Implementing sticky sessions with socket.io and cluster module

[crupeng-1]

Hi,
I have an issue where socket sessionIds are not being remembered thanks to the same user connecting to different clusters causing errors. I could force socket.io to use webSockets only which solved the problem, but that isn't a good solution as it makes my connections unstable. All the Implementations with sticky sessions were not explicitly using the node.js cluster module. For more details see here: https://stackoverflow.com/questions/68142293/using-socket-io-sticky-sessions-with-node-js-cluster-module

[darrachequesne]

You can use the cluster adapter and the @socket.io/sticky package for this:

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) => {
    /* ... */
  });
}

Documentation:

• https://socket.io/docs/v4/using-multiple-nodes/#Using-Node-js-Cluster
• https://socket.io/docs/v4/cluster-adapter/

[crupeng-1]

Thank you so much, my code finally works now!