The design strategy for a SOA does not start from the "bottom-up" as is often the case with a Web services-based approach. You must remember that SOA is more strategic and business-aligned. Web services are a tactical implementation of SOA. A number of important activities and decisions exist that influence not just integration architecture but enterprise and application architectures as well. They include the activities from the two key views of the consumer and provider described in Figure 4 below.
Figure 4 shows the activities that are typically conducted by each of the roles of provider and consumer. Note that the provider's activities are a superset of the consumer's activities (for example, the provider would also be concerned with service identification, categorization, and so forth). In many cases, the differentiation of the roles comes from the fact that the consumers specify the services they want, often search for it, and once they are convinced of the match between the specification of the service they are looking for, and that provided by a service provider, they bind and invoke the service as needed. The provider, in turn, needs to publish the services they are willing to support; both in terms of functionality and most importantly in terms of the QoS that consumers will require. This implicit contract between consumer and provider might mature into an explicit contract in terms of SLAs; negotiated either electronically or through business and legal venues.
The activities described above can be depicted to flow within the service-oriented modeling and architecture method, as shown in Figure 5 below.
The process of service-oriented modeling and architecture consists of three general steps: identification, specification and realization of services, components and flows (typically, choreography of services).
Service identification
This process consists of a combination of top-down, bottom-up, and middle-out techniques of domain decomposition, existing asset analysis, and goal-service modeling. In the top-down view, a blueprint of business use cases provides the specification for business services. This top-down process is often referred to as domain decomposition, which consists of the decomposition of the business domain into its functional areas and subsystems, including its flow or process decomposition into processes, sub-processes, and high-level business use cases. These use cases often are very good candidates for business services exposed at the edge of the enterprise, or for those used within the boundaries of the enterprise across lines of business.
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