Multiprocessing uses a subprocess / fork / exec / wait framework
which gives us 100% control over the process running. It would also
allow the engine cancel checkpoints instead of relying on the
checkpoints themselves to implement a cancel method.
That's one item we need to discuss and get other people's opinion.
With the current model of having the engine
suggest the checkpoint should quit and trust that the checkpoints
will behave and quit at it's earliest convinience,
the engine really has no control over whether the checkpoint. The
good thing about doing it this way is that the checkpoint
can come to a "good" stopping point and quit. The disadvantage is of
course that the engine has no control
over what a checkpoint does.
With the MP module, and the engine controlling the checkpoint, one
approach could be to roll back to the last successful dataset snapshot
and end execution there. I'm also not suggesting removing the
ability of the individual checkpoints to control execution. MP has
similar IPC controls that can be used to tell the engine, "Hey!
Something's wrong! Can you stop?"
We could certainly have separate cancel() and kill() methods if we used
MP, which would allow for both, or add a timeout to cancel() which, if
the checkpoint didn't cease by that time, the engine could forcibly shut
it down. I definitely think the MP module is worth exploring. I think
the only potential for hangup is finding out how easy it is to
coordinate DOC updates across the separate processes, but that seems
like it would be surmountable.
- Keith
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