Question: cookies useless in cluster balancer ?

[welelay]

I've put a lot of console.log in balancer/route.js and can't figure out how cookies are used in cluster.

cluster-endpoint cookie

It seems endpointHash retrieved from cookies is always null in route.js#43.
I can't find a place in cluster where a cookie is set to just 'cluster-endpoint' so it seems to be useless to check for it.

I'm using cluster to load-balance between 2 instances of the same app (2 machines, both export the 'web' service).
By ignoring cookies, a new instance is picked at random for each request from the same client.
I thought it would assign the same client the same instance.

cluster-endpoint::web cookie

The client dutifully sends a cluster-endpoint::web cookie on each request, and the server responds with a same or different one at random.
Only when the service is the ui service is the cookie read, it seems utils.js#L91, but the client never sends /web/sockjs/info queries so again it seems to be useless.

Is it due to my configuration, or to support microservices, or a leftover ?
Please help me understand..