YES (since version 0.1.0)
Inter-server communication	4.1.0	✅ YES (since version 0.1.0)
Broadcast with acknowledgements	4.5.0	✅ YES (since version 0.2.0)
Connection state recovery	4.6.0	❌ NO

Installation

npm install @socket.io/cluster-adapter

Usage

With Node.js cluster

const cluster = require("cluster");
const http = require("http");
const { Server } = require("socket.io");
const numCPUs = require("os").cpus().length;
const { setupMaster, setupWorker } = require("@socket.io/sticky");
const { createAdapter, setupPrimary } = require("@socket.io/cluster-adapter");

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  const httpServer = http.createServer();

  // setup sticky sessions
  setupMaster(httpServer, {
    loadBalancingMethod: "least-connection",
  });

  // setup connections between the workers
  setupPrimary();

  // needed for packets containing buffers (you can ignore it if you only send plaintext objects)
  // Node.js < 16.0.0
  cluster.setupMaster({
    serialization: "advanced",
  });
  // Node.js > 16.0.0
  // cluster.setupPrimary({
  //   serialization: "advanced",
  // });

  httpServer.listen(3000);

  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on("exit", (worker) => {
    console.log(`Worker ${worker.process.pid} died`);
    cluster.fork();
  });
} else {
  console.log(`Worker ${process.pid} started`);

  const httpServer = http.createServer();
  const io = new Server(httpServer);

  // use the cluster adapter
  io.adapter(createAdapter());

  // setup connection with the primary process
  setupWorker(io);

  io.on("connection", (socket) => {
    /* ... */
  });
}

With PM2

See the associated documentation.

With recluster

cluster.js

const cluster = require("cluster");
const http = require("http");
const { setupMaster } = require("@socket.io/sticky");
const { setupPrimary } = require("@socket.io/cluster-adapter");
const recluster = require("recluster");
const path = require("path");

const httpServer = http.createServer();

// setup sticky sessions
setupMaster(httpServer, {
  loadBalancingMethod: "least-connection",
});

// setup connections between the workers
setupPrimary();

// needed for packets containing buffers (you can ignore it if you only send plaintext objects)
// Node.js < 16.0.0
cluster.setupMaster({
  serialization: "advanced",
});
// Node.js > 16.0.0
// cluster.setupPrimary({
//   serialization: "advanced",
// });

httpServer.listen(3000);

const balancer = recluster(path.join(__dirname, "worker.js"));

balancer.run();

worker.js

const http = require("http");
const { Server } = require("socket.io");
const { setupWorker } = require("@socket.io/sticky");
const { createAdapter } = require("@socket.io/cluster-adapter");

const httpServer = http.createServer();
const io = new Server(httpServer);

// use the cluster adapter
io.adapter(createAdapter());

// setup connection with the primary process
setupWorker(io);

io.on("connection", (socket) => {
  /* ... */
});

Options

Name	Description	Default value
heartbeatInterval	The number of ms between two heartbeats.	5_000
heartbeatTimeout	The number of ms without heartbeat before we consider a node down.	10_000

Latest releases

Version	Release date	Release notes	Diff
0.3.0	October 2025	link	0.2.2...0c43124
0.2.2	March 2022	link	0.2.1...0.2.2
0.2.1	October 2022	link	0.2.0...0.2.1
0.2.0	April 2022	link	0.1.0...0.2.0
0.1.0	June 2021	link

Complete changelog: https://github.com/socketio/socket.io/blob/main/packages/socket.io-cluster-adapter/CHANGELOG.md