Client Server communication in Java - which approach to use?

Question

I have a typical client server communication - Client sends data to the server, server processes that, and returns data to the client. The problem is that the process operation can take quite some time - order of magnitude - minutes. There are a few approaches that could be used to solve this.

1. Establish a connection, and keep it alive, until the operation is finished and the client receives the response.
2. Establish connection, send data, close the connection. Now the processing takes place and once it is finished the server could establish a connection to the client to send the data.
3. Establish a connection, send data, close the connection. Processing takes place. client asks server, every n minutes/seconds if the operation is finished. If the processing is finished the client fetches the data.

I was wondering which approach would be the best way to use. Is there maybe some "de facto" standard for solving this problem? How "expensive" is opening a socket in Java? Solution 1. seems pretty nasty to me, but 2. and 3. could do. The problem with solution 2. is that the server needs to know on which port the client is listening, while solution 3. adds some network overhead.

Answer 1

I see an immediate problem with option 2. If the client is behind a firewall, he might very well be allowed to connect and do the request, but the server might be prevented to connect back to the cilent.

As you say, option 1 looks a bit nasty (not too nasty though, could work well), so among the options listed, I would go for option 3. Perhaps the server could estimate the time that's left of the processing, and hint the client, in each poll, of when it's about time to check back.

Answer 2

1. is good enought
2. will not work at many situations, for example wne client is under firewall, NAT, and so on. Server usually accepts incoming connections from everywhere, desktops usualy not
3. better than 1 just because you will haven't problems when connection is lost
4. solutions 1+3 - make long waiting connections, with periodical sleep and reconnect after. I mean: connect to server, wait 30 sec for data, if no data received, sleep for 10 sec, loop.

Opening sockets is sometimes expensive, but not so expensive that your data processing.