How to convert batch application to server app?

Question

I have an application that performs some processing task in batch mode: data in, processing, data out. The application can be accessed through a well-documented API but is not thread-safe -- one cannot run the processing function in two threads concurrently.
In order to make that processing accessible to a program written in another language I would like to encapsulate it into some form of server.
I would like to find some minimum overhead solution where the other program sends the data to the server app in some way, waits for the processing to complete and reads back the result. One option would be to use the HTTP protocol with a POST request, another to write the data to a database row and send some command over a socket.
My main problem is: which solution is the easiest to implement and use and the one that allows to serialize the processing with the least effort. The solution should make it easy to prevent deadlocks or similar nuisances.
Are there some standard patterns for doing this or what would you Wizards recommend?

Answer 1

Thanks for the answers!
I must admit that I am still rather clueless in general about the issues involved here.
My immediate need is to send text files to the java application and get back processed text files. At the moment I have to do this from perl, but in the future, other languages (mostly scripting languages) might also need to do this.
So what I am trying to achieve is this: the client should be able to access the Java application/service as if it was some synchronous write/read operation (similar to a HTTP post request waiting for a response). There might be several clients working in that way but the the server should make sure that they get cleanly serialised: only one text file can be processed at any one time because the API is not thread safe.
I also cannot change the API itself -- just wrap into something that will do what I described. The more lightweight,easy to use, and easy to implement the solution the better.
For example, is there some kind of mini HTTP server and some serialization mechanism in Java that would make it possible to run the Java application disguised as a web service? Are there disadvantages to this and other solutions that are more suited?
I have looked at JMS and it does make a bit of a heavyweight impression to me. Also I'd prefer a solution where the clients use some mechanism that is more general, e.g. HTTP or the file system or maybe even a RDBS table (not sure what the possible traps with regards to deadlocks, timeouts etc. are there).