Cannot establish multiple connections to the same server at a time

[kmaehashi]

Derived from discussion in jubatus/jubatus#507 and https://gist.github.com/kmaehashi/8516915

When using session pool feature of msgpack-rpc, only one connection per server is allowed at a time.

[y-oda-oni-juba]

I think we have two approaches for this issue:

1. use 'multimap' instead of 'map' to implement session pool
   ( session pool have more-than-one sessions per server )

   pros:

   • maybe simple
   • performance: any clients can communicate same server completely simultaneously

   cons:

   • resources: in high-load situation, many sockets will be created

2. support 'pipeline' properly in msgpack-rpc
   ( one client can send a request to a server while other clients wait response from
   same server and same connection )

   pros:

   • resource saving

   cons:

   • hard-way?

   • performance: msgpack-rpc protocol doesn't support 'chunked' message.
     while a client is sending/receiving a huge message, other clients
     must wait it.

     ( But, in most cases, servers and proxies are running in same local network.
     we can ignore this issue? Or, multiple proxies on one client PC avoid this issue? )

[kmaehashi]

Thanks for the follow-up @y-oda-oni-juba! I think the first idea is better, because:

• For the only drawback of the first idea, "in high-load situation, many sockets will be created", users can workaround this by adjusting socket pool size.
• The second idea is a huge architecture change, which is almost rewriting the original msgpack-rpc code.

Could you start trying with the first idea, @gintenlabo?