[openstack-dev] [mistral] Mistral Custom Actions API Design
dougal at redhat.com
Tue Mar 14 10:38:50 UTC 2017
On 14 March 2017 at 10:21, lương hữu tuấn <tuantuluong at gmail.com> wrote:
> On Tue, Mar 14, 2017 at 10:28 AM, Dougal Matthews <dougal at redhat.com>
>> On 13 March 2017 at 09:49, lương hữu tuấn <tuantuluong at gmail.com> wrote:
>>> On Mon, Mar 13, 2017 at 9:34 AM, Thomas Herve <therve at redhat.com> wrote:
>>>> On Fri, Mar 10, 2017 at 9:52 PM, Ryan Brady <rbrady at redhat.com> wrote:
>>>> > One of the pain points for me as an action developer is the OpenStack
>>>> > actions. Since they all use the same method name to retrieve the
>>>> > underlying client, you cannot simply inherit from more than one so
>>>> you are
>>>> > forced to rewrite the client access methods. We saw this in creating
>>>> > actions for TripleO. In the base action in TripleO, we have
>>>> actions that
>>>> > make calls to more than one OpenStack client and so we end up
>>>> re-writing and
>>>> > maintaining code. IMO the idea of using multiple inheritance there
>>>> would be
>>>> > helpful. It may not require the mixin approach here, but rather a
>>>> > change in the generator to ensure the method names don't match.
>>>> Is there any reason why those methods aren't functions? AFAICT they
>>>> don't use the instance, they could live top level in the action module
>>>> and be accessible by all actions. If you can avoid multiple
>>>> inheritance (or inheritance!) you'll simplify the design. You could
>>>> also do client = NovaAction().get_client() in your own action (if
>>>> get_client was a public method).
>>>> If you want to do that, you need to change the whole structure of base
>>> action and the whole way of creating an action
>>> as you have described and IMHO, i myself do not like this idea:
>>> 1. Mistral is working well (at the standpoint of creating action) and
>>> changing it is not a short term work.
>>> 2. Using base class to create base action is actually a good idea in
>>> order to control and make easy to action developers.
>>> The base class will define the whole mechanism to execute an action,
>>> developers do not need to take care of it, just only
>>> providing OpenStack clients (the _create_client() method).
>>> 3. From the #2 point of view, the alternative to
>>> NovaAction().get_client() does not make sense since the problem here is
>>> subclass mechanism,
>>> not the way to call get_client().
> It is hard to me to understand what Thomas wants to say but i just
> understood based on what he wrote:). Sorry for my misunderstanding.
>> I might be wrong, but I think you read that Thomas wants to use functions
>> for actions, not classes. I don't think that is the case. I think he is
>> referring to the get_client method which is also what rbrady is referring
>> to. At the moment multiple inheritance wont work if you want to inherit
>> from NovaAction and KeyStone action because they both provide a
>> "_get_client" method. If they has a unique name "get_keystone_client" and
>> "get_nova_client" then the multiple inheritance wouldn't clash.
>> Sorry Dougal but i do not get your point. Why the get_client could not be
> used through instance since it has context?
In Mistral we have various OpenStack action classes. For example
NovaAction and GlanceAction (and many others in that file). If I want
to write an action that uses either Nova or Glance I can inherit from them,
client = self._create_client()
# ... do something with the client and return
However, if I wanted to use use two OpenStack clients, which I admit is a
special case and I think one that only TripleO uses (that we know of).
class MyNovaAndGlanceActioin(NovaAction, GlanceAction):
nova = self._create_client()
glance = self._create_client() <- doesn't work because they both
access the same method on NovaAction.
If the method was called "create_nova_client" and "create_glance_client"
then you could inherit from both without any conflict.
However, based on the reply Thomas sent earlier, I think we should consider
something like this when the OpenStack actions are moved to mistral-extra.
nova = NovaAction.client(context)
This is slight adaptation changes "_create_client" to "client" and makes it
a class method that accepts the context. I think this would provide a very
clear interface. I also can't think of any advantage of inheriting from
NovaAction, there is no state shared with it, so we only want it to create
the class for us.
>> Thomas - The difficulty with these methods is that they need to access
>> the context - the context is going to be added to the action class, and
>> thus while the get_client methods don't use the instance now, they will
>> soon - unless we change direction.
>>> @Renat: I myself not against to multiple inheritance too, the only thing
>>> is if we want to make it multiple inheritance, we should think about it
>>> more thoroughly for the hierarchy of inheritance, what each inheritance
>>> layer does, etc. These work will make the multiple inheritance easy to
>>> understand and for action developers as well easy to develop. So, IMHO, i
>>> vote for make it simple, easy to understand first (if you continue with
>>> mistral-lib) and then do the next thing later.
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