health

const (
	// DefaultPoolWait is how long one probe waits for a connection. It is
	// under the one second a readiness probe is given by default.
	DefaultPoolWait = 500 * time.Millisecond
	// DefaultPoolStarvedFor is how long the pool must go without handing one
	// out before the replica reports not ready, so a busy pool that frees a
	// connection now and then is not taken out of service.
	DefaultPoolStarvedFor = 30 * time.Second
)

Pool starvation (#2169). A replica whose database pool hands out no connection serves nothing that reads the database, yet the process state alone kept /readyz at 200 while one did. A pool every connection of which is in use, and stays so, takes the replica out of service. A database that is down is not this: its pool opens no connections, and /readyz leaves an upstream outage to the dependency probes so it never takes every replica out at once.