[erlang-questions] Stopping a master process and all its workers

[Jesper Louis Andersen]

On Thu, Apr 12, 2018 at 4:47 PM Torben Hoffmann <torben.lehoff@REDACTED>
wrote:

> Are there any subtleties that I need to cater for? Have I given enough
> information for this question to make sense?
>
>
Yes:

* What is the API accessing this tree? If we start stopping the tree, how
are those API calls going to behave while the tree is being closed down?

* Many such trees needs some kind of "connection draining phase" where they
finish their current work, but doesn't start up new work while they are
being drained.

* If you dynamically start/stop workers, then you might be able to set the
number of workers to the special case of 0 and then stop the tree.

* Surely, there is a supervisor on top of `top_sup` and it it the one who
needs to terminate its child. Consider that some supervisor in your
application has to be "permanent/persistent" over the lifetime of the
application, so you always have a point to which you can "hang" your
workers. This allows you to use supervisor:terminate_child/2, but do note
its documentation about restarting: your child is likely to be temporary,
which means you need to have some kind of management for this if restarts
happen in the system.

* Dynamic alteration of the state should be logged: "worker state was
changed from 8 workers to 0", but it shouldn't report such an event as an
ERROR in the syslog sense. This is INFO/NOTICE level.

Final important comment:

Do extensive tests of the failure scenario! Graceful recovery is nice, but
if you don't test it somewhat, you are essentially sacrificing a goat on
the altar of the god of your choice and you pray to said god that things
end up being nice for you.
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