I am trying to create a network architecture which has a single server and multiple clients. The clients can connect and disconnect at any time so they need to announce their existence or shut-down to the server. The server must be able to send data to any particular client.
What is the best scalability protocols/architecture to use for this?
Currently I use a REQ/REP so that clients can 'login' and 'logout', and a SURVEY socket so that the server can send data to all clients. The message sent has an ID for the particular client it wants to process the message.
Is this good, or is there something better?
Sounds more like you need publisher subscriber. With both 0MQ and nanomsg you don't need to do anything in particular to manage connection / disconnection, the library does that for you.
However if you want more sophisticated message management (such as caching outgoing messages just in case another client chooses to connect) then you will have to manage that yourself. You might use a single push pull from the clients for them to announce their presence (they'd send a message saying who they were), followed by more push pulls from the server to each of the clients to send the messages from the cache that you have. Fiddly, but still a whole lot easier than programming up with raw sockets.
Using req rep can be problematic - if either end crashes or unexpectedly disconnects the other can be left in a stalled, unrecoverable state.