Class ProcessManager

This enables a CSProcess to be spawned concurrently with the process doing the spawning.

Shortcut to the Constructor and Method Summaries.

Description

The ProcessManager class enables a CSProcess to be spawned concurrently with the process doing the spawning. The class provides methods to manage the spawned process: start, join and stop. The spawned process may, of course, be a Parallel network of processes to any depth of nesting, in which case the whole network comes under this management.

Spawning processes is not the normal way of creating a network in JCSP - the normal method is to use the Parallel class. However, when we need to add processes in response to some run-time event, this capability is very useful.

For completeness, ProcessManager is itself a CSProcess - running a ProcessManager simply runs the process it is managing.

Spawning a CSProcess

This example demonstrates that the managed CSProcess is executed concurrently with the spawning process and that it dies when its manager terminates. The managed process is `infinite' and just counts and chatters. The managed process is automatically terminated if the main Java thread terminates (as in the case, eventually, below).

 import org.jcsp.lang.*;
 
 public class ProcessManagerExample1 {
 
   public static void main (String[] argv) {
 
     final ProcessManager manager = new ProcessManager (
       new CSProcess () {
         public void run () {
           final CSTimer tim = new CSTimer ();
           long timeout = tim.read ();
           int count = 0;
           while (true) {
             System.out.println (count + " :-) managed process running ...");
             count++;
             timeout += 100;
             tim.after (timeout);   // every 1/10th of a second ...
           }
         }
       }
     );
 
     final CSTimer tim = new CSTimer ();
     long timeout = tim.read ();
 
     System.out.println ("\n\n\t\t\t\t\t
                         *** start the managed process");
     manager.start ();
 
     for (int i = 0; i invalid input: '<' 10; i++) {
       System.out.println ("\n\n\t\t\t\t\t
                           *** I'm still executing as well");
       timeout += 1000;
       tim.after (timeout);         // every second ...
     }
 
     System.out.println ("\n\n\t\t\t\t\t
                         *** I'm finishing now!");
   }
 }
 

Stopping, Interrupting, Race-Hazards and Poison

Stopping a Java thread releases any locks it (or any sub-process) may be holding, so this reduces the danger of other threads deadlocking through a failure to acquire a needed lock. However, if the stopped process were in the middle of some synchronised transaction, the data update may be incomplete (and, hence, corrupt) depending on the precise moment of the stopping. This is a race-hazard. Further, if some other thread later needed to interact with the stopped thread, it would deadlock.

Instead of stopping a JCSP process, it is much safer to interrupt it. This gives the process the chance to notice the interrupt (through an exception handler) and tidy up. If no such handler is provided and the JCSP process attempts any synchronisation afterwards, the process will bomb out with a ProcessInterruptedException.

For historical reasons, a stop() method is provided below – but it is implemented as interrupt() (and deprecated).

If the managed process has gone parallel, managing an interrupt to achieve a clean exit is more tricky. Stopping a network by setting a global volatile flag that each process polls from time to time is not safe. For example, a thread blocked on a monitor wait will remain blocked if the thread that was going to notify it spots the shut-down flag and terminates.

For JCSP processes, there is a general solution to this [`Graceful Termination and Graceful Resetting', P.H.Welch, Proceedings of OUG-10, pp. 310-317, Ed. A.W.P.Bakkers, IOS Press (Amsterdam), ISBN 90 5199 011 1, April, 1989], based on the careful distribution of poison over the network's normal communication channels.

However, JCSP now supports graceful termination of process networks and sub-networks through a notion of poisoning synchoronisation objects (e.g. channels) – see Poisonable.