Difference between revisions of "EIG:Remoting"

Latest revision as of 01:10, 24 January 2013

Remoting

When we talk about services, we really talk about Java objects. We just call them services because of the way we use these Java objects. Google for "SOA Patterns" for more information on this topic. These Java objects are POJO's, as far as ECF is concerned. There is no need to implement anything from ECF in order for the "service" to be exposed as a remote service. The process to make a Java object (slash service) available on a remote machine is the mechanism we call "Remoting".

Creating an OSGi Service

OSGi Services should be defined by/registered under a specific interface.

public interface MyService {
 
    public String hello(String name);
 
}

There should then exist some implementation of this service:

public class MyServiceImpl implements MyService {
 
    public String hello(String name) {
        return "Hi " + name + "! My name is Budhead.";
    }
 
}

Registering an OSGi Remote Service

The registration of an OSGi Remote Service is nearly the same as with any regular service in an OSGi Framework (e.g. Eclipse Equinox). The only additional thing you need to do is setting some service properties. For the Eclipse Communication Framework (ECF) only two properties are required: The "service.exported.interfaces" property marks the service for export (as an OSGi Remote Service) and defines under which interfaces this service can be exported. The "service.exported.configs" property is a list of configuration types (endpoint types) that should be used to export the service. The other properties are optional settings for the distribution provider.

Service Properties

Example

public class Activator implements BundleActivator {
 
    private MyService myService = null;
    private ServiceRegistration myServiceRegistration = null;
 
    private static BundleContext context;
 
    public static BundleContext getContext() {
        return context;
    }
 
    public void start(BundleContext bundleContext) throws Exception {
        Activator.context = bundleContext;
 
        // Instantiate a service
        this.myService = new MyServiceImpl();
 
        // Register the service instance as an OSGi Remote Service
        Properties props = new Properties();
        props.put("service.exported.interfaces", "*");
        props.put("service.exported.configs", "ecf.r_osgi.peer");
        this.myServiceRegistration = bundleContext.registerService(MyService.class.getName(), this.myService, props);
    }
 
    public void stop(BundleContext bundleContext) throws Exception {
        // Unregister the service
        if(this.myServiceRegistration != null) {
            this.myServiceRegistration.unregister();
        }
        this.myServiceRegistration = null;
        this.myService = null;
    }
 
}

Using OSGi Remote Services

If you would like to use OSGi Remote Services this is as simple as with regular services in the framework. You don't need to do anything different as usual, all remote services discovered by an ECF Discovery Provider are registered with the service registry of your OSGi Framework. So you either use the "getServiceReference(s)" method of your bundle context or create a service tracker for your service.

If you have local and remote services of the same type (e.g. registered under the same interface) in your OSGi Framework, local services can be filtered out by using an LDAP filter on the property "service.imported".

Example: Service Reference

ServiceReference myServiceReference = bundleContext.getServiceReference(MyServiceInterface.class.getName());
if(myServiceReference != null) {
    MyService myService = (MyService) bundleContext.getService(myServiceReference);
    if(myService != hull) {
        System.out.println(myService.hello("Beavis"));
    } else {
        throw new ServiceException("Service object is null.");
    }
} else {
    throw new ServiceException("Service reference is null.");
}

Example: Service Tracker

public class MyServiceTracker extends ServiceTracker {
 
    public MyServiceTracker() {
        super(Activator.getContext(), MyService.class.getName(), null);
    }
 
    @Override
    public Object addingService(ServiceReference reference) {
        MyService myService = null;
        if(reference != null) {
            myService = (MyService) Activator.getContext().getService(reference);
            if(myService != null) {
                System.out.println("MyServiceInterface has come.");
            } else {
                throw new ServiceException("Service object is null.");
            }
        } else {
            throw new ServiceException("Service reference is null.");
        }
        return myService;
    }
 
    @Override
    public void removedService(ServiceReference reference, Object service) {
        System.out.println("MyServiceInterface has gone.");
    }
 
}

Example: Service References with Filter

Filter filter = bundleContext.createFilter("(service.imported=*)");
ServiceReference[] myServiceReferences = bundleContext.getServiceReferences(MyService.class.getName(), filter.toString());

Asynchronous Remote Services (not OSGi standard)

Usualy Remote Procedure Calls operates synchronously. ECF provides an API for asynchronous calls that are executed in some other thread and don't block. This is sometimes desirable, for example, if your user interface thread is doing the calling, as remote methods can/could block for a much longer time than would be acceptable for users.

This asynchronous Remote Service implementation is no OSGi standard. For background on the discussion about asynchronous remote services invocation in general and possible future standardization by the OSGI standards organization see Peter Kriens blog entry, and Scott Lewis' blog entry.

For the Implementation of an asynchronous Remote Service the only thing you need to do is implementing an additional interface for your service. This interface should extend the IAsyncRemoteServiceProxy interface with an "Async" added to the interface name. This interface then should map all of the methods of your original interface with "Async" added to the method name and an additional callback parameter. Also this interface must be located in the same(!) package as the original interface of your service. The last parameter of the asynchronous method should then be an instance of the IAsyncCallback<?> interface with the original return type of your method as generic type parameter.

Another approach (next to IAsyncCallback<?>) to achieving asynchronous invocation of remote services are futures. Future objects allow the consumer to perform operations in between initiating a remote call and receiving a result.

Both of these approaches can be used seperate and also togehter.

public interface MyServiceAsync extends IAsyncRemoteServiceProxy {
 
    public void helloAsync(String name, IAsyncCallback<String> callback);
    // and/or
    public IFuture helloAsync(String name);
 
}

Note that with this you will have direct dependencies to ECF bundles. Also some of the distribution providers still don't handle the registration of multiple interfaces correctly.

Example

if(myService instanceof MyServiceAsync) {
    ((MyServiceAsync)myService).helloAsync("Beavis", new MyServiceCallback());
}

public class MyServiceCallback implements IAsyncCallback<String> {
 
    @Override
    public void onSuccess(String result) {
        System.out.println(result);
    }
 
 
    @Override
    public void onFailure(Throwable e) {
        e.printStackTrace();
    }
 
}

Example (Future)

This initiates a remote call, performs some additional local operations and then gets a result from the future.

IFuture future = ((MyServiceAsync)myService).helloAsync("Beavis");
// Do other local operations here...
String result = (String) future.get();
System.out.println(result);