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Entry  01 Jan 2009, Konstantin Olchanski, Info, odb "hot link" magic explored 
    Reply  14 Jan 2009, Stefan Ritt, Info, odb "hot link" magic explored 
Message ID: 553     Entry time: 14 Jan 2009     In reply to: 546
Author: Stefan Ritt 
Topic: Info 
Subject: odb "hot link" magic explored 

KO wrote:
note 1: I do not completely understand the ss_suspend_xxx() stuff. The best I can tell is it creates a number of udp sockets bound to the local host and at least one udp rpc receive socket ultimately connected to the cm_dispatch_rpc() function.

The ss_suspend_xxx() stuff is indeed the most complicated thing in midas an I have to remind myself always
on how this works. So let me try again:

The basic idea is that for a high performance system, you cannot do the inter-process communication via
polling. That would waste CPU time. Inter-process communication is necessary for for buffer manager
(producer notifies consumer when new events are there), for the RPC mechanism (odbedit tells mlogger to
start a run) or for ODB hot-links. To avoid polling, the inter-process communication works with sockets (UDP
and TCP). This allows to use the select() call, which suspends the calling process until some socket
receives data or a pre-defined time-out expires. This is the only portable method I found which works under
unix and windows (signals are only poorly supported under windows).

So after creating all sockets, ss_suspend() does a select() on these sockets:

_suspend_struct[idx].listen_socket Server side for any new RPC connection (each client is also a RPC server which gets contacted directly during run transitions for example
_suspend_struct[idx].server_acception.recv_sock Receive socket (TCP) for any active RPC connection
_suspend_struct[idx].server_acception.event_sock Receive socket (TCP) for bare events (bypassing RPC layer for performance reasons)
_suspend_struct[idx].server_connection->recv_sock Outgoing TCP connection to mserver. Used for example for hot-link notifications from mserver
_suspend_struct[idx].ipc_recv_socket UDP socket for inter-process notification

For each socket there is a dispatch function, which gets called if that socket receives some data. Hope this sheds some light on the guts of that.
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