CLUSTER parameter* == CLUSTER=[protoList]:ServerListServerList == [/R,]Server[,ServerList]
Server == Host[..Port]
The CLUSTER parameter defines an ordered set of alternative or
complemental servers (origin or proxy).
It is referred when DeleGate failed to connect or authenticate with
an upstream proxy server or an origin server.
It is applied to proxy server specified in
MASTER, PROXY, SSLTUNNEL, SOCKS,
or an origin server specified in SERVER or
in the right-hand of MOUNT.
If a list is prefixed with "/R", the servers in the list are tried
in random order (the first server to be tried is selected randomly and
other servers are tried in the round-robin order).
It could be useful for load balancing among equivalent (proxy) servers.
The retrial by this parameter is commonly applied to servers of any protocols
in the phase of establishment of a TCP connection to a server.
The retrial covers the authentication phase for several protocols.
In HTTP origin/gateway servers, the retrial may be caused depending on
the response from the server, including the response code 503
(Service Unavailable) and 404 (Not Found) for example.