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Putting All of the Pieces Together
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The final step after defining the architecture and implementing the individual pieces is to put everything together into a working solution that can be called a REST-Based Model View Controller pattern. From an architectural perspective, a Java servlet or ASP.NET handler will interact with a Parent implementation. The Parent implementation is what pulls everything together and defines the model, view, and controller. Putting it all together, the architecture would appear similar to Figure 11-8.
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Figure 11-8. Architecture with all of the pieces assembled The UML diagram in Figure 11-8 looks complicated, but it can be separated into two blocks of functionality. There is an inner circle of interfaces and an outer circle of implementations. The inner circle has the types Request, Result, Parent, and Command. The outer circle has the types Amazon, Google, SearchResult, SearchRequest, ParentImpl, HttpServlet, and AsynchronousServlet.
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CHAPTER 11 REST-BASED MODEL VIEW CONTROLLER PATTERN
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Parent is the core of the entire system and is the bridge that binds all the pieces together. However, to keep it simple for Parent, Parent knows about only the inner circle of types. In the diagram, Google, Amazon, and HttpServlet know about the outer circle of types (SearchRequest, SearchResult) that are passed across the bridge.
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Implementing a Parent
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Implementing the Parent interface is a two-step process because the Parent interface plays the central role of processing the data. Let s consider the context. The Parent interface instance is responsible for executing the Command implementations, gathering the results, and making the request information available. Through all of these responsibilities, the Parent interface cannot use specific types but must use the general defined types. Additionally, the Parent interface implementation has to function whether the request is asynchronous or synchronous. The first step when implementing Parent is to define a base class that provides a certain amount of common functionality. The second step is then to create either an asynchronous or synchronous implementation. You need to separate an asynchronous implementation from a synchronous one because of how the results and threads are managed. Implementing the Base Class Before the synchronous and asynchronous Parent implementations are outlined, the first step is to outline the base type. The class ParentBase implements Parent, and a subset of the implemented functionality is outlined as follows (the remaining pieces will be explained in a moment):
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public abstract class ParentBase implements Parent { private List _commands = new LinkedList(); public void addCommand( Command cmd) { _commands.add( cmd); } public Iterator getCommands() { return _commands.iterator(); } public void clearAllCommands() { _commands.clear(); } The code excerpt shows that the individual local client instances (Command) are managed in a LinkedList. To add a local client, the method addCommand is used. To remove all local client instances, the method clearAllCommands is used. Because we are coding in a managed code environment, removing the Command instances does not equate to deleting them. They will be deleted when there are no references to the local client instances. This is important because when the local client instances are cleared, the threads referencing the local client instances will still be executing. It would be very inappropriate to have to wait until all of the old local client instances have finished executing, or to stop the execution in midstream. The remaining functionality implemented by ParentBase relates to executing the local clients through the Command interface. The execution of the local clients is on a per thread basis. Each local client is allocated a thread so that the individual executions can occur concurrently. Some readers may comment that spinning off an individual thread in a heavily multithreaded environment is inefficient. Granted, the statement is true, but consider the context, where the
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CHAPTER 11 REST-BASED MODEL VIEW CONTROLLER PATTERN
greater cost is the waiting time of the network communications. Therefore, to be robust, it is best if each local client waits individually and indicates to the controller when they have completed. While an individual thread is waiting, it is not consuming resources, and therefore having multiple waiting threads is not a problem for the server. Following is the implementation of the command execution: protected List _runningThreads = new LinkedList(); public abstract void addResult(Result result); public void processRequest( Request request) { Iterator iter = _commands.iterator(); _runningThreads.clear(); while( iter.hasNext()) { Command cmd = (Command)iter.next(); cmd.setRequest( request); cmd.assignParent( this); Thread thrd = new Thread((Runnable)cmd); _runningThreads.add( thrd); thrd.start(); } } public void processRequest( String impl, Request request) { Iterator iter = _commands.iterator(); _runningThreads.clear(); while( iter.hasNext()) { Command cmd = (Command)iter.next(); if( cmd.getIdentifier().compareTo( impl) == 0) { cmd.setRequest( request); cmd.assignParent( this); Thread thrd = new Thread( (Runnable)cmd); _runningThreads.add( thrd); thrd.start(); break; } } } The data member _runningThreads is a list of threads that are executing. The list is required by the synchronous or asynchronous controller implementations to know when a thread has completed. The method addResult, which is used to add a result to the controller, is defined as abstract because the synchronous or asynchronous implementations define their own way of managing the results. You will see this difference shortly. The processRequest methods are used to execute the Command interface instances. There are two versions of the processRequest method. The version with a single parameter executes all local clients. The version with two parameters executes a specific local client. Regardless of whether a single local client or all local clients are executed, they are executed on their own threads. This keeps the architecture simple so you don t have to deal with too many architectural variations.
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