A rule that skips servers with "tripped" circuit breaker and picks the
server with lowest concurrent requests.
This rule should typically work with ServerListSubsetFilter
which puts a limit on the
servers that is visible to the rule. This ensure that it only needs to find the minimal
concurrent requests among a small number of servers. Also, each client will get a random list of
servers which avoids the problem that one server with the lowest concurrent requests is
chosen by a large number of clients and immediately gets overwhelmed.