25 Nov
2011
25 Nov
'11
10:14 a.m.
On Thu, Nov 24, 2011 at 22:22, Vincent Massol <vincent(a)massol.net> wrote:
On Nov 24, 2011, at 8:15 PM, Anca Luca wrote:
All what both of you said are true. But I think that what is worrying me
and Anka most finally, is that priority defeat the goal of a traditional
observer or listener pattern, since it will induce some coupling between
sibling listeners. You can almost (since priority are on listeners and not
listener/event pair) get the behavior you want for a defined distribution,
but changes in any sibling listeners of the same event may defeat your
initial behavior due to the "loose" but "still" coupling priorities
will
introduce.
On 11/24/2011 05:31 PM, Vincent Massol wrote:
> On Nov 24, 2011, at 4:55 PM, Denis Gervalle wrote:
>
>> On Thu, Nov 24, 2011 at 16:25, Vincent Massol<vincent(a)massol.net>
wrote:
>>
>>> On Nov 24, 2011, at 4:06 PM, Denis Gervalle wrote:
>>>
>>>> On Thu, Nov 24, 2011 at 13:58, Vincent Massol<vincent(a)massol.net>
>>> wrote:
>>>>> Hi devs,
>>>>>
>>>>> Summary:
>>>>> ========
>>>>>
>>>>> I'd like to add the notion of Priority to Event Listeners. The
reason is
>>>>> that in some cases it's
important that some listeners execute before
>>> others.
>>>>> The problem at hand:
>>>>> =================
>>>>>
>>>>> Here's a typical use case: When receiving the
ApplicationStartedEvent,
>>> we
>>>>> have lot of code that needs to initialize. Initialization order is
>>>>> important (you can compare it to run levels in OS): for example some
>>> init
>>>>> must happen after the Database initialization has happened.
>>>>>
>>>>> Note that another solution exists for this use case: some
>>> initializations
>>>>> could introduce their own events (such as a DatabaseStartedEvent)
and
>>> other
>>>>> init could listen on those events instead of the generic
>>>>> ApplicationStartedEvent. However I can see several drawbacks to
this:
>>>>> * it's less generic than
the priority solution
>>>>> * it means creating more events
>>>>> * but more importantly it means that modules will have strong
>>> dependencies
>>>>> (at maven level) on each other whereas it's not necessary and
shouldn't
>>> be
>>>>> the case. In our example use case: it means that inits that must
happen
>>>>> after database is started
will need to depend on oldcore (which is
>>> where DB
>>>>> is started ATM)
>>>>>
>>>>> Proposal:
>>>>> ========
>>>>>
>>>>> * Don't break backward compat in Observation module
>>>>> * Introduce a PrioritizedEventListener interface that adds a
>>> getPriority()
>>>>> method
>>>>> * Modify ObservationManager implementation to take into account
>>> priorities
>>>>> * In order to make it simple I propose to have only a single
priority
>>> per
>>>>> Listener and not a priority per event supported by a given listener
>>>>>
>>>>> General Context
>>>>> =============
>>>>>
>>>>> To give some context here's what I'd like to do on the medium
term:
>>>>>
>>>>> * Step 1: Introduce notion of priority in EventListeners
>>>>> * Step 2: Refactor XWiki init to use an EventListener on AppStarted
with
>>>>> low priority
>>>>> * Step 3: Refactor wiki macros to use an EventListener on AppStarted
>>> with
>>>>> priority value lower than at step2
>>>>> * Step 4: Write an EventListener for the new UI Extensions with a
>>> priority
>>>>> value higher than the one of step2<-- this is the initial goal
that
>>> led
>>>>> me to make this proposal ;)
>>>>>
>>>>> WDYT?
>>>>>
>>>> Sounds good if not overkill for the goal.
>>>> An simpler alternative would be to have more than a single AppStarted
>>>> event, like there is more than one starting level in Linux.
>>>> Let say one level before XWiki, the one during, the one after ? is
there
>>> so
>>>> many other use case ?
>>>
>> Well, I have said +1 anyway, but...
>>
>>
>>> Yes this is very close to the other solution I explained above.
>>>
>> Surely.
>>
>>
>>> But it's far less generic and introduces knowing stuff you don't
really
>>> need to know. For me it's a
"poorman" implementation of priorities.
>>>
>> Well, it depends on the pursued goal. What I mostly dislike in priority
>> systems, it the management of priorities. When you say that you are
>> listening on a let says DatabaseStarted event, you clearly explain
what
you
>> really need. On the other hand, when you
say that you want to be run at
>> priority 100 of the AppStartedEvent, without another document saying
that
>> starting at 100, database is ready, and
ensuring this in the database
>> module, you do not really know what it means really.
> Yes it's static dependency vs loose dependency. One is statically typed
the other is documentation.
>
> But that's the only way I know to not have modules depend on each other.
>
> I like the static way of course but what you proposed while it's good
for
this specific use case doesn't solve all other use cases, which is why
I'm ambivalent about it and why I think the generic solution is a bit
better (I could be convinced otherwise with enough arguing and if enough
devs prefer the non generic way ;)).
I sort of agree with Dennis on this one. Numbered priorities always have
conflicts, and in order to make sure you place yourself in the proper place
(before this stuff, after this other stuff) you need to check (in the code?
in a documentation about priority numbers?) the numbers that stuff are
using.
Or did I understand the proposal wrong and
it's not about priority
numbers?
It's really a good idea BUT, as we've discussed before, numbered
priorities are always very tricky, since you can never be sure what it
means when you put 100 or 0 or some other number x (what do other listeners
declare? will my listener be before or after a certain other listener?).
If you can propose a better solution I'm all for it. I just don't know a
better solution.
FYI I've reviewed the following I could think about:
* Specific events added by other listeners (as I mentioned in my first
mail)
* Ability to say 'before' and 'after' other events
* Priority numbers
And by very far the one that I've found with the most pros is the priority
number solution (which we use in several other places in xwiki code base
whenever we need order: macros, etc).
I would think all the number-based priority
levels endup being used as a
static list-based priority levels (there are a few
well-known values that
are used),
Yes that's the idea. Special numbers are documented and are "API"s. Except
they are "decoupled APIs" (vs the static coupling you get with the other
solutions).
since you can't really use a continuous
interval (you need "before this
standard listener" or "after this
other standard listener", if we're
talking about non-standard listeners, numbers are not much more helpful
since you can never know what will be all the priorities declared by all
those listeners and where would yours be placed).
That's not true. In practice this still works and this is what makes
numbers powerful vs other solutions. They're flexible and you can always
(ok, almost always) tune the number to get the behavior you wish to have
which is definitely not even imaginable with the other 2 solutions I listed
above since they're not "dynamic" solutions and cannot adapt to something
unknown from the onset.
Now I'm open to hear about a better solution. I
just don't see one.
I do not know much myself, I am not sure these exists really. In a similar
case, the usual solution is to introduce a intermediate controller that
will divide the single event into smaller ones, allowing listeners a more
granular notification. This is static priorities, but these does not
introduce coupling between listeners.
So basically, what I was saying is that introducing flexible priority
in listeners may cause more problems than solutions. Since I have already
given my +1, I will keep it, but I would not have done so, and I would
have preferred the more usual way of adding more events, especially for the
current UC you have to solve, which is mainly concentrated on the single
AppStartEvent.
Denis
Thanks
-Vincent
--
Denis Gervalle
SOFTEC sa - CEO
eGuilde sarl - CTO
Thanks,
Anca
>
> Thanks
> -Vincent
>
>>> I don't know about other use cases right now but I can imagine a lot
of
>>> them (for example, some extensions
that needs open office to be
>>> initialized, etc). We need to remember that we're developing a
platform and
>>> not just for XE's needs. The
extra amount of work is negligible IMO
not to
>>> do it (btw I have a working version
locally already, I just need to
fine
>>> tune it a bit for improved
performance maybe).
>>>
>>
>>
>>> Thanks
>>> -Vincent
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