On Apr 5, 2012, at 6:51 PM, Anca Luca wrote:
> On 04/05/2012 06:42 PM, Vincent Massol wrote:
>> Hi Sergiu,
>>
>> On Apr 5, 2012, at 5:55 PM, Sergiu Dumitriu wrote:
>>
>>> Hi devs,
>>>
>>> Currently, requesting a component instance without a hint will look for the
>>> implementation that uses the "default" hint, which makes it difficult to
>>> change the implementation in an XWiki instance. Sure, it is easy as long as
>>> all the implementations use the "default" hint, but choosing the default
>>> between alternative implementations that should all still be usable by
>>> themselves is not possible.
>>>
>>> Also, "default" is not really a good hint, since it describes the state of
>>> the implementation, not the technology, the aspect that makes it different
>>> from the others. It would be better to name each implementation with a
>>> proper hint.
>>>
>>> I propose to define a mapping that can specify which hint is the default
>>> for a component. In a text file, META-INF/component-defaults.txt, we'll
>>> keep componentinterface=defaulthint mappings. For example:
>>>
>>> com.xpn.xwiki.store.XWikiStoreInterface=hibernate
>>> com.xpn.xwiki.store.migration.DataMigrationManager=hibernate
>>>
>>> And then, when we lookup the current storage implementation, we don't need
>>> to check what is the configured hint in xwiki.cfg (or xwiki.properties), we
>>> can just request the default implementation.
>>>
>>> If there's no mapping for a component, we'll continue to use the "default"
>>> hint.
>>>
>>> I'm not sure where exactly to keep such files. We bundle a components.txt
>>> file in each jar containing component implementations. We could do the same
>>> for the components we consider the platform defaults, and allow overrides
>>> in the WEB-INF/classes/META-INF/component-defaults.txt file. Still, this
>>> means that whenever platform defaults change, we need to keep another
>>> special section in the release notes, to let users know about these
>>> changes, so that they can manually revert to the old default if they need
>>> to.
>>>
>>> In the future we could change existing components to give proper hints
>>> instead of "default", where such a change is applicable.
>>>
>>> Another idea is to not use "default" at all, and instead go for a generic
>>> "xwiki", "xe", "xwiki-platform" or something like that whenever there's
>>> just one implementation for a component and we can't find another hint to
>>> describe it.
>>>
>>> WDYT?
>> This is not really how it's been designed ATM. Whenever you wish to use a
>> different implementation of a component you use a component implementation
>> with the same role and same hint. You then make it available in your
>> classpath. (Of course you can also do this at runtime simply by registering
>> a new implementation over the old one).
>>
>> To decide which implementation is used you use a priority order, as
>> described on:
>> http://extensions.xwiki.org/xwiki/bin/view/Extension/Component+Module#HOverrides
>>
>> I'd be curious to know your exact use case and understand why the current
>> mechanism doesn't work for it.
>
> One usecase I see is that you have multiple implementations and you want to
> change the default one for a specific running instance of xwiki.
This is supported already.
> Overwrite mechanism only allows you to say which impl should be used from the
> _components with the same hint_. However, you cannot change the hint of a
> component at configuration time, so if you have a standard distr of xwiki and
> you want to use ldap authentication, let's say (if only auth was impl with
> components), unless you do some java to add the default hint to the ldap
> implementation and then to specify that this one has priority over all the
> default ones, I don't see how you can re-wire the default.
Yes that's because the hint is not a configuration param. It's an intrinsic
information about what it represents. When you have:
@Named("ldap")
public class DefaultLDAPAuthenticator implements Authenticator
...
It means that (Authenticator,"ldap") means the LDAP Authenticator
implementation and not something else. If you want to change that
implementation you need to write a component with (Authenticator,"ldap").
Now if what you want is way to have *several* implementations of the same Role
at once in the classloader you need to use different hints and if what you want
is pick one implementation from those various implementation and make it your
default you need to configure somewhere which one to use. We use that
everywhere in the XE.
2 examples:
* Macros. In this case it's the user who says which ones to use by using the
"hint" as the macro name telling XWiki which macro implementation to use:
{{hint .../}}
* Storage implementation. In this case we want only one default storage impl to
be used. What we do is have a Storage Manager which is in charge of deciding
which implementation to use. In this case the selection is based on a
configuration param which contains the hint of the implementation to use (for
ex: xwiki.store.main.hint=hibernate).
I don't see the limitation. It's even more powerful than what you suggest
(which is static rewiring of component implementations) since in this case we
support dynamic rewiring based on any rule.
I still fail to see the use case that would make what you suggest required.
Thanks
-Vincent
_______________________________________________
devs mailing list
[email protected]
http://lists.xwiki.org/mailman/listinfo/devs
