On 04/06/2012 08:51 AM, Vincent Massol wrote:
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.
Ok, so you write a factory (call it manager if you want, I can also call
it proxy if I want) that has the hint default and that actually does
nothing but give you the implementation that was configured at default
with one or the others of the means.
So if you want the user to have a choice (the dude installing xwiki and
having it running), you must _always_ write these factories, as soon as
there is more than one implementation, and have it read configuration
from some place.
If somebody builds something based on xwiki platform and adds an extra
implementation and wants to be able to change that, he needs to write
his new implementation + this factory (+ a bit of magic to actually
overcome the fact that the old implementation was "default" and now his
new implementation must be default but also needs to be able to dispatch
to the old default -- here I so much agree with sergiu default shouldn't
exist).
In addition, most of these factories will work exactly the same: they
will look for a param in the configuration file and use that hint as the
default. Of course we can provide an
AbstractXWikiConfigurationBasedGenericComponentImplementationManager to
make it easy for people to write their own.
It is covering the use case, but it's a pain. In addition, it really
depends on the fact that the person that wrote the multiple
implementations thought of whether there should be something that
dispatches between them or not (because somebody at some point might
wanna change it).
Since all these managers will be the same, why not add an "implicit
implementation" of this in the CM, by just allowing a hint to be
configured in the config file, and the CM would read it on the fly?
This doesn't mean that you cannot write your own if you want, that
dispatches based on the moon phase, for example.
Note that I push more towards dumb-simple things, although I understand
why it's a complex topic.
Thanks,
Anca
Thanks
-Vincent
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