Now thinking of it, I just realized that this concept of True state
executing repeatedly is actually a good thing.
In many cases, there is a need to query external resource for its
status. This is often implemented as a polling loop. So in charm, I can
implement it as
@when("prep")
def prep():
# are we there yet?
is_ready = prep_test()
if is_ready:
remove_state("prep")
set_state("do.sth")
@when("do.sth")
def do_sth():
# do this
pass
# move on
remove_state("do.sth")
set_state("next")
On 07/27/2017 03:56 PM, Cory Johns wrote:
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 <fx...@lenovo.com
<mailto:fx...@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 <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.
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Lenovo USA
Phone: 5088011794 <tel:5088011794>
fx...@lenovo.com <mailto:fx...@lenovo.com>
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Feng xia
Engineer
Lenovo USA
Phone:5088011794 <tel:%28508%29%20801-1794>
fx...@lenovo.com <mailto:fx...@lenovo.com>
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