Distibuted OSGi

DOSGi helps in interaction between services which exists in different services in the same way as when they are co-hosted.

Specifications for DOSGi: http://www.osgi.org/Specifications/HomePage

Well Known implementations are

Apache CXF implementation : http://cxf.apache.org/distributed-osgi.html

Eclipse ECF implementation : http://wiki.eclipse.org/Distributed_OSGi_Services_with_ECF

Apache Tuscany implementation


Apache CXF implementation
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http://cxf.apache.org/roadmap.html : Nothing more other than web services even in roadmap.

DOSGi Webinar : http://download.progress.com/open/adobe/prc/psc/042809_fuse_osgi_webinar/index.htm

Some answers to the questions on the webinar

http://coderthoughts.blogspot.com/2009/05/questions-from-rfc-119-webinar.html

Exposing service -has to mention the interace which can be remotely invoked.

Based on this Apache CXF creates a wsdl - which can be called by any web service client

(RFC 119) complaint discovery service - http://cxf.apache.org/dosgi-discovery.html
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If discovery service is present - it will aurmtically detect , else we need to statically configure end points.

To enable automatic hooking of these remote services we need to have Service Registry Hooks -OSGi 4.2 spec(RFC 126)

Service Registry Hook Functionality(RFC 126 -OSGi spec 4.2) -Should be part of OSgi Core

Service Hook Functionalities
----------------------------

1. On Demand Registration of remote Services

2. Limiting Service Visibility

3. Service Proxification




How does it work [Refer Attached Diagram]
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Interface bundles are present in both the interacting containers.

Both teh containers have RFC 119 complaint discovery service and RFC 126 complaint registry hook

Service implementer registers with teh service with local OSGi container.

If this service has to be remotely accessed it needs to mention the property of remote interface.

Once this interface is mentioned the Discovery Service comes into picture which creates a remote end point for the service

It also register some meta data info with teh discovery service.

On the client side(in another Vm) when it expresses its interest in consuming this service , this information is passed
from the local osgi registry into the Registry hook which consults the discover service.

Now the discovery service will see if any one has remotely registered and if so creates a proxy for this service which will be reffered by the local osgi registry.

Current Apache CXF doesnt have either discovery service nor registry hook , hence end points need to be statically mentioned.

Apache is working on a discovery service under withthe ZooKepper Project

http://hadoop.apache.org/zookeeper/

http://wiki.apache.org/hadoop/ZooKeeper/ProjectDescription


[I think ZooKeeper is an alternative to Jgroups for sharing registry information between two nodes]

Working Demo is present in the link I have mentioned above, but without discovery service or service registry hook and it has a web service based implementaion for communication.

So to use Apache CXF we need minimum of these version - Equinox 3.5 / Felix 1.4.1 or later

Dmversion of equinox currntly- Dm 1.0.2 uses equinox 3 and 2.0 version uses equinox 3.5

Eclipse ECF implementation
----------------------------

http://wiki.eclipse.org/Distributed_OSGi_Services_with_ECF

http://eclipsesource.com/blogs/2008/12/19/rfc-119-and-ecf-part-1/

http://wiki.eclipse.org/Getting_Started_with_ECF%27s_RFC119_Implementation

Here changing the protocol for communication between services in diffrent VM's looks very simple

They claim to support the following protocols

Distribution

* r-OSGi (http) -r-OSGi is based upon a TCP-based protocols
* XMPP
* ECF Generic
* Skype
* Java Messaging Service (JMS)
* JavaGroups