How can it perform a rolling update without downtime?

[rikuson]

I'm hosting socket.io with a Redis stream adapter on ECS, and I'm encountering two issues when trying to perform a rolling update.
I don't use sticky session and set transports: ["websocket"] .

Connection state recovery is a feature which allows restoring a client's state after a temporary disconnection, including any missed packets.

Does this feature only apply to client-side disconnections? What if the server crashes and restarts? Is there a way to restore a client's state in this case?

Based on my investigation, it seems that the system treats reconnection as a new connection with a new socket ID and doesn't map the state to the reconnected client.

Additionally, during the rolling out of pods, the server triggers events: "leave-room", "disconnect" and "delete-room" with showing the log below.

socket.io:client client close with reason transport close

There is my application code that erases some states from Redis in "disconnect" event handler.
Ideally, pods die before triggering the "disconnect" event.

Is there a standard solution for these issues?

[darrachequesne]

Hi!

Does this feature only apply to client-side disconnections?

No, it also works when the Socket.IO server is cleanly stopped:

process.on('SIGTERM', () => {
  io.close();
});

The connections will be closed with a server shutting down reason, which is in the list of recoverable reasons:

socket.io/lib/socket.ts

Lines 50 to 57 in 9a2a83f

	const RECOVERABLE_DISCONNECT_REASONS: ReadonlySet<DisconnectReason> = new Set([
	"transport error",
	"transport close",
	"forced close",
	"ping timeout",
	"server shutting down",
	"forced server close",
	]);

So the connection state will be persisted in the store (if the process has enough time to do so) and restored upon reconnection.

[rikuson]

Thank you!

No, it also works when the Socket.IO server is cleanly stopped:

Right, calling close method kept the state.

There is my application code that erases some states from Redis in "disconnect" event handler.

For this problem, maybe I can do something in the SIGTERM handler.