Hi,
So let's agree to disagree for now. This may be related to my
personal way of comparing stuff which is pretty much limited to:
1. understand what the requirements are.
2. understand how the technologies support these requirements. I do
not need all the bells and whistles that a technology offers to
fulfill the requirements. Moreover comparing stuff based on
technology differentiators not clearly linked to the requirements is
pointless.
3. assess best way forward based on above scoring.
Key steps are 1 and 2 where stuff offering all the bells and whistles
may well be scored as good as other stuff (I clearly do not support
over-bloated stuff...).
Obviously, I am keen to be proven wrong and adjust accordingly. So
far, I am still saying that the main challenge is to properly tune
export/import of dependency declarations. For me, the technology is
not the core issue and switching is not going to resolve problems.
Thanks,
Gianny
On 11/03/2009, at 7:11 PM, Guillaume Nodet wrote:
On Wed, Mar 11, 2009 at 08:57, Gianny Damour
<gianny.dam...@optusnet.com.au> wrote:
Hi,
FWIW, I believe that improving the configuration style to simplify
the means of creating a bunch of objects in the kernel has more
benefits than swapping the classloading infra. On paper OSGi may
appear as superior from a classloading isolation perspective;
however, I believe the current CLing design is nearly up to par
with the OSGi one and that the main challenge is to properly tune
export/import dependency declarations.
I have to disagree with that. The CLing mechanism is very
different in Geronimo (from what I recall) and OSGi. Geronimo uses
a multi-parent classloader style with some nice features to be able
to hide / never override + parent or self-first delegation.
OSGi CLind is very different: the first one is that you don't
really have parent classloaders: the classloader for a given OSGi
bundle is calculated wrt to the constraints expressed in the OSGi
manifest using imported packages or required bundles.
Let's take an example:
bundle A needs api packages from bundles B and C
implementation classes from bundle B and C needs something from
bundle D but with different versions
OSGi will be able to handle that because of non tree-like CLind
mechanism: if bundle A is wired to bundle B, it does not have to
see all the requirements from bundle B, and same for C. Therefore,
bundle A can be wired to both B and C without problems because it
will not see bundle D at all (so there's no conflicts between the
two versions of bundle D).
OSGi has a much more powerful CLing mechanism where you can express
lots of different constraints. The drawback is that establishing
the classloader can take a bit of time, so going to OSGi most
certainly leads to a big slowdown at startup while creating the
classloaders.
Also, OSGi does not really play nicely with the usual JEE way to
discover implementations through the MANIFEST/services entries.
That's kinda what we've tried to solve in servicemix specs, though
I'm not sure if that really applies everywhere because I would
imagine the classloaders for EARs are not really OSGi classloaders ...
I certainly don't want to say OSGi is not the way to go, just want
to make the point that there are benefits but also drawbacks.
The JAXB approach to turn xml plans to a bunch of objects is
certainly interesting. I believe it is still a technology limiting
decision whereby a lot of custom code will have to be implemented
to support various style (factory methods or beans et cetera) of
configurations. I have been bouncing around this idea a while back
and here it is again. Why do we want to define a XML language to
create a bunch of objects when scripting can do that for us?
I believe that xbean-spring is still unnecessary noisy when
compared to something like the Spring Bean Builder (http://
www.grails.org/Spring+Bean+Builder).
If there is an interest in a scripting approach, then I can
investigate further.
Thoughts?
Thanks,
Gianny
On 11/03/2009, at 6:54 AM, David Jencks wrote:
So as mentioned below I'm starting to look into the osgi
classloading bit, sort of "from the bottom".
Another approach to many of these issues is perhaps "from the top",
from the point of view of going from a presumably xml plan to a
bunch of objects.
I've long thought that it must be possible to leverage jaxb to do
most of the heavy lifting here. In particular sxc is some code we
can presumably actually extend to do stuff like constructor
dependency injection. So another avenue that could perhaps be
approached in parallel would be to investigate sxc, jaxb, xbean-
spring, xbean-reflect, the blueprint service schema, and jsr299
requirements and see what we can come up with.
For instance, it might be possible to have a large part of the
blueprint service functionality in jaxb-enabled objects that jaxb
instantiates from the xml. The "init" method could deal with
feeding the metadata into the blueprint service core. Maybe we can
get sxc to use xbean-reflect to create the objects.
So far this is more or less wild speculation in my head... but I
think it would be a lot of fun to investigate.
thanks
david jencks
On Mar 4, 2009, at 4:56 PM, David Jencks wrote:
Geronimo has been around for a while and despite the many good
features gbeans and the geronimo kernel are not catching on big
time. I think we want to consider taking action now to avoid
ending up being dragged down by supporting a dead container. Here
are a few thoughts.
Actual problems with geronimo:
- gbeans are too restrictive. It's too hard to instantiate other
peoples components as gbeans. GBeans don't support common patterns
like factory methods, factory beans, etc etc, and require the
component to be instantiated directly by the gbean framework.
- it's too hard to get the classloaders to work. The most common
problem is a class cast exception due to loading the same jar in
two plugins. NoClassDefFound errors from an optional jar in a
child classloader are also really annoying.
Really good things about geronimo I haven't seen elsewhere (at
least in one place):
- gbean dependencies work across plugins. Dependencies are a
unified system, not per-plugin.
- gbean dependencies are resolved in the ancestors of a plugin, not
server wide. This means that you can't make a partially specified
dependency ambiguous by deploying additional plugins. I consider
this an extremely important feature for predictability.
- plugin dependencies allow assembly of a server from the explicit
dependencies which are normally the same as the maven dependencies.
Other projects and specs that have stuff we should look into:
maven. Maven has a lot better infrastructure for dealing with
dependency resolution from partial transitive dependency
specification than we do. We should look into using more of their
infrastructure.
osgi. osgi has a lot of similarities to geronimo. The osgi
classloading model is getting a lot of people excited. The import-
bundle idea is pretty much the same as our classloader model where
every jar is a plugin. I don't know if people are really using the
allegedly recommended method of specifying imports and exports and
letting the osgi runtime figure out where they come from; this
seems worth investigating to me. Also, we get periodic inquiries
about when we are going to support osgi and the was ce folks get
even more.
osgi blueprint service (rfc 124) This appears to be a simple wiring
framework for a single plugin. IIUC it uses the osgi service
registry for component dependencies between bundles.
xbean-spring. I'd be reluctant to try to implement a blueprint
service that didn't provide the xbean-spring capabilities really well
ee6 dependency injection. EE6 is going to have a pretty
sophisticated dependency injection service which we'll need to
support anyway. We should try to figure out how much of the core
we can assemble using it.
Other great stuff we have:
xbean-reflect, xbean-finder, xbean-spring
These ideas have been floating around in my head for a long time
and I've chatted with various people about them occasionally.
While more discussion is certainly needed on everything here I need
to do some implementation to understand much more. So, what I'm
planning to do:
Dave's crazy work plan...
- Try to use the osgi classloader. I think this involves putting
the classloader creation in Configuration into a service.
Configurations will turn into osgi bundles. I'll put the Kernel in
the osgi ServiceRegistry so the Configuration bundle activator
should be able to use it to resolve cross-plugin dependencies.
- try to figure out how maven dependency resolution fits into osgi.
- see if eclipse p2 is relevant for provisioning geronimo repositories
at this point I think geronimo would be running on osgi, still
using gbeans.
- look into relaxing the gbean framework so it is more plugin-at-a-
time rather than gbean-at-a-time
- see how that differs from the blueprint service, ee DI, and xbean-
spring. Try to support all of these at once.
Thoughts? Counter proposals? Anyone interested?
many thanks
david jencks
--
Cheers,
Guillaume Nodet
------------------------
Blog: http://gnodet.blogspot.com/
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