Port address already in use error.

I'm reading the documentation but can not find what requirements does an application need to meet in order to be managed by PM2 in cluster mode?

I'm trying to run 'gatsby serve' in cluster mode, but when attempting to 'reload' the application, I see errors in the logs related to the port already being in use:

Error: listen EADDRINUSE: address already in use 127.0.0.1:9000
    at Server.setupListenHandle [as _listen2] (node:net:1330:16)
    at listenInCluster (node:net:1378:12)
    at GetAddrInfoReqWrap.doListen (node:net:1516:7)
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:73:8)
Emitted 'error' event on Server instance at:
    at emitErrorNT (node:net:1357:8)
    at processTicksAndRejections (node:internal/process/task_queues:83:21) {
  code: 'EADDRINUSE',
  errno: -98,
  syscall: 'listen',
  address: '127.0.0.1',
  port: 9000
}

The application doesn't start after reload command, but PM2 shows the status as "online," which is incorrect.

Start by reproducing gatsby serve under PM2 cluster mode, then run a reload and inspect the reported EADDRINUSE logs alongside PM2's online status. Done means the application reloads without the port conflict, or PM2 accurately reports the failed application state.