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Tutorial: Creating Custom Distribution Providers

Revision as of 15:47, 16 March 2016 by (Talk | contribs) (Step 1: Create and Register a Namespace)


The ECF project provides an implementation of the OSGi R6 Remote Services and Remote Service Admin specifications. The RSA specification defines two major subsystems: discovery and distribution. Discovery concerns finding remote services exported by other processes on the network. The distribution subsystem is responsible for the actual communication of invoking a remote call: serializing remote method parameters, communicating with the remote service host via some network transport protocol, unmarshalling and invoking the service method with provided parameters, and returning a result to the consumer.

ECF's implementation of RSA defines an API to create new distribution providers. This API is declared in the ECF remote services API, provided by the org.eclipse.ecf.remoteservices bundle. Custom distribution providers implement a portion of this API and then will be used at runtime to supply the necessary functions of the distribution provider.

This tutorial will describe the creation of a simple custom distribution provider using the relevant portions of the ECF remote services API.

Remote Service Containers, IDs, and Namespaces

ECF has the concept of a 'container' (IContainer), which is an object instance that implements the remote services API and represents a network-accessible endpoint.

Containers have unique transport-specific ID. Some examples of transport-specific container IDs:

Each ID instance must be unique within a Namespace. Distribution providers must define a new Namespace that enforces the expected syntax requirements of the endpoint identifier.

Step 1: Distribution Provider Namespaces

The first thing a distribution provider must do is to register a new type of Namespace. ECF provides a number of Namespace classes that can be extended to make this easy. For example, the URIIDNamespace can handle any ID syntax that can be represented as a URI, like all of the above. Here is an example Namespace class that extends the URIIDNamespace:

public class Example1Namespace extends URIIDNamespace {
	private static final long serialVersionUID = 2460015768559081873L;
	public static final String NAME = "ecf.example1.namespace";
	private static final String SCHEME = "ecf.example1";
	private static Example1Namespace INSTANCE;
	 * The singleton instance of this namespace is created (and registered
	 * as a Namespace service) in the Activator class for this bundle.
	 * The singleton INSTANCE may then be used by both server and client.
	public Example1Namespace() {
		super(NAME, "Example 1 Namespace");
		INSTANCE = this;
	public static Example1Namespace getInstance() {
		return INSTANCE;
	public String getScheme() {
		return SCHEME;

In bundle Activator start, the singleton Example1Namespace is created and registered:

// Create and register the Example1Namespace
context.registerService(org.eclipse.ecf.core.identity.Namespace.class, new Example1Namespace(),  null);

The same Example1Namespace type must be used by both servers and clients, and so it typically makes sense to define the Namespace in a small common bundle, which can be deployed on both servers and clients. For the source code for this example, see the bundle here.

Note that other type of ID syntax can be easily supported by either inheriting from other Namespace classes (e.g. LongIDNamespace, StringIDNamespace, UUIDNamespace, etc.), or creating one's own Namespace subclass.

Step 2: Distribution Provider Server

In addition to registering a new Namespace, distribution providers must implement and register the IRemoteServiceDistributionProvider interface using the whiteboard pattern.

For example, here is the code for registering a new custom distribution provider server

public static final String SERVER_ID_PARAMETER = "id";
public static final String SERVER_ID_PARAMETER_DEFAULT = "tcp://localhost:3333";
// register this remote service distribution provider
   new RemoteServiceDistributionProvider.Builder().setName(ProviderConstants.SERVER_PROVIDER_CONFIG_TYPE)
      .setInstantiator(new RemoteServiceContainerInstantiator(ProviderConstants.SERVER_PROVIDER_CONFIG_TYPE,
         ProviderConstants.CLIENT_PROVIDER_CONFIG_TYPE) {
         public IContainer createInstance(ContainerTypeDescription description,Map<String, ?> parameters) {
            // Create and configure an instance of our server 
            // container type
            return new Example1ServerContainer(getIDParameterValue(Example1Namespace.getInstance(), parameters,

Notes for the above code

The RemoteServiceDistributionProvider.Builder class is used to create the IRemoteServiceDistributionProvider instance.

The setName(ProviderConstants.SERVER_PROVIDER_CONFIG_TYPE) method on the Builder sets the distribution provider's name (ProviderConstants.SERVER_PROVIDER_CONFIG_TYPE==ecf.example1.provider.dist.server).

This name corresponds to the OSGi Remote Service config type, and must uniquely identify the distribution provider. When a remote service is registered, the standard service property service.exported.configs may be specified so that the remote service will be exported via this distribution provider rather than some other distribution provider. For example:

Hashtable serviceProperties = new Hashtable();

The underlying ECF RSA implementation will export the serviceInstance via the ecf.example2.provider.dist.server distribution provider.

The other required part of the distribution provider is the container instantiator:

      .setInstantiator(new RemoteServiceContainerInstantiator(ProviderConstants.SERVER_PROVIDER_CONFIG_TYPE,
         ProviderConstants.CLIENT_PROVIDER_CONFIG_TYPE) {
         public IContainer createInstance(ContainerTypeDescription description,Map<String, ?> parameters) {
            // Create and configure an instance of our server 
            // container type
            return new Example1ServerContainer(getIDParameterValue(Example1Namespace.getInstance(), parameters,

This code uses the RemoteServiceContainerInstantiator class. The two constants in the constructor (ProviderConstants.SERVER_PROVIDER_CONFIG_TYPE,ProviderConstants.CLIENT_PROVIDER_CONFIG_TYPE) sets up the instantiator so that these two config types (server and client for distribution provider) are associated with each other so at remote service consumer import time the ECF RSA implementation can select the appropriate client config type associated with the exporting server config type. For example, when ecf.example1.provider.dist.server is used as the exporting config type, then on the client/consumer the selected config type will be: ecf.example1.provider.dist.client
The createInstance method then creates an instance of the appropriate container type. This method allows the container to be created and configured based upon a Map of parameters. As per the RSA specification, if the remote service registration has service properties with keys of the form:
<config type>.<param>
then the provided parameters Map will contain entries of the form
key: <param>=<value>
For example, if the remote service is registered:

Hashtable serviceProperties = new Hashtable();

then the parameters Map will contain an entry with key="id" and value="tcp://localhost:3333". Such values can be used to configure the provider. In the case of the Example1 provider above, the id parameter is used to create an ID via the call to getIDParameterValue. Other distribution providers can define arbitrary parameters used to configure the underlying transport such as host, port, bind address, path, etc.
At the appropriate time during export, the createInstance method will be called and the Example1ServerContainer instance created. Here is the implementation of Example1Container

 * Instances of this class are created via the container instantiator specified
 * by the RemoteServiceDistributionProvider whiteboard service in the Activator.
 * @see Activator#start(org.osgi.framework.BundleContext)
public class Example1ServerContainer extends AbstractRSAContainer {
	 * Create an Example1ServerContainer.  The given id must not be null,
	 * and should be created using the Example1Namespace.  The server's ID
	 * will then be automatically used to provide the to
	 * remote service clients via the RSA-created EndpointDescription.
	 * @param id
	public Example1ServerContainer(ID id) {
	 * When the ECF RSA impl is requested to export a remote service and 
	 * this container instance is selected, a remote service registration will be
	 * created by RSA and this method will then be called to actually export the given
	 * registration via some communications transport.  This should trigger the appropriate 
	 * networking initialization (e.g. opening listener on socket), given the info in the
	 * registration.
	 * The given registration will not be <code>null</code>.
	 * If the transport would like to insert properties into the EndpointDescription
	 * for this endpoint, a Map of name (String) -> Object map should be returned.  All
	 * the values in the Map should be Serializable.   This provides a mechanism for
	 * distribution providers to include arbitrary private properties for use by 
	 * clients.
	 * Note that keys for the Map should be unique to avoid conflicting with any other properties.
	 * If values are provided for either OSGI RemoteConstants or ECF Remote Services Constants
	 * then these new values will <b>override</b> the default values in the EndpointDescription.
	protected Map<String, Object> exportRemoteService(RSARemoteServiceRegistration registration) {
		// TODO Auto-generated method stub
		return null;
	 * When this remote service is unregistered, this method will be called by ECF RSA
	 * to allow the underlying transport to unregister the remote service given
	 * by the registration.  Necessary clean-up/shutdown of transport should be completed
	 * before returning.
	protected void unexportRemoteService(RSARemoteServiceRegistration registration) {
		// TODO Auto-generated method stub

Once this container instance is created the ECF RSA implementation will then call exportRemoteService(RSARemoteServiceRegistration registration) and expect that upon successful completion that the distribution provider will have made the remote service described by the given registration will have been made available for remote access. The exportRemoteService implementation is therefore where the distribution provider should actually make the remote service available over the network (e.g. by listening on socket, joining communication group, etc). When the remote service is unregistered, the unexportRemoteService(RSARemoteServiceRegistration registration) will be called to allow the transport to close and clean up any network resources associated with the previous export.
Each method will be called for each remote service export, with a unique RSARemoteServiceRegistration passed in by the ECF RSA implementation.
For the exportRemoteService method, the implementation can return a Map of service properties (or null). If a Map instance is returned, the values in the given map will be merged with the other endpoint description properties defined/required by ECF's RSA implementation. For example, one required ECF remote service property is The default value of the is the exporting container's ID...e.g. by default will be set to: "tcp://localhost:3333" because the Example1Container's ID is set by construction to "tcp://localhost:3333". However, if the returned Map is non null, and has the key, the value in the returned map will override the default value. e.g.

	protected Map<String, Object> exportRemoteService(RSARemoteServiceRegistration registration) {
		// TODO setup transport and export given remote service registration
                Map<String,Object> result = new HashMap<String,Object>();
                // This will override the value in the RSA endpoint description
                // This will add a brand new property to the RSA endpoint description
		return result;

The complete source for the distribution provider server is available here.

Service-Level Concerns

Although frequently of secondary concern to service implementers, service-level concerns are typically of primary importance to service consumers. For example, before I can use any REST-based service, I have to know about (discover) the service and needed meta-data: the service location (i.e. it's URL), what methods exist and arguments are required, and what can be expected in response. Additionally, what version of the service is being accessed, what credentials/security are required, what are the run-time qualities of the remote service?

OSGi Remote Services

In recent releases the OSGi Alliance has defined a specification called Remote Services (chapter 100 in the enterprise spec). Along with the Remote Service Admin specification (chapter 122 in enterprise spec), Remote Services defines ways to express answers to service-level concerns in an implementation-independent way, similar to what JAX-RS does for transport-level concerns. For example, the Remote Service spec allows meta-data for service type, service version, service endpoint/URL identification, security, and quality of service properties to be communicated between service implementer and service consumer without referring to any Remote Services implementation. Further, Remote Services/RSA defines a way to dynamically discover and un-discover remote services, allowing for easy consumer support for networked and frequently unreliable services.

Several implementations of OSGi Remote Services exist, including open source ECF, CXF, Amdatu and commercial implementations.

OSGi RSA has the concept of a distribution system responsible for exporting a local service and making it available for remote access. According to the specification, many distributions systems may be used even for a single service. ECF's implementation is unique because it has an open API for creating/adding new distribution providers. Currently, ECF committers have implemented the following distribution providers

Jax-RS with OSGi Remote Services

Since ECF's RSA implementation implements the OSGi RS and RSA specifications, and Jersey implements the Jax-RS specification, it's possible create and export a remote service that deals with both the transport and service-level concerns in a way completely standardized, and not dependent upon either the OSGi RS/RSA implementation (e.g. ECF), nor on the distribution system provider implementation (Jersey).

For example, here is some example OSGi code for exporting a remote service using the Jax-RS/Jersey provider:

BundleContext bundleContext;
Dictionary<String, String> props = new Hashtable<String,String>();
// osgi rs property to signal to distribution system 
// that this is a remote service
// specify the distribution provider with osgi rs property
props.put("service.exported.configs", "ecf.jaxrs.jersey.server");
// as per spec, <provider>.<prop> represents a property intended for use by this provider
props.put("ecf.jaxrs.jersey.server.alias", "/jersey");
// register (and export) HelloImpl as remote service described by Hello interface
bundleContext.registerService(HelloWorldService.class, new HelloWorldResource(), props);

HelloWorldResource implements a HelloWorldService interface which is defined

// The Java class will be hosted at the URI path "/helloworld"
public interface HelloWorldService {
    // The Java method will process HTTP GET requests
    // The Java method will produce content identified by the MIME Media
    // type "text/plain"
    public String getMessage();

Note that the HelloWorldService has exactly the same Jax-RS annotations as the HelloWorldResource implementation class.

With the Jax-RS/Jersey distribution provider, the above registerService call will dynamically export the remote service (via Jersey) so that remote clients (Java/OSGi-based or not) can access getMessage by sending a GET to an URL like this:

curl http://localhost:8080/jersey/helloworld/1

Would respond with "Hello World"

The use of Jax-RS annotations to implement an ECF distribution provider, and the use of OSGi Remote Services has several important advantages for the service implementer and the service consumer:

  1. An ability to flexibly handle both transport-level and service-level concerns in a way appropriate to the application
  2. A clean separation between service contract (HelloWorldService interface), and service implementation (HelloWorldResource)
  3. A clean separation between application concerns from transport or service-level concerns
  4. Alternative implementations of Jax-RS and/or OSGi RS/RSA may be substituted at any time without application changes

Using the Remote Service

Since the HelloWorldService is exported via Jax-RS/Jersey provider, it may be accessed by any client (e.g. javascript, java, curl, etc) that can access via http calls. If, however, it is an OSGi client (or a non-OSGi Java Client) ECF Remote Service can automatically construct a proxy for the remote service, and make the proxy available as an OSGi Service on the client. For a description and example of doing this, see Exposing a Jax REST Service as an OSGi Service.

A More Complete Example

A more complex example exists in the Jax-RS/Jersey Provider repo. The remote service host/server is in the bundle. The remote service consumer/client is in com.mycorp.examples.student.client bundle. Notice that neither of these bundles has references to ECF, Jersey, or even OSGi classes, but rather only to Jax-RS standard annotation types (javax.*) and model classes defined in the com.mycorp.examples.student bundle.

Background and Related Articles

Tutorial: Exposing a Jax REST service as an OSGi Remote Service

Getting Started with ECF's OSGi Remote Services Implementation

OSGi Remote Services and ECF

Asynchronous Proxies for Remote Services

Static File-based Discovery of Remote Service Endpoints

Download ECF Remote Services/RSA Implementation

How to Add Remote Services/RSA to Your Target Platform

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