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 <fx...@lenovo.com
<mailto:fx...@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 <tel:5088011794>
fx...@lenovo.com <mailto:fx...@lenovo.com>
Lenovo.com
Twitter | Facebook | Instagram | Blogs | Forums
--
Juju mailing list
Juju@lists.ubuntu.com <mailto:Juju@lists.ubuntu.com>
Modify settings or unsubscribe at:
https://lists.ubuntu.com/mailman/listinfo/juju
<https://lists.ubuntu.com/mailman/listinfo/juju>
--
Alex Kavanagh - Software Engineer
Cloud Dev Ops - Solutions & Product Engineering - Canonical Ltd
--
Feng xia
Engineer
Lenovo USA
Phone: 5088011794
fx...@lenovo.com
Lenovo.com
Twitter | Facebook | Instagram | Blogs | Forums
--
Juju mailing list
Juju@lists.ubuntu.com
Modify settings or unsubscribe at:
https://lists.ubuntu.com/mailman/listinfo/juju
