5-1. IMPLEMENTING A UNIVERSAL WEB SERVICE ARCHITECTURE in Font

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CHAPTER 5 5-1. IMPLEMENTING A UNIVERSAL WEB SERVICE ARCHITECTURE
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Figure 5-4. Resulting architecture that doesn t work In theory, this approach is sound, but it suffers from being too complex. The problem of the approach is a question of focus. In the example of the trader subsystem, the focus is the subsystem, and the Web service layer is an add-on to the trader subsystem. In theory, the Web service add-on is not even needed, because the trader subsystem manages everything. In the context of an Ajax Web service application, this is the wrong approach because the focus is the Web service, and it should not be an add-on.
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The solution to the trader subsystem is to neither define nor implement a trader-neutral subsystem using a specific technology, as Figure 5-4 showed. The trader subsystem is converted into a series of Web services that you can assemble into a subsystem. A Web service based trader subsystem still requires an interface, but the interface is defined at the HTTP Web service level. In architectural terms, the ITrader implementations converted into Web services appear as Figure 5-5. The architecture shown in Figure 5-5 relies on reusing the already existing implementations of ProviderTrader1 and ProviderTrader2 directly from the class TradeServlet. Each implementation provides a set of methods, properties, and result sets that the trader-neutral subsystem defines. Using a Web service, the methods, properties, and result sets are converted into something that is Web service compatible. Then at a higher level, another technology assembles the Web services into a trader subsystem. Taking a step back and thinking about the overall architecture, you get an architecture similar to Figure 5-6.
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CHAPTER 5 5-1. IMPLEMENTING A UNIVERSAL WEB SERVICE ARCHITECTURE
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Figure 5-5. Modified trading system architecture
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Figure 5-6. Big picture of modified trading system architecture
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CHAPTER 5 5-1. IMPLEMENTING A UNIVERSAL WEB SERVICE ARCHITECTURE
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Figure 5-6 shows an Ajax client referencing two separate instances of the trading application. TradeServlet implements version 1.0 of the trading application, and TradeServlet2 implements version 2.0 of the trading application. To switch the Ajax client from version 1.0 to version 2.0 is trivial and only requires specifying a new server. Of course, this assumes that the data formats of version 1.0 and version 2.0 are compatible. On the server side, TradeServlet could be using a socket connection to access the trading information, and TradeServlet2 could be the middleware replacement. The Web service has become the abstraction, and the code behind the Web service is irrelevant from the perspective of the Web service consumer.
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Implementing the Trader-Universal Web Service
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The trading Web service is an example of what to expect when implementing a complete Web service solution. This recipe covers these remaining pieces: Defining the URLs Identifying the formats that can be sent and received How to support relative URLs Defining the URLs for the Trader Application Let s continue with the evolution of the trader Web service and outline the important pieces, namely the URLs and supported data formats. The trader application exposes the following base URLs: /services/controller /services/realtime /services/orders /services/orders/trader123/order345 /services/historical /services/controller is the base URL used to manage the engine on the server side. The controller URL lets you reset the code behind the Web service manually. For example, in the case of the trader application, /services/controller would connect the Web service implementation to the socket that provides the trading interface. You could use the URL /services/controller/start to start the server code, and you could use the URL /services/controller/stop to stop the server code. However, this approach wouldn t be usable, because it would seem that the identifiers start and stop are resources, which they are not. Compare it to a light switch. A light switch is a single resource that has two states: on and off. It does not have two resources, one for each state. You use query parameters to start and stop the server code. To start the server code, you execute the verb POST on the URL /services/controller with the computer graphics interface (CGI) parameters action=start. To stop the server code, the URL remains the same, as does the verb, but the CGI parameters change to action=stop. The verb POST is appropriate, because you re executing some server-side process, and what the process should do depends on the data sent. Calling the verb GET on the URL /services/controller returns the status of the server code.
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