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) and their functionality. OSGi
heavily utilises ClassLoaders to limiting visibility, avoiding
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 software doesn't need to be
concerned.
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 who 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 from within an OSGI bundle utilises your jini service
remotely, it must lookup your service from the Jini Lookup Service, it
might do this during bundle activation, or it might do so during
execution. When a client deactivates its bundle, it also stops using
your service (It doesn't actually depend on packages from your proxy
bundle, only packages from the interface bundle/s).
Bundles containing Jini services, should register with a Jini service
registrar during bundle activation and de-register when deactivated.
They don't need to be registered with the OSGi service registry. Notice
at no point here have I required any mappings to OSGi services. That
would be a distraction abstraction ;) If I want to, there's nothing
stopping me from using an OSGi service from within my 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. If you must
provide a Jini service to a local software component that utilises an
OSGi service, create an adapter bundle that looks up and utilises a Jini
Service. Your adapter bundle can register an OSGi service with the OSGi
service registry. Other OSGi bundles using your OSGi service don't need
to be aware that the Jini service even exists, however you do need to
take the network fallacies / latencies into account.
There isn't a requirement to create an OSGi service. Your local bundle
utilising Jini Services can be made to handle the 8 fallacy's of
distributed computing and deactivate itself if suitable Jini Service are
no longer available. This way, local components that are directly
dependant on your functionality are deactivated properly also and don't
hang waiting, you'll want to write to the local service log before doing
so, try to clean up gracefully.
If you have created an OSGi Service and registered it with the OSGi
Registry, other OSGi bundles won't depend on your bundle. Instead of
shutting down your bundle, you might choose to look for another jini
service, while doing this you de register your OSGi service, until you
find another suitable Jini service.
I wouldn't recommend that Smart proxy's register an OSGi service
directly, I suppose that someone might find a reason to do it, however
there is nothing stopping a Jini Smart proxy from utilising the local
OSGi services.
Simple Reflective Proxy's will never directly cause any other bundle to
become deactivated by the OSGi framework. Bundles that utilise Jini
services must be designed with the 8 network fallacies in mind.
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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