Hi Paul,
Just a quick suggestion regarding your comment: "/we check for
outstanding scheduled Runnables and then sleep for 5 seconds/".
ExecutorService exposes /shutdown/ and /awaitTermination/ methods to
basically do exactly what you need - graceful shutdown of the thread pool.
Cheers
On 15/06/2016 07:26, Paul Hoadley wrote:
Hello,
We have an app that uses a ScheduledExecutorService to delay some interactions
with another web service. The basic use case here is that a user logs in,
performs some action that results in a Runnable being sent to the
ScheduledExecutorService with a delay so that it’s executed later, and then the
user logs out. This all works fine, except where it doesn’t.
One place where it doesn’t work fine is if we want to update the running app
and terminate an instance—any app instance might be holding Runnables that are
scheduled to execute later, and yet have no active sessions and would thus be
candidates for a shutdown via “refuse new sessions”. Until now, we’ve been
keeping track of the scheduled Runnables somewhat indirectly (the details don’t
matter), and it’s been reasonably successful. However, it occurred to me that I
might be able to use gracefulTerminate() to wait until there are no more
outstanding Runnables in the queue. In development, this works just fine: using
a while loop we check for outstanding scheduled Runnables and then sleep for 5
seconds. When there are none, we call super.gracefulTerminate() and the app
shuts down.
My questions:
1. I assume that setting "refuse new sessions” on an instance in JavaMonitor
will eventually send a SIGTERM to the app which will be caught by the graceful
shutdown mechanism. (Hitting the stop button in Eclipse, for example, doesn’t do
that and so gracefulTerminate() isn’t called. You have to simulate it by sending
SIGTERM to the Java process from a terminal.) That is, will this even work in
production? (I can’t imagine the answer here is no, but would be good to know if
anyone’s doing/done it.)
2. The Javadocs for gracefulTerminate() talk about "last minute cleanup”, but I
assume we can wait an arbitrary amount of time here. We could conceivably have
delays of _hours_ on some of the Runnables in the queue—is this going to bother
JavaMonitor (or anything else)? (I assume JavaMonitor will continue to honour
“refuse new sessions”, for example.)
Obviously we will test this out, but if someone is doing anything like this, it
would be great to hear about it. Finally, yes, there are probably much better
ways to achieve the outcome here (database-backed Quartz Scheduler, for
example), but this was the solution that fit into the budget constraints.
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