SCA Async/Conversational services Part 1: Internals of Tuscany SCA

Sometime ago I wrote about developing applications with SCA on a JBI-based infrastructure, using simple SCA services for that.

I’m coming back again with two separate SCA blog posts discussing the usage of more complex services, asynchronous and conversational services:

• In this post, I provide an example client-server project that implements a conversational and asynchronous service in Tuscany SCA, digging into the internals of the implementation for handling these services using Web Services and JMS bindings. I found this information quite difficult to find.
• The second post looks into the situation where the usage of a Tuscany SCA runtime is not possible in the client side, but we still need a way to make use of these type of services from a more standard web service client. For the specific case of a web service binding, and knowing the internals of Tuscany I will use a standard jax-ws client to consume a Tuscany SCA conversational and asynchronous service.

Both posts use a common example project, a Counter service that notifies to the client each time the count is updated (every second) via an asynchronous callback. The service is conversational and allows clients to stop the count and restart it through separate independent service calls. Also, multiple clients can be run in parallel, each one with its own counter service instance. Version 1.6 of Tuscany SCA is used in this sample.

The source code of the sample counter service for this post can be found here.

SCA asynchronous and conversational service definition

Let’s refresh how to define a service in Tuscany SCA that is both conversational and defines a callback interface for asynchronous calls to the client. Good references for this are the SCA specifications, the Tuscany SCA documentation and the sample projects included in the distribution. However, while I could find several callback projects on the Tuscany SCA samples there are none that exercise conversations.

Below, you find the interface definition of the SCA service in Java for the CounterService (counter-callback-ws-service maven project).

I use the @Callback and @Conversational annotations to define our service. For the @Callback annotation we need also to define the callback interface class. Therefore, for our counter service we have two interfaces, one for the service which implementation is done in the server side, and another one for the callback which implementation is done in the client side.

/**
 * The remote service that will be invoked by the client
 */
@Remotable
@Callback(CounterServiceCallback.class)
@Conversational
public interface CounterService {
@OneWay
    void startCounting(int maxcount);
    
    @OneWay
    void stopCounting();
@OneWay
    void restartCounting();    
    
    @OneWay
    void shutdownCounting();     
}
The Callback interface is as simple as:
/**
 * The callback interface for {@link CounterService}.
 */
@Remotable
public interface CounterServiceCallback {
void receiveCount(int count, int end);
}
The server side implementation of the service looks like:
/**
 * This class implements CounterService and uses a callback.
 */
@Service(CounterService.class)
@Scope("CONVERSATION")
public class CounterServiceImpl implements CounterService {
	@ConversationID
	protected String conversationID;
/**
	 * The setter used by the runtime to set the callback reference
	 * 
	 * @param counterServiceCallback
	 */
	@Callback
	public void setMyServiceCallback(CounterServiceCallback counterServiceCallback) {
		System.out.println("setCounterServiceCallback on thread "
				+ Thread.currentThread());
		this.counterServiceCallback = counterServiceCallback;
	}
On the implementation, the @Service SCA annotation is used to identify the SCA service being implemented and the @Scope which specifies a visibility and lifecycle contract an implementation has with the SCA runtime. Possible values of @Scope are STATELESS, REQUEST, CONVERSATIONAL and COMPOSITE. For this specific service we need to use CONVERSATIONAL, instructing Tuscany that conversation state must be kept in order to correlate interactions between a client and the service. For a description of the rest of scopes, refer to the SCA Java Annotations and API reference specification.
Last, we use the @Callback annotation to instruct Tuscany SCA where to inject the Callback reference to be used by the service.
The server side SCA composite is very similar than the Tuscany SCA official callback-ws-client and you can find it under the "src/main/resources/counterws.composite". The binding used for the sample is a Web Service binding.
Tuscany SCA Counter Service Client
On the client side (counter-callback-ws-client-SCA maven project) we need to define a client interface that makes use of the counter service. To make it simple, the interface has a method that returns the reference to the Counter Service and an additional helper method to get the current count recorded in the client based on the service callbacks. As in the server side, we have to make sure to define the interface as @Conversational and the implementation with @Scope("CONVERSATION") so Tuscany populates the service call with conversation information.
Also, to simplify the sample code, the shared interfaces between client and server (i.e. CounterService and CounterServiceCallback) are included in both projects separately. Ideally, these interfaces would be on a separate interfaces library used by both projects.
/**
 * The client component interface.
 */
@Conversational
public interface CounterServiceClient {
    
    public CounterService getCounterService();
    
    public int getCount();
    
}
The client composite is as listed below. It defines the reference to the counter Service and the associated callback. The url defined in the callback is used by Tuscany SCA in the client side to start the listening callback web service:
targetNamespace="http://counterws"
	name="counterws-client">
callbackInterface="com.tsl.sca.tuscany.demo.server.CounterServiceCallback" />
For the testing, a unit test is setup (WebServiceSCAClientTest.java),  exercising the counter service, callbacks and conversations.
Running the counter service sample project
To run the counter service sample, extract the source code and:

• Run the server side, executing the run-server.sh script in the counter-callback-ws-service directory. The server component should start and wait for client requests.
• …
  Nov 8, 2010 4:11:23 PM org.apache.coyote.http11.Http11Protocol start
  INFO: Starting Coyote HTTP/1.1 on http-8086
  Callback server started (press enter to shutdown)
  Nov 8, 2010 4:11:23 PM org.apache.tuscany.sca.http.tomcat.TomcatServer addServletMapping
  INFO: Added Servlet mapping: http://localhost:8086/CounterServiceComponent
• Go to the client project, counter-callback-ws-client-SCA and execute the tests with:
• mvn clean test
• On the client side you should see how the client starts the counting service, receives the callbacks and how the counting is stopped and later restarted, continuing the count where it was left.
• …
  Nov 8, 2010 4:11:28 PM org.apache.tuscany.sca.http.tomcat.TomcatServer addServletMapping
  INFO: Added Servlet mapping: http://localhost:1999/callback
  Starting Count and waiting 5 seconds for counts…
  CounterServiceCallback — — Received Count: 0 out of 30
  CounterServiceCallback — — Received Count: 1 out of 30
  CounterServiceCallback — — Received Count: 2 out of 30
  CounterServiceCallback — — Received Count: 3 out of 30
  CounterServiceCallback — — Received Count: 4 out of 30
  Stopping Count and waiting 5 seconds for no counts…
  Restarting Count and waiting 5 seconds for counts…
  CounterServiceCallback — — Received Count: 5 out of 30
  CounterServiceCallback — — Received Count: 6 out of 30
  CounterServiceCallback — — Received Count: 7 out of 30
  CounterServiceCallback — — Received Count: 8 out of 30
  Stopping the Client Node
  Nov 8, 2010 4:11:44 PM org.apache.tuscany.sca.node.impl.NodeImpl stop
  …
• On the server side, you should see the client call and the associated conversationID.
• …
  INFO: Added Servlet mapping: http://jmatute-laptop:8086/CounterServiceComponent
  setMyServiceCallback on thread Thread[Thread-2,5,main]
  ConversationID = d8a05f47–064b-4832–9cca-7ad035bd36ee. Starting count
  Sleeping …
  Sleeping …
  Sleeping …
  Sleeping …
  Sleeping …
  ConversationID = d8a05f47–064b-4832–9cca-7ad035bd36ee. Stopping count
  Sleeping …
  Sleeping …
  Sleeping …
  Sleeping …
  Sleeping …
  ConversationID = d8a05f47–064b-4832–9cca-7ad035bd36ee. Restarting count
  Sleeping …
  Sleeping …
  Sleeping …
  Sleeping …
  Thread interrupted.

Having reached this point, let's look into how Tuscany SCA is handling the conversations and callbacks and how the required information is included in the underling transport bindings (WS or JMS).
Internals of conversational and asynchronous services in Tuscany SCA
Tuscany SCA conversational services (i.e. marked with @Conversational) make use of conversation IDs in order to keep track and correctly map multiple invocations associated with a certain conversation.
SCA and Tuscany asynchronous services are similar to those defined, for instance, in BPEL where the forward and callback service calls are two complete separate service invocation calls. Therefore, Tuscany also requires additional information to know where the callback needs to be send and optionally provide a application correlation id for the callback call.
Summarising, the below table contains the information used by Tuscany SCA for conversation and callback services:
Tuscany informationDescriptionconversationIDUUID identifying the current conversation. This is used by Tuscany SCA to associate accordingly the service instance associated to that conversation.callbackLocationThe location of the callback service. This, depending on the binding used might be a URI for Web Service binding or a Queue name for JMS.callbackIDApplication-specified callback ID, that is used by the application to correlate callbacks with the related application state.
The above information needs to be mapped to the specific binding and here is where no standarisation exists, making difficult to have interoperable solutions:

• WebService Binding : for the Web Service binding, WS-Addressing headers are used in order to store the Tuscany SCA conversational/async required information. Below it is shown an example of a Tuscany Web Service conversational and asynchronous/bidrectional invocation:
• The WS-Addressing Address contains the Callback URI to be used for the callback invocation. This is normally setup by the client to notify the server where to send the callbacks.
• The WS-Addressing ReferenceParameters contains the other two information fields under specific Tuscany Namespace, the CallbackID and the ConversationID.
• http://localhost:1999/callback
  
  45a963da-2074–4bb2-b9ee-f721e2ec753b
  309b8322–1dc2–4c51-a4db-73d65edae391
  
  
  
  
  
  30
• JMS Binding : For JMS, Tuscany SCA uses JMS Message properties to store the required information. A screenshot (see below) of ActiveMQ console shows an Tuscany SCA JMS Message for conversational and async/bidrectional service.
• CallbackID, scaConversationID and scaCallbackQueue, Tuscany SCA propietary JMS message properties to hold the information.
• [caption id=”attachment_1185" align=”aligncenter” width=”569" caption=”Tuscany SCA JMS Message”]

• [/caption]

Conclusion
In this first post I have presented a Tuscany SCA example that covers both conversational and asynchronous scenarios, not currently available in the official Tuscany SCA samples, and looked into the internals of Tuscany SCA used to handle these services. This provides the basis for the next post, where I will be using this information to extend the project code with a non-SCA JAXWS-based web service client as an approach to consume these services without a Tuscany SCA runtime.