TLDR: What is the upper-bound on how long I should wait to guarantee that a GCE instance has been removed from the load-balancing path and can be safely deleted?
Details: I have a relatively standard setup: GCE instances in a managed instance group, global HTTPS load balancer in front of them pointed at a backend service with only the one managed instance group in it. Health checks are standard 5 seconds timeout, 5 seconds unhealthy threshold, 2 consecutive failures, 2 consecutive successes.
I deploy some new instances, add them to the instance group, and remove the old ones. After many minutes (10-15 min usually), I delete the old instances.
Every once in a while, I notice that deleting the old instances (which I believe are no longer receiving traffic) correlates with a sporadic 502 response to a client, which can be seen only in the load-balancer level logs:
I've done a bunch of logs correlation and tcpdumping and load testing to be fairly confident that this 502 is not being served by one of the new, healthy instances. In any case, my question is:
What is the upper-bound on how long I should wait to guarantee that a GCE instance has been removed from the load-balancing path and can be safely deleted?
