Reactive when_file_changed decorator question

[Merlijn Sebrechts]

Thank you for this. I proposed the idea of events here:
https://github.com/juju-solutions/charms.reactive/issues/49

I think it would solve a lot of use-cases and bring some clarity in how
these non-state predicates are handled.

2016-01-28 20:15 GMT+01:00 Cory Johns <cory.johns at canonical.com>:

> Merlijn,
>
> You are correct, the order of handlers is undetermined, so it is entirely
> possible for the execution order to be: handler1, handler2, handler3,
> handler2.
>
> The predicates, including the @when_changed test, will be checked again
> after each round of dispatch, so handler2 should definitely be called again
> after handler3.  If you're not careful, this can lead to inefficiencies in
> your charm, which has been discussed previously here, and on issues on the
> charms.reactive repo (
> https://github.com/juju-solutions/charms.reactive/issues).  Stuart
> Bishop, in particular, has raised several very good concerns about ensuring
> intelligent handling of expensive operations such as apt-get calls or
> service restarts, when dealing with constructs such as these.
>
> While checking the code to verify my answers here, I realized that the
> logic for managing re-invocation of state handlers (so that, e.g.,
> @when('A') doesn't just keep matching and executing over and over in a
> given hook invocation, while still allowing it to run again if the A state
> changes) will override non-state predicates such as @when_file_changed.
> That means that if you combine @when and @when_file_changed, handler2 might
> *not* get reinvoked the second time (at least not until the next hook).  I
> opened https://github.com/juju-solutions/charms.reactive/issues/49 to
> track this.
>
> There are also a couple of other open issues regarding @when_file_changed
> that you should be aware of:
> https://github.com/juju-solutions/charms.reactive/issues/25
> https://github.com/juju-solutions/charms.reactive/issues/44
>
> I'm still not sure what the right solution for these issues is, so input
> on those is welcome.
>
> On Thu, Jan 28, 2016 at 9:55 AM, Merlijn Sebrechts <
> merlijn.sebrechts at gmail.com> wrote:
>
>> Hi Cory
>>
>>
>> I have a question about the reactive handlers. How exactly do
>> when_file_changed handlers get dispatched? Consider the following:
>>
>>
>> @when('init')
>> def handler1():
>>     change_file('file1')
>>     set_state('A')
>>
>> @when_changed('file1')
>> def handler2():
>>     restart_service()
>>
>> @when('A')
>> def handler3():
>>     change_file('file1')
>>
>>
>> The order in which handlers are dispatched is random afaik, so there is
>> the possibility that handler2 is executed before handler3. This would mean
>> file1 is changed again after executing handler2. Will handler2 be executed
>> again?
>>
>>
>>
>> Kind regards
>> Merlijn Sebrechts
>>
>>
>>
>>
>
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