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Design Patterns for Building Service-Oriented Web Services
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essage-oriented Web services are the building blocks for service-oriented applications. In the previous chapter, you learned how message-oriented Web services are constructed, and what sets them apart from traditional RPC-style Web services. The main difference is that messages typically include complex types that are defined using custom XML schema files. Message-oriented Web services are effective at executing operations, whereby the input parameters feed into a process rather than dictating the process. In contrast, procedure-style method calls are straightforward operations with a strong dependency on the input arguments. For example, the message-oriented StockTrader Web service provides a PlaceTrade operation that accepts the trade specifications, executes a complex trade operation, and then returns the details of the trade encapsulated in a complex data type (the Trade object). The simple input parameters trigger a complex operation and cause a complex type to be returned. There is no direct correlation between the input parameters and the complexity of the operation. In contrast, one example of a procedure-style Web method is a simple arithmetic Add operation that accepts two numeric input parameters. This Web method has nothing complicated happening internally, nor does it require that a complex data type be returned. What you get out of the method is directly correlated to what you send into it. In this chapter, we need to make another conceptual leap, this time from messageoriented Web services to service-oriented Web services. Messages do not go away in this new architecture; they are just as important as ever. What is different is that Web services are not the central player in the architecture.
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Service-oriented Web services act more as smart gateways for incoming service requests than as destinations in and of themselves. Let s revisit the complex SOA diagram from 1, reprinted here as Figure 4-1.
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CHAPTER 4 DESIGN PATTERNS FOR BUILDING SERVICE-ORIENTED WEB SERVICES
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Figure 4-1. Complex SOA Notice that Web services are not the ultimate endpoint destinations in this architecture. Instead, their purpose is to authenticate and authorize incoming service requests, and then to relay the request details to back-end business components and workflows for processing. This fact by no means diminishes the importance of their role; it just switches perspectives. Web services have certain unique properties that make them essential to this architecture: Web services process SOAP messages. Web services provide accessible (and discoverable) endpoints for service requests. Web services (optionally) authenticate and authorize incoming service requests. In this role they selectively filter incoming service requests and keep out unauthorized requests. (This feature is technically optional but it is an important available feature with WSE 3.0, and so is listed here as an essential property). In contrast, other components in the architecture, such as the business components, do not have any of these properties. They do not expose publicly accessible endpoints. They
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CHAPTER 4 DESIGN PATTERNS FOR BUILDING SERVICE-ORIENTED WEB SERVICES
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do not process SOAP requests directly. And they do not have the same ability to filter out incoming service requests based on security tokens. Note that business components can implement custom security checks through mechanisms such as code access security (CAS) and Active Directory checks, but these options are not comparable to the available mechanisms for Web services, which can accept encrypted and signed requests, and which inspect several aspects of the request directly, not just the identity of the caller. So we have established that Web services play a unique role in SOA, one where they are an important support player rather than the ultimate destination endpoint. But what does this translate to in practical terms, and how is it different from before The implication is that you need to build Web services differently to maximize the effectiveness of their role in SOA applications. This includes the following: A renewed emphasis on breaking out Web service code-behind into separate class files and assemblies: This includes abstract IDC files (based on the applicable WSDL document). It also includes generating a dedicated assembly for encapsulating custom data type definitions (so that common data types may be used across multiple services and components using a common reference assembly). Delegation of all business process logic to back-end business components: The Web service code-behind should be focused exclusively on preprocessing incoming request messages and then relaying the request details to the appropriate back-end business component. The Web service code-behind should not handle any business processing directly. A focus on new kinds of service-oriented components: SOA architecture creates a need for different kinds of service components that may have no equivalent in other architectures. For example, SOA applications rely heavily on service agent components, which act as the middleman between separate Web services and which relay all communications between them. (You will learn how to build a service agent component in the section Design and Build a Service Agent later in this chapter.) Be forewarned: some of the material in this chapter may strike you as unusual or unorthodox and certainly more complex than you are used to seeing with Web services development. This is not surprising given that SOA applications are still relatively new. Recall that it took several years for the n-tier architecture model to become fully formed and to gain wide acceptance as a standard. SOA will also go through an evolution. Some ideas will gain acceptance, while others will fall by the wayside. This chapter quite likely contains some of both, so read the chapter, absorb the material, and take with you as much or as little as you like. The primary requirement that SOA imposes on a system is that its business functionality must be accessible through more than one type of interface and through more than one kind of transport protocol. Enterprise developers have long understood the need to separate out business functionality into a dedicated set of components. In 3, the StockTrader Web service implemented its business logic directly, based on an IDC file (defined in a separate, though embedded, class file). This approach is incorrect from an SOA perspective for two reasons:
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