Available in all subroutines.
Whether or not this backend, or recursively any of the backends under this director, is considered healthy.
The random director has the additional constraint that the
must be met by the healthy backends under the director. The health state is
determined by healthcheck results, whether there is room for a new connection
to be made to the backend based on the number of currently used connections
and the backend's
max_connections setting, and any applicable saintmode