Hi Cory
I think this is another symptom of the deeper issue that the reactive
framework treats events like states. 'config.changed' is an event. The
following code is something that intuitively seems correct. We want to
reinstall when the config has changed while the service is installed.
However, it will still have the unwanted side effect you stated earlier.
@when('installed', 'config.changed.install_source')def reinstall():
install()
Please note that *your fix will only work when the service is installed
during the first config-changed hook*. If a service is installed during a
subsequent config-changed hook, you will again have the same issue. This
can happen when you have config options such as "(bool)install_plugin-x"
and "(string)plugin-x-source".
Anticipating these kind of conflicts requires a thorough understanding of
both the reactive framework and hooks. You are correct in thinking that
these conflicts should not happen. If we require every Charmer to have full
understanding of these things, we might miss out on valuable contributions.
I would urge you to seriously consider making the differentiation between
events and states. For people who have used hooks it might seem logical
that config.changed is active during an entire hook. Newcomers might have
more difficulty understanding this.
So my suggestion is:
- An event is only active right after the event happens.
- A handler can only be added to the queue when his events + his states
are active
- A handler will be removed from the queue only when one of his states
becomes inactive. Events of handlers that are in the queue are not
'rechecked'.
Another use-case for this:
@when('service.running', 'configfile.changed')
def restart_service()
1. When the config file changes, and the service is running, restart the
service.
2. When the config file changes and the service is not running, don't
restart the service.
3. When the config file changed before the service was running, and now we
start the service, don't restart the service.
4. When the config file changes, the service restarts, and the config file
changes again, we want to restart the service again.
1 and 2 are currently possible. 3 and 4 would be if 'file.changed' would be
an event.
Kind regards
Merlijn Sebrechts
2016-04-22 23:02 GMT+02:00 Cory Johns <[email protected]>:
> I have proposed https://github.com/juju-solutions/layer-basic/pull/61 as
> a slight change to how the config.changed states from the basic layer
> work. Currently, the changed states are set during the first hook
> invocation, under the assumption that the values were "changed" from
> "nothing" (not being set at all). However, this is slightly problematic in
> a case like the following, where we expect install() to only be called
> once, unless the value has changed after the fact:
>
> @when_not('installed')def install():
> # do install
> set_state('installed')
> @when('config.changed.install_source')def reinstall():
> install()
>
>
> The proposal adds new states, config.new, and changes config.changed to
> not be set the first time. You could get the old behavior by saying
> @when_any('config.new.foo', 'config.changed.foo').
>
> Is anyone depending on the current behavior? Are there any objections to
> this change?
>
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