ConfigProvider notification

[Tom Bentley <tb...@redhat.com>]

Colin asked that discussion about the future of the ConfigProvider
notification mechanism happen on the list (rather than in
https://issues.apache.org/jira/browse/KAFKA-9635 where I originally asked).

The background that I've pieced together is that the subscribe family of
methods were originally added in KIP-297 (which was focussed on adding
ConfigProviders for Kafka Connect), but subsequent discussion for KIP-421
(which was about enabling their use elsewhere–brokers, producers, consumers
etc) decided that using these methods to dynamically reconfigure e.g. a
broker was not desirable because there was no way to do so atomically if
multiple files were being changed.

I'm sure we all agree it would be nice to have a notification feature for
ConfigProviders, but the atomicity issue hasn't gone away. I suppose that
what's needed is a way to indicate that all the modifications to all the
config provider sources are now complete so that it is safe to start the
reconfiguration.

I'm happy to do some work on this and come up with a concrete proposal, if
no one is already working on it?

Cheers,

Tom