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From David Jencks <>
Subject Re: Whence the geronimo kernel?
Date Fri, 13 Mar 2009 07:44:10 GMT

On Mar 12, 2009, at 10:02 AM, David Jencks wrote:

> On Mar 12, 2009, at 3:25 AM, Gianny Damour wrote:
>> On 12/03/2009, at 4:29 AM, David Jencks wrote:
>>> On Mar 11, 2009, at 1:46 AM, Gianny Damour wrote:
>>>> 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.
>>> I agree.  I doubt Guillaume has seen your additions to  
>>> classloading in trunk which allow you to not export packages from  
>>> a classloader.  I haven't tried to prove it mathematically but I  
>>> think that with this feature the classloading models for geronimo  
>>> and osgi are equivalent in that you can express the same ability  
>>> to access classes with either of them.  Of course, the notation  
>>> you use to express this and the specific information included in  
>>> the configuration is different.
>>> I think I probably have the most experience with classloading  
>>> problems in geronimo and the only real problem that arises is  
>>> loading a jar in two different classloaders.   This can be solved  
>>> by a classloader-per-jar model which offers no theoretical  
>>> problems to set up in geronimo but practically would take a lot of  
>>> work (and maven projects to build a plugin per jar).  So I think  
>>> we'll have to see what kind of problems we get with trying to  
>>> actually use OSGI.
>> Hi,
>> Thinking more about this, I believe we can expedite the  
>> implementation of a classloader-per-jar model. Under the hood of a  
>> MultiParentClassLoader we can replace the current implementation of  
>> find class and resources contracts by an implementation which  
>> delegates to a bunch of URLClassLoaders (one per jar). These bunch  
>> of URLClassLoaders are global classloaders, i.e. shared across all  
>> the configs/MultiParentClassLoaders. The core challenge is to  
>> create them in a hierarchy respecting the maven dependency  
>> declarations. So, we could install the pom of the dependencies in  
>> the repo and lazily parse them when MultiParentClassLoader are  
>> created to build this global and share tree of URLClassLoaders.
> IMO the danger here is that the maven pom may not exist or may be  
> wrong.  OSGI has the same problem in that the vast majority of  
> released jars don't have osgi manifests.  I think I saw a rumor that  
> spring spent a lot of effort osgi-ifying a lot of commonly used jars  
> to try to solve this problem.
> I also don't know if there are situations in which a small number of  
> closely related jars need to be loaded in a single classloader,  
> perhaps because one of the jars is "optional" but if present the  
> "main" jar needs access to its classes.  I think there was an osgi  
> feature that looked sort of like this.
>> I just started to work on it and I will post back my findings (i  
>> should be able to complete this over the week-end). Even if we  
>> switch to an OSGi kernel, part of this work may hopefully still be  
>> useful.
> Unless you are pretty sure we won't have the kind of problems with  
> bad community metadata suggested above it might be a good idea to do  
> this in a sandbox branch?

Thinking about this more I really expect the code will be easy  
compared to straightening out the dependencies :-(

As a concrete example, the JACC spec needs the servlet and ejb specs  
but in our maven poms we've excluded them as dependencies so as to  
make it easier to upgrade the jars independently.  While (especially  
with maven 3) we can probably put in the dependencies with version  
ranges, they aren't there now.  Getting the server to work again may  
be time consuming.  I really hope I'm wrong :-D

david jencks

> thanks
> david jencks
>> Thanks,
>> Gianny
>>> One thing I'd really like actual user data on is how people  
>>> actually specify osgi classloading info in real life.  I'm very  
>>> aware that in theory you are supposed to specify the package  
>>> imports and exports for your bundle but I've been told that in  
>>> real life everyone with a serious osgi project actually specifies  
>>> the jar dependencies they want using require-bundle.
>>> thanks
>>> david jencks
>>>> Thanks,
>>>> Gianny
>>>> On 11/03/2009, at 7:11 PM, Guillaume Nodet wrote:
>>>>> On Wed, Mar 11, 2009 at 08:57, Gianny Damour <

>>>>> > 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 (

>>>>> ).
>>>>> 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:
>>>>> ------------------------
>>>>> Open Source SOA

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