What is the best practice on socket programming -- do I do a close every time or leave it open?

Question

I haven't found a clear answer on this one.

I have a client/server application in Java 7. The server and client are on seperate computers. The client has a short (1 line of 10 characters) command to issue to the server and the server responds (120 character string). This will be repeated every X seconds--where X is the rate in the configuration file. This could be as short as 1 second to Integer.MAX_VALUE seconds.

Every time that I've created a client/server application, the philosophy has been create the connection, do the business, close the connection and then do whatever else with the data. This seems to be the way things should be done--especially when using the try with resources programming.

What are the hiccups with leaving a socket connection hanging out there for X seconds? Is it really a best practice to close down and restart or is it a better practice for the socket to remain connected and just send the command every X seconds?

Answer 1

I think the answer depends a bit on the number of clients you expect to have.

If you will never have very many client connections open, then I'd say leave the connection open and call it good, especially if latency is an issue - even on LANs, I've seen connections take several milliseconds to initialize. If you expect hundreds or thousands of clients to connect and do this, however, I would reconnect every time. As others have said, leaving non-blocking sockets open will often mean you have a thread left running, which can take several megabytes of stack space on a per-thread basis. Do this several thousand times and you will have a big problem on most machines.

Another issue is port space. Just because the TCP/IP stack gives us 65535 total ports doesn't mean all are usable - in fact, most local firewalls will prohibit most from being used, so even if you had enough memory to run thousands of simultaneous threads, you could very likely run out of ports if you leave a lot of connections open simultaneously.