Long running process remote termination? - linux

Question

I am developing an application that allows users to run AI algorithms on the server remotely. Some of these algorithms take a VERY long time. It is set up such that AJAX calls supply the algorithm parameters and launch a C++ algorithm on the server. The results and status of the computation are tracked via AJAX calls polling status files. This solution seems to work well for multiple users concurrently using the service, but I am now looking for a way to cancel the computation from the user's browser. I have a stop button that stops the AJAX updating service and ceases any communication between the browser and the running process on the server. The problem is that the process still runs, and I would like to free up the server resources when the user cancels the operation. Below are some more details.
The web service where the AJAX calls hit are run under the user 'tomcat' and can be listed by ps -U tomcat. The algorithm executions are all child processes of 'java' and can be listed by ps --ppid ###.
The browser keeps a record of the time that the current computation began (user system time, not server system time).
Multiple computations may be going on at once from users connected from different locations, resulting in many processes under the same name and parent process.
The restful service executes terminal commands via java runtime.exec().
I am not so knowledgeable about shell scripting, so any help would be greatly appreciated. Can anyone think of a way to either use java process object or shell script/awk to locate a process via timestamp (maybe the closest timestamp to user system time..?) or some other way?
Thanks in advance.
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Is there even a way in java to get a handle for a given process if you have the pid...? Doesn't seem like it.
--edit
I cannot change the source code of the long running process on the server. :(

Answer 1

Your AJAX call should be manipulating some sort of a resource (most conveniently a text file) that acts as a semaphore to the process, which in every iteration of polling checks whether that semaphore file has been set to the stop status. If the AJAX changes the semaphore file to stop, then the process stops because your application checks it and responds accordingly. Which in turn means that the functionality needs to be programmed into your Java AI application rather than figuring out what the PID is and then killing it at the OS level. That, of course, assumes you have access to the source code of the app.
Of course, the semaphore does not have to be a file but can be a value in the DB etc., whichever suits your taste and configuration.

Answer 2

I have finally found a secure solution. From the restful java service, using Process p = Runtime.getRuntime().exec() gives you a handle on the running process. The only way, however, to get the pid is through a technique called reflection.
Field f = p.getClass().getDeclaredField();
f.setAccessible(true);
String pid = Integer.toString(f.getInt(p));
How unbelievably awkward...
Anyways, due to the passing of p from the server to the client being impossible, and the insecurity of allowing a remote call to kill an arbitrary server process by a pid passed by parameter, the only logical strategy I could come up with was to write the obtained pid to a process-unique file indicated by the initial client timestamp, and to delete this file upon restful service function return. This unique file can be used as a termination handle via yet another restful service which reads the file, and terminates the process with pid equal to the contents of the file. This

Answer 3

You could keep the Process instance returned by runtime.exec and invoke Process.destroy to kill the subprocess. Not knowing much about your webservice application I would assume you can keep the process instances in a global session map that maps users to process lists. Make sure access to this map is thread-safe. Also it only works if you have one webservice process that allows to share such a global session map across different requests.
Alternatively take a look at Get subprocess id in Java.