Is a set state called multiple times?

[Cory Johns]

fengxia,

It's probably more enlightening to think of them as "flags" rather than
states.  (Indeed, the next release of charms.reactive will deprecate
calling them states and will provide set_flag, remove_flag, etc. functions
instead.)

On Thu, Jul 27, 2017 at 3:29 PM, fengxia <fxia1 at lenovo.com> wrote:

> Alex,
>
> Thank you for the detailed explanations and examples.
>
> After reading Tilman's and Cory's replies, I think the confusion is at
> continuous evaluation (thus execution) of a True state. So a pair of @when
> and @when_not will result in one of them being executed over and over
> despite adding a remove_state("myself") in the @when block.
>
> I'm still trying to grasp the idea of this "state" instead of treating it
> as an event handler.
>
> So for states, I usually draw a state machine diagram. In this case, it
> feels rather unnatural that all True states will inherently loop to
> themselves.
>
> But I don't what alternative is in charm's context.
>
> On 07/27/2017 04:13 AM, Alex Kavanagh wrote:
>
> Hi
>
> On Thu, Jul 27, 2017 at 2:37 AM, fengxia <fxia1 at lenovo.com> wrote:
>
>> Hi Juju,
>>
>> Once I set a state, set_state("here"), I want to make sure its @when will
>> only be executed ONCE (when "here" from False->True).
>>
>> So my thought is to remove_state("here") in its @when("here") code block.
>> If I don't, will this @when be called multiple times if I don't reset this
>> state? What's the good practice here?
>
>
> You have a couple of options here depending on the nature of the handler.
>
>
>    1. If, in the lifetime of the unit's existence, the handler only has
>    to execute ONCE.  (and I mean EVER), then there is a @only_once decorator
>    that can be used.  It can be used in combination with other decorators to
>    set up a condition, but it guarantees that the handler will only be called
>    once.  However, what you probably want is ...
>    2. Use a @when_not('flag') and then set it the 'flag' in the body of
>    the handler.
>
> The first would look something like:
>
> @when('some-condition-flag')
> @only_once
> def do_something_only_once_when_some_condition_flag_is_set_
> for_the_first_time():
>      ... do something once ...
>
> The second treats a flag as a 'have I done this yet' condition, and allows
> you to reset the flag at some other point in the charm's life cycle so that
> you can do it again.  'installed' is a good example of this:
>
> @when_not('installed-something')
> def do_install_of_something():
>     ... do the installation ...
>     # when it is fully successful, set the installed-something flag.
> Don't set it early as
>     # if it errors, a future handler invocation may be able to continue
> the installation.
>     set_state('installed-something')
>
>
> @when(some other conditions indicating do an upgrade)
> def do_upgrade():
>      ... set upgrade sources, or other pre upgrade actions
>      remove_state('installed-something')
>
> In this situation, hopefully you can see that we can re-use
> 'do_install_of_something()' when we do upgrades.
>
> I think it's useful to think about states (flags) as being a 'memory' that
> something has happened, and use them to either gate on not doing things
> again, or to trigger the next action is a graph of actions that need to
> take place to get the charm's payload to the desired operational state.  I
> tend to name them, and use them, to indicate when something has happened,
> rather than when it hasn't, and so tend to use @when_not('some-flag') on
> the handler that eventually sets that flag.
>
> Hope that this helps.
> Alex.
>
>>
>>
>> --
>> Feng xia
>> Engineer
>> Lenovo USA
>>
>> Phone: 5088011794
>> fxia1 at lenovo.com
>>
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>>
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>
>
>
> --
> Alex Kavanagh - Software Engineer
> Cloud Dev Ops - Solutions & Product Engineering - Canonical Ltd
>
>
> --
> Feng xia
> Engineer
> Lenovo USA
>
> Phone: 5088011794 <(508)%20801-1794>fxia1 at lenovo.com
>  	
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