Why sequential calls of fetchSockets() return different result (Postgres Adapter)?

[mezinov95]

We have several Socket.io servers deployed in k8s clusters and working concurrently. We use Postgres Adapter. Clients connect to websocket server randomly. And for business purposes we need to detect if any user online right now. That's why we need to fetchSockets() and send some message like ping with acknowlegement.
And the problem I encountered with is:

const sockets = await this.server.in('my-room').fetchSockets()
console.log(sockets.length); // prints 0 although there is definitely one connected client on another server

// If I sequentially call fetchSockets() several times more the result is different. Even though no clients have connected or disconnected. For example:
const sockets2 = await this.server.in('my-room').fetchSockets()
console.log(sockets2.length); // prints 1

[mezinov95]

UPD:
I found a one-line workaround. The source code study of cluster-adapter.ts helped
After the server is initialized, we send an arbitrary service message to other servers so that they understand how many servers are currently in the cluster
After sending the service message, heartbeat messages will start to be sent periodically between the servers and each server will know about other active servers
For completeness of context, look at the methods serverSideEmit() -> publish() -> scheduleHeartbeat() sequentially
If service message is not sent, servers will not know about each other and, in particular, fetchSockets() method will return only local sockets

In my code it looks like (we use Nest.js as backend framework)

afterInit(server: Server) {
    server.use(async (socket, next) => {
      const username = getUsernameFromSocket(socket);
      socket.join(username);
      
      next();
    });

    server.serverSideEmit('server.init'); // Sending service message helps
}