[UIMA-6412] Stop using ThreadGroup in CPMEngine

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 3.3.0SDK
• Component/s: Collection Processing
• Assignee: Richard Eckart de Castilho
• Reporter: Richard Eckart de Castilho
• Created: 19/Jan/22 08:37
• Updated: 18/Feb/22 16:05
• Resolved: 10/Feb/22 08:23

Description

The CPMEngine internally uses a ThreadGroup to manage the worker threads. This causes a problem with concurrency features present in modern Java versions, e.g. the static system-wide ForkJoinPool.commonPool() which is used by concurrency features by default when no other pool is specified. But even if a custom pool is used, the problem persists. The problem is that threads launched from within the context of a thread that is part of a thread group inherits that thread group as its parent. Consequently, worker threads launched e.g. by the ForkJoinPool.commonPool() become part of the CPM thread group. The common pool worker threads are long-lived and re-used. the CPM thread group however refuses to shut down as long as there are still threads in it. That essentially blocks the use of many concurrency features within the scope of a CPE.

The best approach would probably be to replace the CPMThreadGroup with some kind of CPMExecutorService which essentially performs the same duties as the thread group but without being a thread group.

• CPMThreadGroup.uncaughtException() - instead of handling the uncaught exceptions at the level of the thread group, an exception handler needs to be installed in the individual spawned threads
• ThreadGroupDestroyer - this is the code that currently waits until all threads in the CPMThreadGroup are finished and then destroys the group to avoid CPMThreadGroup-instances piling up over time. I guess the replacement would be to call shutdown() on the replacement executor service. shutdown() causes the executor service not to accept any new tasks and once all running tasks are done, it cleans itself up.
• worker threads must not be started by callling start() explicitly but rather by submitting them to the executor service
• The CPE allows configuring a custom output queue (instance of BoundedWorkQueue. There is one subclass of BoundedWorkQueue provided by UIMA which is the SequencedQueue and it internally spawns a new ExpirationTimer thread. In the current implementation, this timer thread would block the thread group from shutting down. However, the task of the ExpirationTimer is just to remove a map entry after a certain amount of time - it is not a long running thread. So I think that it doesn't matter that it won't block the execution service from shutting down and also that we do not have to change the API for registering custom output queues to pass the execution service into the custom queues. There might be custom queues which rely on the thread group being blocked, but I somewhat doubt it - and if so, those will then have to change their implementation... I'd still call this a feature release change and not a major release change.
• This removes the public CPMThreadGroup class and replaces it with a CPMExecutorService. Again this is a change of an API that is in principle public and such changes should only be done in major releases. But again, I doubt that anybody really depends on this particular API. I think it is mainly public because the class is used by another class in a different package and otherwise it should be considered internal. So - IMHO - fine for a feature-level release.

Issue Links

• supercedes: UIMA-1906 Daemon thread prevents CPE exit
• links to: GitHub Pull Request #165