19 May
2010
19 May
'10
2:44 p.m.
On May 19, 2010, at 2:02 PM, Guillaume Sauthier wrote:
Yes, that's actually what I meant by "installing remote jars" (the jars +
their deps) but I should have been more explicit :)
Yes that could be done. We'd need to use the Constants.FRAMEWORK_SYSTEMPACKAGES_EXTRA
config property and manually list all packages (this is a pain!). I guess an alternative
would be to create a xwiki core bundle that is empty except for Bundle-Classpath in its
manifest, generated by the maven bundle plugin and pointing to WEB-INF/lib/*.jar.
Note: I like what I saw in OSGi and I still think one goal of xwiki should be to integrate
with OSGi (or with the JSR294 to be more generic, if that JSR is implemented one day...
;)). It's just that for the Extension Manager use case, I'm not sure we need OSGi
from the onstart.
Thanks for the info Guillaume,
-Vincent
PS: I'm still discovering all this and I'm not yet 100% sure of what I say.
There's a little nagging thought in the back of my head that tells me that going with
OSGi now is the best thing to do even though we might not need all its power (and
complexity) right now but that it would be easier to shift now than later... ;)
Hi Vincent
Here are some comments inlined...
Le 18/05/2010 19:24, Vincent Massol a écrit :
yep
Hi devs,
I've explored enough of OSGi to get a handle of how it works and what it would mean
to use it for XWiki as the basis for our Extension Manager (Thomas helped me with this
too).
The executive summary is that I don't think we need to use OSGi right now for the
Extension Manager and we can defer its usage for later.
Let me start by listing what would OSGi bring for XWiki:
1) Known framework with good documentation, books, etc
2) Users would be able to write extensions using OSGi programming model (without using
XWiki's programming model), this also means the ability to reuse OSGi bundles inside
XWiki
As OSGi use the service layer (µservice as we should say now) to interoperate,
everything that is service can talk with another service. It doesn't matter if your
µservice was produced by hand, with iPOJO, Blueprint (any OSGi component model you want).
So if the XWiki programming model produces/consume
µservices other µservices can be used too, they just have to conform to the µservice
API/Interface.
yep
3) ClassLoader
isolation and thus ability to have several versions of classes
4) OBR (Bundle Repository) for downloading and installing remote jars
OBR is not something that will help to download and install remote bundles.
With a "naked" gateway, you can always download and install remote bundles
(http://....).
The goal of OBR is to help installling a *set* of bundles with dependencies between
them.
For example, if Bundle A depends on Bundle B, you should have to manually download and
install 2 Bundles.
With OBR, you just "say": "I want A installed" and B will be
installed too. Now here are
the problems found with OSGi so far:
A) Impedance mismatch with the webapp/WAR model (classloader and visibility issues). Two
examples:
- it would mean having a special directory somewhere, where to put the bundles, and this
means not going through the standard WAR deployment process.
- if we want to keep WEB-INF/lib in order to have a standard WAR deployment then it would
mean keeping for ex the core jars in WEB-INF/lib and using OSGi only for extensions.
However doing this is not really possible since it would mean extension (OSGi bundles)
would have Imports on core classes but this won't work since core classes won't be
OSGi bundles
In that case, It's possible to have the webapp launching an OSGi gateway.
You could have the core classes in the WEB-INF/lib/*.jar (or WEB-INF/classes) and have a
selected set of packages (from the core) exported by the gateway (through the System
Bundle). So your extensions (standard bundles) can use the packages from the core. The only thing you cannot do with that case is to
update the core classes (since they are not really in a bundle, but considered as part of
the JDK packages).
Yes but that's acceptable I think and is not our primary need.
B) In practice
it's very hard to use OSGi with only a part of the modules being OSGi bundles and
another part being standard external libraries. Everything must be OSGified for it to work
well.
You're absolutely right, it's easier when everything is OSGified.
In addition we might want not need the features
listed above:
1) Not really needed especially since we have the XWiki programming model that we
don't want to loose right now
2) is a nice to have but not a requirement for the Extension Manager
3) in practice it's going to be very hard to support multiple versions of classes.
Imagine for example an application A which is providing a macro in a wiki page.
Application B requires version 1.0 of A and application C requires version 2.0 of A. How
do we support that? OSGi won't help here since this is about wiki pages. It's only
helping for jars. The jar isolation could also easily be implemented using a custom URL
classloader without too much problem IMO, but I'm inclined to say that we don't
need to support multiple versions in a first version of the Extension Manager.
Agree, OSGi will help you to bind a bundle to an exported package using a specified
version (may be a version range).
If what you produce is a wiki page, not some java code, it will be more difficult to
requires a given package version...
As OSGi targets Java, it's more usable (and there are more tools available) if you
write directly Java code: Bnd can automatically deduce your imports and service-oriented
component models handles dynamic service dependencies for you.
Cheers
--Guillaume
> 4) We can easily use the Maven 3 Embedder to download and install maven artifacts
into a local repository. This is convenient since we already deploy our
applications/modules to a maven repository. Note that the only "difficult" part
here might be searching into a remote repository; I don't know if a maven api exists
for this (but probably). In the worst of cases we could use the Nexus REST API to provide
this feature for now.
>
> And here's what I think we should do right now:
> i) focus on the 2 features we need for the Extension Manager:
> -- ability to install an application containing not only wiki pages but also jars
> -- ability to install an application from a remote repository
> ii) then once i) is done focus on the upgrade part of the extension manager
>
> Implementation note:
> - To implement i) we can do this easily I believe using a custom URL ClassLoader (we
already have one btw used in the script macro) that we pass to the Component Annotation
Loader when a new JAR is installed (in order to register the components in that jar) + set
that custom URL ClassLoader as the new thread context classloader (for code that would use
the thread context classloader to load classes).
>
> WDYT?
>
> Thanks
> -Vincent
>
> PS: I'd be interested to hear from OSGi experts what they have to say about this
since I'm an OSGi newbie and there might be stuff I have not seen/realized.
> PPS: I'm just realizing that the proposal in this mail is close to
Pomegranate's idea: http://www.caucho.com/projects/pomegranate/