It's mentioned numerous times in the Apache documentation for mod_proxy
Connection pool worker retry timeout in
seconds. If the connection pool worker to the backend server is in the
error state, Apache will not forward any requests to that server until
the timeout expires. This enables to shut down the backend server for
maintenance, and bring it back online later. A value of 0 means always
retry workers in an error state with no timeout.
BalanceMember inherits all keys provided by
ProxyPass, so you can use
#Retry balance members every 60 seconds
BalanceMember http://app1/ retry=60
status=+H implys a balance member will be a 'hot standby'. So with the config
BalanceMember http://app1/ retry=30
BalanceMember http://app1-backup/ retry=60 status=+H
When app1 goes down, the hot standby will be used instead. After 30 seconds Apache will retry with app1, if it fails it will switch back over to the hot standby. 5 seconds later, app1 comes back online, but Apache will still use app1-backup until the timeout has expired, then after another 30 seconds when Apache retrys the main server it will succeed. Under normal usage, app1-backup will never be used.