There were quite a number of questions on the webinar and I didn't get to answer all of them, so I'm providing some answers here. For those of you who missed the webinar, you can still view the recorded version of the Distributed OSGi webinar here.
The source code of the GreeterDemo that I used during the demo session can be found in the Apache CXF demos SVN here: http://svn.apache.org/repos/asf/cxf/dosgi/trunk/samples/greeter for details on how to run the demo see the demo walkthrough page on the CXF Distributed OSGi website.
Below are the questions asked during the webinar, and an attempt to answer them. Feel free to send comments if I missed anything…
Q: Does it use RMI?
A: The wire-protocol and data-binding used are not specified by Distributed OSGi. They are up to the implementation. The Apache CXF implementation used in the demo uses SOAP/HTTP but other implementations could be written to use other wire protocols, such as RMI.
Q: Why don't you use JaxWS?
A: It's all about choice for the developer. JaxWS is a specific programming model for Web Services in Java. Distributed OSGi is for those who like the OSGi Services programming model, and would like to stay within that programming model to distribute OSGi services. It's not incompatible with other programming models, such as JaxWS, JaxRS, CORBA etc and can be used alongside them.
Q: Can I used Distributed OSGi services from Visual Basic?
A: Yes you can! Visual Basic programs can invoke on Web Services that are defined by WSDL. If you use the Apache CXF Distributed OSGi implementation it will generate a WSDL for your OSGi service which you can use from your Visual Basic client.
Q: Is there an implementation of Distributed OSGi available in Open Source?
A: Yes, the Apache CXF implementation, Eclipse ECF implementation and Apache Tuscany implementation are all open source and freely available to everyone.
Q: Can I use Distributed OSGi services from an Ajax browser client?
A: Yes, this is possible. I wrote an example of this on this blog a little while ago, see here: http://coderthoughts.blogspot.com/2009/02/distributed-osgi-powered-ajax-webapp.html.
Q: Can i use cxf with equinox, specifically pre 3.5?
A: The short answer is no, but you can use the current betas of Equinox 3.5 (builds since December). Any implementation of Distributed OSGi will need the Service Registry Hooks. Since this is a new API, it is only available in Equinox 3.5 and Apache Felix 1.4.1 or newer.
Q: Why is it based on webservices?
A: The design of Distributed OSGi is not based on Web Services and you may find implementations that don't use web services at all. The Apache CXF implementation is based on Web Services. Web services provide a standards-based approach for distributing software and allow integration with other (non-OSGi) systems. Although not required per se by an implementation of RFC 119, ‘Legacy integration' is one of the use cases that drove the design of Distributed OSGi, and by using Web Services this use-case can be addressed.
Q: You are using CXF for cross-jvm/osgi communication and mentioned that the rfc does not define the communication protocol. Does that mean that it is possible to implement a custom communication protocol?
Q: How can I specify which protocol(s) to be used for some remote service?
A: Use intents mechanism for that. The intents mechanism allows you to declaratively specify constraints on the remote service. In short, you can put a property on the service that you want to remote that declares using what binding/protocol you the service should be exposed. So you could do something like: osgi.remote.requires.intents=JMS which means that the service should only be exposed over JMS. If the Distribution Software (DSW) can't satisfy the intent, it will not be able to expose the service. Although some values (for SOAP and JMS) have been defined in the RFC for this property, the actual possible values depend on the Distribution Software implementation used.
Q: How do I set the discovery service dynamically without the remote-services.xml?
A: Use an RFC 119 compliant Discovery implementation, these don't use the remote-services.xml files. We're currently working on one in the Apache CXF codebase that is based on Apache Zookeeper, but other implementations, such as the one based on SLP at Eclipse ECF are also available.
Q: Can I specify more than one protocol for one service? For example Web Services, CORBA, RMI, etc.
A: If you specify the protocol to use using the osgi.remote.requires.intents mechanism as described above, you can only specify one. E.g. specifying that the service has to satisfy both the RMI and CORBA intent at the same time is a contradiction, however you can register the same POJO twice with the OSGi Service registry, once for each protocol.
Q: Is Discovery service a separate OSGi container?
The RFC 119 Discovery architecture typically uses a centralized Discovery Server (potentially a replicated one, like we have with the CXF/Zookeeper implementation). The Discovery Service in the OSGi container provides a standardized interface to such a Discovery Server. The Distribution Software (DSW) interacts with the Discovery Service in the local OSGi service registry, which then communicates with the centralized Discovery Server.
Q: Service Registry hooks- limiting visibility of service-This is similar to access control the service- what are your views on this? Thanks
A: I forwarded this question to BJ Hargrave from IBM, the main author of the Service Registry Hooks RFC, he provided the following answer: "It is more about controlling visibility to services (but I guess some people may call that access [see http://blog.bjhargrave.com/2009/03/i-am-visible-but-am-i-accessible.html]). But the primary purpose is not to secure the system since without a Security Manager installed, there is no actual security. Its primary purpose is to control service visibility to influence the services a bundle sees so that it selects a desired service rather than another service. For example, the proxy for X rather than X. "
Q: so if they [OSGi Services] don't implement any signature, how are they identified within the registry?
A: OSGi Services don't need to implement a particular OSGi signature or interface, but they would typically implement an interface that relates to their purpose. In the presentation I mentioned a Purchase Service, which implements a foo.bar.PurchaseService interface. Normally services are looked up via their interface, but you can also look up services in the OSGi Service Registry purely on other properties that they might have.
Q: When you say 'Dynamically come and go', can you elaborate on that what does that really mean?
A: Example: A running OSGi system. A consumer in that system might be using a service that satisfies its lookup criteria. While the system is running another service that also satisfies these criteria could arrive (either because a new bundle that creates this service is installed or because an existing bundle registers a new service). Depending on the consumer's requirements, it could now potentially be using both services, the old and the new one. At a later time one of the services might go away at which point the consumer will only be using the one that's left. All (properly developed) OSGi service consumers are capable of dealing with dynamics like this.
Q: is the WSDL, automatically generated by some tool?
A: Yes, the Apache CXF DOSGi implementation does this dynamically at runtime, based to the Java interface of the service. You don't have to run a tool for this with CXF, although other implementations might require the running of a tool to achieve something like this.
Q: What does pojo mean?
A: Plain Old Java Object.
Q: I suppose that the parameters of the service exposed are of type 'primitive' like int, string, double, DateTime but cannot be of type by example SessionFactory ?
A: It depends on the DOSGi implementation used. If you want your code to be suitable for a number of DOSGi implementations it's best to restrict the use of data types to primitives, collections and arrays. The CXF DOSGi implementation also supports classes and interfaces as part of the signature as long as they in turn are defined in terms of the data types mentioned.
Q: Can you highlight the security capabilities that are provided by distributed OSGi?
and Q: Can we secure the communication between remote OSGI servers (and authenticate, authorise access to the services)?
A: Security is a Quality of Service that DOSGi implementations could provide. The intents mechanism can be used to only select remote services that are secure, e.g. by requesting the ‘confidentiality' intent, or by requesting a custom intent (such as AcmeSecurity) that your deployer defines with the appropriate security configuration. For more information see chapter 5.5.3 of RFC 119 (in http://www.osgi.org/download/osgi-4.2-early-draft3.pdf).
Q: How do local discovery services know about other discovery services?
A: The Discovery Service registered in the OSGi container communicates with a centralized Discovery Server. This Discovery Service needs to be configured with the connection details of the central discovery server.
Q: what implementations of distributed OSGi will be available in Equinox 3.5? Anything faster than webservices?
A: Equinox 3.5 does not have any Distributed OSGi implementations, but Eclipse 3.5 provides the ECF implementation. I guess I'm wondering what really the performance difference is that you are looking for. Have you measured the performance of CXF webservices versus something else? Could you provide us with some target figures? I know that CXF performs pretty well…
Q: Can you specify multiple instance endpoints for the same service?
A: On the server side one remote service typically maps to one remote endpoint (although exceptions are possible, especially in case the service implements multiple interfaces some implementations will give you one endpoint per interface). If you want more endpoints you can always register more OSGi services for the same POJO (although I'm not sure what the use-case for that would be).
On the consumer side, the consumer can be informed of multiple remote endpoints satisfying the OSGi Service Lookup criteria, in which case the consumer could get multiple client-side proxies to the remote services.
Q: Can you talk more about discovery with ZooKeeper and how that would work?
A: In the CXF project I started implementing the RFC 119 Discovery implementation using Apache ZooKeeper as the underlying Discovery mechanism. ZooKeeper is based on a replicated central server that holds a virtual file system. Consumers connecting to the ZooKeeper server will all view the same filesystem which holds the discovery information. The way this works in practice is that you have your cluster (1 or more) of zookeeper servers running. The CXF Discovery implementation bundle is installed in your OSGi containers. It will communicate with the ZooKeeper server for the discovery information. The net effect is that the remote service consumers will see the remote services without needing any extra configuration.
Q: Question : Is there other org.osgi.remote.type foreseen (e.g. POJO) ? What is the particularity of the type 'POJO' except its name ?
A: The POJO configuration type is specific to CXF. There is one configuration type specified in RFC 119: SCA, which is optional. Using SCA as a way to configure Distributed OSGi will give you portability on the configuration level. CXF does not implement the SCA configuration type yet…
Q: Question : Will it be possible using 'blueprint or spring DM' to integrate remote OSGI with Spring XML file ?
A: Yes - there is a demo that comes with the CXF DOSGi implementation that uses Spring-DM to create a remote OSGi service and on the consumer side it uses Spring-DM to create the consumer. Distributed OSGi is orthogonal to any OSGi component model and should therefore work fine with any of them. See here for the demo code: http://svn.apache.org/repos/asf/cxf/dosgi/trunk/samples/spring_dm
Q: is it possible to load balance across distributed OSGi bundles?
A: Yes – if multiple remote instances exist the consumer could be informed about all of them: the consumer will get a service proxy for each remote instance. The consumer can use these to implement some load-balancing algorithm.
Q: Why does a copy of the interface and class have to live on the client?
A: Only the interface needs to be available to the service consumer. The implementation class does not. I think this is a very natural thing to do. If I write a consumer to a service, like a PurchaseService, I need to know the interface to that PurchaseService, how am I otherwise going to use this in a meaningful way? The interface is the contract between the consumer and the service provider. This is also the case for local OSGi Services. They can't use an OSGi service if they don't have access to the interface.
Q: It seems like you are cheating if you get a copy of the interface and class in your client code prior to starting the demo.
A: No I'm not cheating, only the interface is present in the consumer. When I run the consumer I can see that the value I'm entering in the consumer running in Felix is echoed in the Equinox server window. The implementation resides in Equinox.
Q: Does distributed OSGi support multiple instance of the same service and thus software based failover rof services across remote services?
A: Yes, see above.
Q: for properties is there a namespace type construct to prevent collisions? or is it inherent based on the scope of a service?
A: No there isn't really a namespace (although the Distributed OSGi properties use the osgi. prefix), but it is good practice to qualify your own properties with your own organization just like a Java package, e.g. com.acme.myproperty to avoid clashes.
Q: Are there standard properties of services, like names, that every OSGI service should define?
A: No. Its absolutely fine to register a service with no properties. OSGi will typically add a few properties, like service.id and objectClass but as a service provider you don't need to worry about these.
Q: How does a Discovery service, choose if the service is deployed both locally and remotely?
A: If a good match is available locally, DOSGi will not look for remote services. If you want to influence your match, forcing either a local or a remote match you can refine your lookup by adding extra criteria in the LDAP filter.
Q: Can the contract by dynamically found and transmitted remotely?
A: You could write a system to do this, as bundles can be installed at any time in a running OSGi system, but the Distributed OSGi standard doesn't define this.
Q: With multiple instances of the same service, would they have the same endpoint, or could they have different endpoints? How would a discovery pick which one?
A: The will have different endpoints. Everything being equal Discovery will simply inform you about all of them. If your consumer only needs to use one of them, which one the consumer will get is undefined. If the consumer has a preference for a particular one it needs to refine its filter to only match that one. You can also have the consumer receive proxies to all remote instances.
Q: Can you turn existing code (POJOs) into distributed OSGI services just using configuration (i.e. no java re-coding)?
A: Yes, OSGi Blueprint/Spring-DM provide a lot of support here. You configure them with an XML file that will instantiate your POJO and register it as an OSGi service.
Q: Are you familiar with the Paremus fabric, distributed OSGi runtime that uses SCA as the "wire-up" model? If so, does it use SCA in the way you described?
A: I'm not really familiar with this product, but I have seen demos of it. The SCA part of RFC 119 is really about defining qualities of service for the distributed services. It's not used for wiring services, the OSGi Service registry is used for that.
Q: Will both local and remote services be returned from a search?
A: There is no difference in the API to use to obtain local services or remote services. However, if appropriate local matches are available, the system won't look for remote services. If you want all local and remote matches you can do lookups with and without the osgi.remote property. The osgi.remote property is only set on proxies for remote services.
Q: Are there any drawbacks, using OSGI distributed services that you may know of?
A: None :) Seriously, yes remote computing is not the same as local computing. There are additional failure scenarios, additional latency, additional security impacts and so on. So if your system works perfect with all your services locally, keep it that way. However, if you want to distribute your OSGi services, then RFC 119 is a good way to do it!