Kirby Bohling wrote:
On Fri, Jul 17, 2009 at 5:21 PM, Andrzej Bialecki<[email protected]> wrote:
Doğacan Güney wrote:
There's no specific design yet except I can't stand the existing plugin
framework anymore ... ;) I started reading on OSGI and it seems that it
supports the functionality that we need, and much more - it certainly
looks
like a better alternative than maintaining our plugin system beyond 1.x
...
Couldn't agree more with the "can't stand plugin framework" :D
Any good links on OSGI stuff?
I found this:
http://neilbartlett.name/blog/osgi-articles
Hi Kirby,
Thanks for your insights - please see my comments below.
Plugins are called Bundles in OSGi parlance, but I'll use plugin as
that's the term used by Nutch.
I have done quite a bit of OSGi work (I used to develop RCP
applications for a living). OSGi is great, as long as you plan on not
using reflection to retrieve classes directly, and you don't plan on
using a library that uses it directly.
Pretty much every use of usage like this:
Class<?> clazz = Class.forName(stringFromConfig);
// Code to create an object using this class...
Will fail, unless the code is very classloader aware. So if you're
going to switch over to using OSGi (which I think would be wonderful),
you'll want to ensure that you can deal with all of the third-party
libraries. I haven't played much with any of the Declarative Services
stuff (I think that was slated for OSGi, but it might have just been
an Eclipse extension).
This is an important issue - so I think we need first to do some
experiments, and continue development on a branch for a while ... Still
the whole ecosystem that OSGI offers is worth the trouble IMHO.
The OSGi uses classloader segmentation to allow multiple conflicting
versions of the same code inside the same project. So having a
pattern like:
Plugin A: nutch.api (Which contains say the interface Parser { })
Plugin B: parser.word (which has class WordParser implements Parser)
Plugin B has to depend on Plugin A so it can see the parser. In this
case, Plugin A can't have code that uses Class.forName("WordParser");
OSGi changes the default classloader delegation, you can only see
classes in plugins you depend upon, and cycles in the dependencies are
not allowed.
If I understand it correctly, this is pretty much how it's supposed to
work in our current plugin system ... only it's more primitive and it's
got some warts ;)
If you want to do that, you end up having to do:
ClassLoader loader = ParserRegistery.lookupPlugin("WordParser");
Class.forname("WordParser", loader);
OSGi has some SPI-like way way to have a plugin note the fact that it
contributes an implementation of the Parser interface. Eclipse builds
on top of it, and that's what Eclipse 3.x implemented the
Extension/ExtensionPoint system on top of. I believe they are called
services in "raw" OSGi.
It's not a huge deal to write that yourself for API's you implement.
The problem is that it can be difficult to integrate really useful
third-party libraries that don't account for this change in
classloader behaviour. At points it can make it very problematic to
use a specific XML parser that has the features you want (or some
library you want to use really wants). Because they do this sort of
thing all the time.
This doesn't sound too much different from what we do already in Nutch
plugins.
I'm guessing that Tika isn't ready for this. Given that it's an
Apache and/or Lucene project, it can probably be addressed. My guess
is that a number of the libraries they depend upon won't be.
I think we would like Tika to function as an OSGI plugin (or a group of
plugins?) out of the box so that we could avoid having to wrap it ourselves.
You can use fragments to get away from that (a fragment requires a
host bundle, the fragment's classes are loaded using the same
classloader as the host), but it doing that defeats a lot of the
reason for using OSGi (at least in terms of allowing you to use
multiple conflicting libraries in the same application).
Thank you again for the comments - I'm a newbie to OSGI, so I'll
probably start with small experiments and see how it goes. If you think
you could help us with this by providing some guidance or help with the
design then that would be great.
--
Best regards,
Andrzej Bialecki <><
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