Which process notify Web Gateway when to switch to secondary/mirror IRIS

Which process notify Web Gateway when to switch to secondary/mirror IRIS? I'm trying to debug a case when the web gateway doesn't know about primary IRIS failure and is trying to send request to that instance which is causing 500 error. Also, no clue why at the same time I see that GET request to /csp/bin/mirror_status.cxw will return 200 OK.

Our setup is 2 web gateways, primary IRIS, secondary/mirror IRIS and an arbiter.