TypeError: Cannot read properties of undefined (reading 'id') when using createAdapter() from @socket.io/cluster-adapter

[michioxd]

Hi, I'm trying to set up Socket.IO with cluster support using PM2, following this docs.

Here's my code:

import { createAdapter } from "@socket.io/redis-adapter";
import { createAdapter as createClusterAdapter } from "@socket.io/cluster-adapter";
import { setupWorker } from "@socket.io/sticky";

const app = express();
export const server = http.createServer(app);
export const io = new Server(server, {
    transports: ['websocket', 'polling'],
    adapter: createAdapter(redisClient, redisSubClient)
});

io.adapter(createClusterAdapter()); // src/server.ts:76:4
setupWorker(io);

While I try to run my code, it showing this error

TypeError: Cannot read properties of undefined (reading 'id')
    at new ClusterAdapter (/home/michioxd/backend/node_modules/.pnpm/@socket.io+cluster-adapter@0.2.2_socket.io-adapter@2.5.5/node_modules/@socket.io/cluster-adapter/dist/index.js:61:34)
    at new <anonymous> (/home/michioxd/backend/node_modules/.pnpm/@socket.io+cluster-adapter@0.2.2_socket.io-adapter@2.5.5/node_modules/@socket.io/cluster-adapter/dist/index.js:40:16)
    at Namespace._initAdapter (/home/michioxd/backend/node_modules/.pnpm/socket.io@4.8.1/node_modules/socket.io/dist/namespace.js:99:24)
    at Server.adapter (/home/michioxd/backend/node_modules/.pnpm/socket.io@4.8.1/node_modules/socket.io/dist/index.js:188:17)
    at file:///home/michioxd/src/server.ts:76:4
    at ModuleJob.run (node:internal/modules/esm/module_job:274:25)
    at onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:644:26)

Command I used to run in PM2:

pm2 start index.js -i max

Thanks!

[Mike-Animal-Counseling]

That stack points to cluster.worker being undefined in the process that executes createClusterAdapter(). Version 0.2.2 reads cluster.worker.id during construction; Node only provides cluster.worker inside a cluster worker.

Log the actual runtime context immediately before that line:

import cluster from 'node:cluster';
console.log({
  isPrimary: cluster.isPrimary,
  isWorker: cluster.isWorker,
  workerId: cluster.worker?.id,
});

In a worker, expect isWorker: true and a worker ID. If not, inspect which entry point/process PM2 is actually launching; adding optional chaining inside the adapter would hide the missing worker context rather than establish it.

There is also a separate configuration issue: supplying the Redis adapter in new Server(...), then calling io.adapter(createClusterAdapter()), replaces the adapter; it does not combine Redis and cluster IPC. Choose one setup:

• For the IPC/sticky-worker example, follow the complete @socket.io/pm2 guide. Its process manager supplies the corresponding primary-side routing and IPC relay; copying only the worker code is insufficient.
• If you want your existing Redis adapter across processes/hosts, keep that adapter and configure session affinity separately while HTTP long-polling is enabled. The adapter documentation explains that Redis does not remove that requirement.

I reproduced the same reading 'id' error with Socket.IO 4.8.1 / cluster-adapter 0.2.2 in a normal Node primary process; adapter construction succeeded in a real cluster.fork() worker. That verifies the missing-worker failure mode, not your particular PM2 deployment.

Disclosure: AI-assisted investigation and writing; the primary/worker comparison was executed locally on Node 24.16.0.