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From Mike McGrady <mmcgr...@topiatechnology.com>
Subject Re: River & OSGi
Date Fri, 06 Nov 2009 14:40:57 GMT
I am getting ready to jump on an airplane, so will but initiate a  
multipart reply.



Sent from my iPhone

On Nov 6, 2009, at 3:32 AM, Peter Firmstone <jini@zeus.net.au> wrote:

> Mike McGrady wrote:
>> First, I would like to know what the "shared" "Service Pattern" is.
> Sharing - Objects moved between jvm's in a distributed environment.

"Shared" in your previous post meant a shared pattern.

>
> Distributed Service Pattern -  Dynamic Runtime Discovery - Lookup of  
> Objects that share a common Interface, receiving a proxy that  
> enables the transmission of as Parameters and Returned Methods using  
> Jeri / RMI / Serialization etc.

The terminology just changed,  "Service Pattern" now has distributed  
added.  Please indicate what this "pattern" is where it occurs in  
Felix.  Looks like a loose and inappropriate use of "pattern" if you  
mean design pattern, like inapropriately calling MVC a design pattern.

>
>>  Second, I would like to note that in my opinion changing Felix,  
>> which is architecturally on firm ground to accommodate Jini is a  
>> huge mistake.
> Why? Felix is designed to be embedded into systems, are you  
> suggesting Felix isn't suitable, would you prefer I use another  
> Modular infrastructure to manage package visibility?  If so which  
> one? The current alternative JSR-294 depends on Java 7 and hasn't  
> been released yet.  I fail to see why utilising mature code as  
> opposed to expending much time reimplementing when something already  
> exists would be a huge mistake?   I have considered extending  
> Classworlds, however this would require jvm restarts when packages  
> needed to be replaced.  The Java Package Version Specification is  
> optional and isn't widely implemented.

OSGi is designed with services that are not inherently remote and are  
dynamic in that they appear and disappear in the course of a runtime.   
This is very different from "managing package visibility".  You need  
to distinguish.modularity, lifecycle management and services and to  
more appreciate their varied levels of abstraction.

>>
>> I am not convinced that this analysis understands the depth of  
>> OSGi.  Certainly the module, lifecycle, service levels of  
>> abstraction in OSGi seem to me to not coincide with the notion of  
>> service in Jini.  This difference is greater when distribution is  
>> considered.  I would suggest that no decisions be made without a  
>> more developed and better expressed alternative than this.  Felix  
>> does not need to compromise it's architectural integrity to  
>> accommodate less mature concepts in Jini.
>
> OSGi segregates bundles of software (jar files with metadata) into  
> separate classloaders, controls visibility of packages between  
> bundles based on compatibility information (metadata) in jar files  
> and enables the bundles to be stopped, replaced and restarted.

This is a serious but not unusual over simplification of OSGi.  This  
is the lowest level of OSGi.

>
> To utilise OSGi, a Jini program must be distributed in a bundle  
> containing dependency information on package version imports.

This is ambiguous in important respects.

> To be made available outside its Classloader (one for each bundle)  
> the visibility of Packages must be controlled, it's public API  
> Packages must be registered ie the bundle started.  Since Java  
> cannot reload class files when a Bundle is upgraded, it must reside  
> in another Classloader, so Classloaders are Bundle and version  
> specific.  The service pattern enables the bundle to be rediscovered  
> by identity locally within the JVM after an upgrade.

I don't think you understand what an OSGi service is.  An OSGi service  
is a contract.

>  A Jini service must be exported and registered to be available for  
> lookup, if it is being managed by an OSGi framework it must first  
> have it's bundle started and registered.

What you mean by "managed" needs to be specified.  JIni registration  
and OSGi registration are not the same of course.

>
> Jini currently loads class files into classloaders based on the  
> local availability of classes or http coadebase URL origin, imposing  
> restrictions on sharing of Objects, this is suboptimal, OSGi can  
> assist in resolving this issue by providing a framework that manages  
> package visibility.  However I don't believe that OSGi alone is  
> enough in a distributed environment

What do you mean here?  OSGi is what it is.

>
> Please provide me with facts so we can have a meaningful discussion,

I have no idea how this relates to this discussion.  What facts about  
what?

> from your reaction I detect your gut feel is telling you this isn't  
> the right approach, my gut feel is that it is. I've based my  
> reasoning on facts, I need some factual information to be convinced  
> otherwise.

What facts?  You have not explained what you mean and you speak very  
loosely where precision is required, but yet make very sweeping and  
important recommendations.  My mind, not my gut, does not like this  
since we do important business with Felix and do not want it fiddled  
with without clarity, understanding and open discussion at depth.

I don't think, again, you understand the nature and proper  
architectural positioning of OSGi services.  Rather, you seem to focus  
on the sorts of things or understandings we could get from sources  
like Wikipedia, I.e., superficial and misguided. Maybe your  
understanding is deep and accurate but we have no way of knowing and  
what you say indicates the opposite.

Ideas - nor facts - are at issue.

I have asked you to to tell me about your view of services on this  
context, which is critical, and have gotten a gloss of liwer level  
functionallity.

Let me remake my request that you explain what you initially meant  
about Jini and OSGi services because it sounds all wrong to me.

Thanks.

>
> No offense intended.

None taken - ditto.

>
> Regards,
>
> Peter.
>>
>> Mike
>>
>> Sent from my iPhone
>>
>> On Nov 5, 2009, at 7:03 PM, Peter Firmstone <jini@zeus.net.au> wrote:
>>
>>> Well,  I've had a good think about it, this is presently the way  
>>> I'd like to utilise OSGi:
>>>
>>> First let me make one thing clear:
>>>
>>> 1. Both OSGi and Jini utilise the Service Pattern.
>>> 2. Jini utilises the Service Pattern for sharing distributed
>>>    pluggable java software.
>>> 3. OSGi utilises the Service Pattern for making java software
>>>    modular, versioned and pluggable without JVM restarts.
>>>
>>> When I refer to Jini I'm referring to the Jini Platform, when I  
>>> refer to River, I'm referring to the Apache River implementation  
>>> of Jini.
>>>
>>> Java Also uses the Service Pattern with SPI for 3rd Party Vendors  
>>> to Provide Pluggable implementations of JVM components or  
>>> extensions, eg JDBC, Encryption software etc. Gregg inspired me by  
>>> pointing out possible PreferredClassLoader changes utilising the  
>>> RMIClassLoaderSPI.
>>>
>>> By embedding an OSGi implementation such as Felix into River in a  
>>> pluggable manner (usage is optional) River can support OSGi  
>>> applications and provide Jini Services to those local applications.
>>>
>>> Doing so will require modification to the Felix Codebase, which is  
>>> probably best managed as a Patch for a Particular Version of Felix.
>>>
>>> Additionally, I figure further modifying to Felix to support  
>>> interactions with Remote Codebase Services utilising Static  
>>> Analysis to identify Package API.  The Static Analysis will be  
>>> used for three reasons, one to identify compatibility between  
>>> software in addition to the Bundle & PackageVersion metadata, two  
>>> to identify later versions of import packages that are compatible  
>>> using a Subset of their API used by a Bundle.  Lastly but perhaps  
>>> most significantly to allow software to evolve without fear of  
>>> runtime exceptions caused by binary incompatibilities.   
>>> Modification to Felix would be done in a manner that didn't break  
>>> compatibility with OSGi.
>>>
>>> Let me make one thing clear, OSGi does not compete with River, nor  
>>> is OSGi's remote service standard intended to compete with  
>>> distributed frameworks, nor are they intending to implement a  
>>> distributed framework, the API is there to be utilised by  
>>> distributed frameworks, OSGi made a statement to this effect.  
>>> Benefits:
>>>
>>> 1. Existing OSGi Applications can be hosted by River.
>>> 2. Existing OSGi Applications can utilise Jini Services without
>>>    modification to OSGi applications, in a pluggable fashion.
>>> 3. The River Platform (with OSGi felix embedded) is the minimum
>>>    installation requirement (apart from Java), all sofware can be
>>>    downloaded on demand from codebase services, guaranteed
>>>    compatible, including additional security benefits from having
>>>    bytecode API identified by Static Analysis from a code base  
>>> with a
>>>    trust relationship.  The codebase would never execute uploaded
>>>    bytecode, just analyse and distribute it.  Hence codebases are
>>>    mediators/proxy's for disconnected or untrusted service providers
>>>    or clients.  All code and API apart from a minimal core platform
>>>    code could evolve dynamically over time.
>>> 4. OSGi enables local bundles to be restarted, this would allow  
>>> River
>>>    to locally update older bundle's when a need no longer exists to
>>>    utilise an older version bundle or if another bundle requires a
>>>    later version, complementing software evolution.
>>>
>>>
>>> I know that this is perhaps a somewhat bold ambition, however I  
>>> believe it would increase the interest in the River project,  
>>> especially from the OSGi community.
>>>
>>> There are plenty of details to work out, such as how to implement  
>>> persistence services for bundles and their import packages over  
>>> restarts, how to coordinate starting and stopping of bundles that  
>>> contain Jini remote services.  How to proxy Jini Lookup services  
>>> from within the OSGi framework to make these services available to  
>>> existing OSGi applications.  OSGi would not be able to work with  
>>> Jini Services that didn't themselves utilise OSGi, Felix wouldn't  
>>> know which bundles were required for compatibility reasons.   
>>> That's where codebase services would come to the rescue, by  
>>> checking for known OSGi bundles that are in fact compatible and  
>>> substituting them.  If a compatible bundle couldn't be found the  
>>> codebase service would have to create a bundle using available code.
>>>
>>> One remaining concern I have is the approach of integrating Felix  
>>> into River.  I'd like to make the required changes within Felix  
>>> Pluggable components if possible, so that the existing felix  
>>> implementation would only require some minor changes that we might  
>>> be able to get them to accept.  That would allow us to have a  
>>> totally independent implementation which could continue to work  
>>> with future versions of Felix, and possibly other OSGi  
>>> implementations, in case word spreads about River in the OSGi  
>>> community.  I can't see that I could use OSGi for replacing OSGi  
>>> system components, but it might be possible using SPI, ironically  
>>> again another Service Pattern.
>>>
>>> I acknowledge that the alternative method of making River a bundle  
>>> within OSGi is possible without changes to OSGi, however it  
>>> doesn't make it possible then for OSGi to utilise the codebase  
>>> services I'm interested in.  If someone can show me that it is  
>>> possible to do so I'll consider the option seriously.
>>> Cheers,
>>>
>>> Peter.
>>>
>>>
>>>
>>>
>>
>

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