title: K8s graceful upgrade of service with long-running connections
body: <p>tl;dr: I have a server that handles WebSocket connections. The nature of the workload is that it is necessarily stateful (i.e., each connection has long-running state). Each connection can last ~20m-4h. Currently, I only deploy new revisions of this service at off hours to avoid interrupting users too much.</p>
  <p>I'd like to move to a new model where deploys happen whenever, and the services gracefully drain connections over the course of ~30 minutes (typically the frontend can find a &quot;good&quot; time to make that switch over within 30 minutes, and if not, we just forcibly disconnect them). I can do that pretty easily with K8s by setting gracePeriodSeconds.
  However, what's less clear is how to do rollouts such that new connections only go to the most recent deployment. Suppose I have five replicas running. Normal deploys have an undesirable mode where a client is on R1 (replica 1) and then K8s deploys R1' (upgraded version) and terminates R1; frontend then reconnects and gets routed to R2; R2 terminates, frontend reconnects, gets routed to R3.</p>
  <p>Is there any easy way to ensure that after the upgrade starts, new clients get routed only to the upgraded versions? I'm already running Istio (though not using very many of its features), so I could imagine doing something complicated with some custom deployment infrastructure (currently just using Helm) that spins up a new deployment, cuts over new connections to the new deployment, and gracefully drains the old deployment... but I'd rather keep it simple (just Helm running in CI) if possible.</p>
  <p>Any thoughts on this?</p>