I have 4 modules :
1. command line interpreter ( CLI )
2. Requestprocessor ( REQPRO )
3. RequestHandler. ( REQHAND)
4. GUI
RequestProcessor (REQPRO ) interact with CLI on TCP/IP. i.e. interface between REQPRO and CLI is TCP/IP similarly
interface between REQPRO and REQHAND is also TCP/IP ,
GUI and REQHAND interact using TCP/IP.

[ CLI ] -------------------[ REQPRO ] ---------[ REQHAND ]---------[ GUI ]
---->Request-----------> <--------Request-------


1. CLI module act as a client and take command from user and pass is on TCP/IP to the REQPRO
which process request and send reply back to CLI.
2. REQHAND takes request from GUI and process it. If there are some critical requests then REQHAND send that
request to REQPRO
and REQPRO process that request and send the result to REQHAND and then REQHAND send back result to the GUI.
The REQHAND and REQPRO will communicate i.e. if i keep REQPRO as server and REQHAND as client then if REQPRO will down
REQHAND will also down.
but i want that REQHAND will always up ( not as client )
so tell me proper mid so that i will choose proper module as client and sever.

I am using C++/Solaris

I think you can have both REQPRO and REQHAND as daemons (will probably require two ports so that every one on them can listen). When they're separate processes you can turn them on and off any time. Then CLI and GUI will be clients.

I would tend to have one program pipe() twice and fork() to become the REQPRO and REQHAND. Then you have only one executable to start and you don't use up any extra tcp port numbers.

What is the reason for having the separate daemons? If the cli and gui make similar requests, you should probably have them appear exactly the same to the daemon.