Hi Tom,
I thought I'd grab this thread again, as it still appears to be an issue and
I'm now in a better position to answer.
Both Jini and OSGi use their implementation of the Publish, Find, Bind
pattern as an abstraction or façade to enable the implementation behind it
to be hot plugged, they both use the term service.
I've realised that there is no need to map a Jini service to an OSGi
service, nor worry about the different semantics between them. Why? You
only need one abstraction, why have an abstraction on an abstraction? And
if you want to get technical each solves a different problem, so one is not
better than the other or in a competitive relationship:
* You use your OSGi service to change out, swap between, iterate
over or group your modules (bundles). It heavily utilises
ClassLoaders for limiting visibility to avoid classpath hell.
* You use your Jini service to abstract away the communication
protocol between two nodes, so they can exchange objects easily.
When the protocol changes, the client doesn't need to worry about
it. It also enables the construction of a distributed system from
the sum of its components and it enables the abstraction of
hardware and software components.
Here's how to take advantage of OSGi from Jini, it is slightly different
from standard OSGi modularity practises, if you don't like it, use Preferred
Classloader instead, or use the existing http URL mechanism, you are not
committed to utilising OSGi, but those the are planning to use Jini Services
over the web will probably want to.
1. Create a bundle containing your service interfaces. The
interfaces will then be in their own ClassLoader, this is
important, so that others can implement the same service interface
without being dependant on and having to download your
implementation bundles.
2. Create your Smart Proxy Implementation, it belongs within its own
bundle, it depends on one or more of the Interface bundle
packages. If the interface bundle extends interfaces from
external packages, (imports other bundle packages containing
interfaces) then you must import those interface packages also
otherwise the you will get a runtime exception. Software on the
client utilising your proxy will need to import the interface
packages and any additional packages that these interfaces
extend.. You also need to specify any permissions your proxy needs.
3. For simple reflective proxies, the proxy will be loaded into the
interface bundle Classloader at the client.
4. Create your Server Service Implementation, it must import the
packages from the Interface and the Smart Client Package bundles
and any other packages that the interface bundle extends.
When a client utilises your service, it's bundle must lookup your service
from the Jini Lookup Service it might do this during bundle activation or it
might do so, at some other time. When that same client deactivates its
bundle that depends on your service (proxy), it also stops using your
service.
Our bundles need to register with the Jini service registrar when activated
and de-register when deactivated. They don't need to be registered to the
OSGi service registry. Notice at no point here have I required any mapping
OSGi service. That would be a distraction abstraction ;) If I want to,
there's nothing stopping me from using an OSGi service from within my Jini
bundles, it's purely up to the implementer.
At no time should we attempt to directly map an OSGi service to a Jini
Service or visa versa, they do not map well due to different lookup
semantics and the 8 fallacy's of distributed computing, instead create a
bundle that utilises a Jini Service, your bundle can register some other
service with the OSGi service registry so that you can plug in an
implementation that does work for an existing local software component,
which doesn't need to be aware that the Jini service even exists. Your
bundle will handle the 8 fallacy's of distributed computing and deactivate
itself if the you cannot rescue your service so that local components
depending on you are deactivated properly also.
Cheers,
Peter.
Tom Hobbs wrote:
"I know you guys are very busy, but it would be nice if the most
experienced Jini/River software engineers were able to dissect the
[OSGi] RFC 119 and provide an assessment as to how or if it is "suited"
for Jini/River. I know it's tough to allocate time to do that though."
Well, in the absence of the most experienced you're left with me. :-)
For added confusion, I don't know a whole heap about OSGi either, so the
follow is a likely mix of over simplification and misunderstanding.
If that sounds useful, continue reading...
This is the complete document, I skipped down to RFC 119 only;
http://www.osgi.org/download/osgi-4.2-early-draft.pdf
The RFC discusses the concept of a "Service Registry" which looks an
awful lot like a River ServiceRegistrar. Delving further into the RFC
it seems to me that we if we can translate from the specified interfaces
that describe an "OSGi Service" to that which describes a "River
Service" then River could slot in quite nicely as a response to this
RFC.
Much of the work feels like translating from what OSGi say service
descriptions and lookups *should* look like and what River says service
descriptions and lookups *do* look like.
The only tricky part, I think, would be how an OSGi component (which
likely extends something else) can be made into a River service such
that it is discoverable in the usual way. This would be an interesting
problem and raises the circumstance where an OSGi service might publish
itself as an OSGi service, but because it's River underneath, would be
discoverable by pure River clients on the same network also.
Looking at how the RFC specifies what a service description is and what
it looks like, I think that there is mileage in River adopting something
similar. It would be nice, in my opinion, to move away from the
quasi-java config files River uses in favour of something else.
XML makes sense because that's what most of the rest of the world uses -
although I personally don't care for it much.
Someone on the Jini-Users (or similar, I can't quite remember) a while
ago was talking about using Groovy classes to describe service
configuration. Something like this sounds pretty neat, but anything
that needs to be recompiled for changes can take affect is likely to be
unworkable for obvious reasons.
Also, building in a mechanism to provide a similar version-sensitive
lookup mechanism would 1) fit with OSGi nicely and 2) be a useful
feature for River all other considerations not-withstanding.
Anyway, that's this layman's interpretation of this OSGi RFC; if only
for a few days or weeks of spare time to spend putting it together.
Tom
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