set_state not setting the state immediately

fengxia fxia1 at lenovo.com
Thu Oct 5 18:50:59 UTC 2017


" An assumption is being made that the state changes get committed
immediately, but these changes are actually transactional and
following the same transactional behaviour as the Juju hook
environment [1]."

To chip in my experience as 6-month into learning charms and writing a 
few simple charms.

1. The "transactional" nature of Juju hook needs better explained. To be 
honest I have no idea what this means, and what implication it has to a 
charm writer. Any reference would be helpful.

2. I like Mike Wilson's approach to provide a list of "set_state_xxx" 
functions so new writer can better guess what this function will do. 
Further, a different name calls for further study why they are 
different, thus learning important concept of whatever Juju thinks charm 
writer needs to understand.

Otherwise, I will expect "set_state" will set that state/flag asap. If 
there is a scanning cycle (which I heard there is some kind of 5-min 
cycle, which document has not sufficiently made it clear to a writer 
either), charm writer needs to have better doc to learn what it means 
for design. I came from embedded system world in which a timer loop is 
common. It calls for a different thinking than user space script user. I 
think such implication should be emphasized more.

3. idempotent

Again, this is a concept me (or many new writer) will fail to grasp. 
Looking at "apt install" as example, my reaction was that the package 
manager is taking care of "doing nothing" if called multiple times. But 
how this translate to my system in which charm is expected to "do 
something"? Does it mean I need a gatekeeper like the package manager so 
to guard these "multiple calls"?

Again, this feels like a work around because "set_state" will call the 
same function block multiple times, which is unintuitive to writer -- 
when I set a state, the action for that state is executed once, not over 
and over again until I turn it off. Further, even "remove_state" doesn't 
take effect immediately, so it feels arbitrary how many cycles a block 
of code is executed. This is a design pattern I'm afraid many are not 
familiar with, so some tutorial examples will be much appreciated.

Best,

Feng




On 10/04/2017 08:59 AM, Marco Ceppi wrote:
> So, I've not actually checked the logs in a while, but if visibility 
> is an issue, it seems reasonable that the reactive framework should 
> print in the log that X flags are being reset due to hook failure. 
> Things like set_flag_immediate has farther reaching consequences than 
> simply stating that flags only get set on success of a hook run.
>
> I know there are further reaching initiatives to alleviate this by 
> decoupling the reactive state engine from the juju hooks system. In 
> this case each successive loop of the reactive runtime could better 
> snapshot state and make failures more granular.
>
> * state is being renamed to flag in the next major version of reactive.
>
> On Wed, Oct 4, 2017 at 8:52 AM Mike Wilson <mike.wilson at canonical.com 
> <mailto:mike.wilson at canonical.com>> wrote:
>
>     So as a new charm writer coming to Juju I would first do this:
>
>     def get_ready():
>         func0()
>         func1_fails()
>
>     Then I would, hopefully, test and notice issues. I would
>     investigate and see that I needed to be idempotent. My next
>     attempt would be to wrap those functions inside state checks with
>     sets after they complete. This would also fail and now the charm
>     creator is left with nothing in the api that can help. They are
>     now off to their own devices to start doing random things to
>     attempt to make this work the way they want it to work. Hopefully,
>     the solution is as straight-forward as touching random files, but
>     we just never know.
>
>     I would expect the name of set_state to be something like
>     set_state_on_success and I would further expect some sort of
>     immediate state thing like set_state or set_state_immediate. This
>     would give the user the tools we know that they need in order to
>     create bug-free charms.
>
>     Now to compound that confusion, we have the issue of a hook can
>     call multiple functions inside the charm code and if any of those
>     functions have something that fails the whole thing is unwrapped.
>     I understand from a Juju perspective why this is the case, but as
>     a user, I would be completely taken by surprise here. The only
>     real fix here is documentation so that we can set expectations,
>     but people will most likely look at examples instead of
>     documentation. This means that we need to make sure to call out
>     any potential issues like this in the example charms we release.
>
>
>     On Wed, Oct 4, 2017 at 6:34 AM Stuart Bishop
>     <stuart.bishop at canonical.com <mailto:stuart.bishop at canonical.com>>
>     wrote:
>
>         On 4 October 2017 at 00:51, Mike Wilson
>         <mike.wilson at canonical.com <mailto:mike.wilson at canonical.com>>
>         wrote:
>         > So the best practice here is to touch a file and test for
>         the existence of
>         > that file before running must_be_called_exactly_once()?
>         >
>         > I think part of the issue here is that without knowing the
>         extent of the
>         > hook it is hard to enforce idempotency as a charm writer.
>         It's easy to look
>         > at the code above and say that is it idempotent since the
>         init function is
>         > wrapped in a when_not and the initialized state is set at
>         the bottom of
>         > init.
>
>         Individual handlers should be idempotent, so it doesn't matter
>         about
>         the extent of the hook, or even if the chained handlers being
>         triggers
>         are running in the same hook. Assume your handlers get called
>         multiple
>         times, because they may be. Yes, it looks idempotent but it
>         isn't. An
>         assumption is being made that the state changes get committed
>         immediately, but these changes are actually transactional and
>         following the same transactional behaviour as the Juju hook
>         environment [1]. I think this can certainly be explained
>         better in the
>         docs, but I can't think of a way to stop this being an easy
>         error to
>         make.
>
>         [1] spot the DBA
>
>         --
>         Stuart Bishop <stuart.bishop at canonical.com
>         <mailto:stuart.bishop at canonical.com>>
>
>     --
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>
>

-- 
Feng Xia

Advisory Engineer
Datacenter Group (DCG), Lenovo US
8000 Development Dr, Morrisiville, NC 27509
W: http://www.lenovo.com

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