Client/Server API with push?

Question

I set up a simple Socket client server architecture with the Java socket API. However, my clients sometimes are initiators or requests, and sometimes my server is the initiator of infos. Example: client want to get infos from server but also want to tell the server something. The server tells all registered clients something but also can accept requests from clients. So both sides are client AND server (depending on what they currently want to do).
The socket API is just a classic client/server API, i.e. a server listens and a client sends requets and get responses. For my server to send requests to all clients, this is not enough. Is the only way to have all clients offering a socket server on their computer, too? so that this 2-way-request-communication works?
This is basically some kind of "push" technology, but i don't want my clients to "poll" the server just to emulate the feature of sending requests from the server to the client. Is there some other (perhaps non-socket) API that offers this?

Answer 1

@sjasja: how is the socket api symmetrical?It is, trust me! Connection initiation isn't, but after that it is. (Let's assume connected TCP/IP.)
The server does bind() and accept(), the client does connect(). After that, the connection is symmetrical: either end can issue any number of read() and write() calls in any order. You write() to a socket, it pops out from read() at the other end (though not necessarily as a single packet; TCP/IP is stream oriented, so write() "packets" can and will be glued together, or fragmented at the whimsy of network routers and friends.)
- accept() //wait for client to send request
- socket.getInputStream() //read request
- socket .getOutputStream() //write responseThose don't read or write; they get streams which you can read from and write to.
In the server, when accept() returns a new connection, you'd start a reader thread for that client. The thread would read() (or readLine() if you implement a line-oriented protocol) and handle the incoming messages. Whenever you want to write to the client, write to the stream returned by socket.getOutputStream(). You may need to synchronize writes if you have several threads that do sequences of write()s that need to arrive at the client sequentially.
As to the difficulty of socket programming: I don't think it particularly difficult. But then I've written dozens of socket programs since my first one in '85... The Java Tutorial socket chapter shows a simple client/server pair; not terribly difficult I think. In particular, check out the KKMultiServer at the bottom of the sample server chapter.