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details should seek a book on that topic. For the .NET developers, I recommend my book, Foundations of Object-Oriented Programming Using .NET 2.0 Patterns (Apress, 2005) as it goes into detail regarding the use of .NET generics.

Finishing the controller implementation means applying the Extension pattern for two levels of abstraction. The first level is the general case of implementing the REST-Based Model View Controller pattern. The second level is the case of implementing a search engine based on the REST-Based Model View Controller pattern. This section focuses on the first level of abstraction. The controller manages the local clients. In the context of the REST-Based Model View Controller pattern, the controller fulfills the abstract role of executing the local clients, managing the local clients, managing the request, and managing the results that will be returned to the client. The controller exposes itself to the local client by using an interface called Parent that is defined as follows: public interface Parent { public void addResult( Result result); public Request getRequest(); public void addCommand( Command cmd); public Iterator getCommands(); public void processRequest( Request request); public void processRequest( String type, Request request); EBVN public String getTransactionIdentifier(); } The methods of Parent use general types such as Result, Command, and Request. Result defines a result generated by a local client. Request defines the HTTP request parameters such as the query string. And the local clients implement Command. There are two variations of the method processRequest. The processRequest with a single parameter will execute a search on all local clients. The processRequest with two parameters has as a first parameter the identifier of the local client that will process the request and generate the results (for example, amazon). The Request and Result interfaces are defined as follows: public interface Result { } public interface Request { } The interfaces have no method implementations and therefore represent pure general types, as illustrated by the Extension pattern example. The local clients implement the Command interface, which is defined as follows: public interface Command { public void setRequest( Request request); public void assignParent( Parent parent); public String getIdentifier(); }

The method assignParent is used to assign the parent controller with the local client. The association is needed when the local client generates a result and wants to pass the result to the controller, which then passes it to the client. The method getIdentifier is used by the implementation of the method Parent.processRequest( String type, Request request) to identify which Command instance is executed.

The search engine local clients (Amazon.com and Google) implement two interfaces: Command and Runnable. The search engine local clients are managed by Parent using the Command interface, but Parent executes the local client by using the Runnable interface. The reason is that the controller executes each local client on its own thread. The reason for an individual thread will be discussed shortly. An example implementation of the Amazon.com search engine local client would be as follows (note that some details have been removed for clarity): public class AmazonSearchCommand implements Command, Runnable { private String _endpoint; private String _accessKey; private String _secretAccessKey; private Parent _parent; public void assignParent( Parent parent) { _parent = parent; } public AmazonSearchCommand( String endpoint, String accessKey, String secretAccessKey) { _endpoint = endpoint; _accessKey = accessKey; _secretAccessKey = secretAccessKey; } public String getIdentifier() { return "amazon"; } } The run method implementation has been removed and was already shown in the section Using Amazon.com to Search for Something. What has been kept are the details relating to the instantiating and configuring of the Amazon.com local client. The method getIdentifier is a hard-coded string that returns the identifier amazon. Normally, hard-coded strings are a bad idea, but because the Amazon.com local client is being referenced, the identifier is not going to change. Let s put it this way: you are not going to reference the Amazon.com local client as Google or Barnes & Noble. The identifier amazon is identical to the URL /search/impl/amazon. The same value is not a coincidence because when the /search/impl is retrieved, the generated links are generated by the controller that iterates the local clients, which in turn are queried by using the method getIdentifier. In the section Using Amazon.com to Search for Something, there were references to configuration items such as the access key. The configuration items are passed to the client using the constructor. The constructor was chosen so that under no circumstances can the