Health check.

the two-minute refresher

• The balancer calls an endpoint on an interval, for example every 30 s with a 5 s timeout.
• A server is marked unhealthy after N consecutive failures and healthy again after M successes.
• Thresholds avoid flapping: one slow response should not eject a server.
• Check what matters: a /health that only returns 200 can hide a dead database connection.