There has been a lot of buzz and hype surrounding the opportunities presented by Service Oriented architecture (SOA) and its implementation as webservices. For many newbies SOA is all about service enabling components to some form of webservice and publishing them using standards. In reality, SOA is not about products and standards that help realize it, for example through webservice, but beyond those like services, their flows, compositions, quality of service, and governance. This brings us to service oriented modelling which is quite different from object oriented modelling in the sense that the later is more use case driven whereas the former is more process driven. In most cases, service oriented modelling is a hybrid of business process modelling, object oriented modelling and enterprise architecture.
To work on SOA projects, one may have to analyze, design and make architectural decisions at various layers like operational system or legacy layer, Enterprise component layer, Services layer, Business process or orchestration layer and the Integration layer.
Legacy layer : This consists of existing custom built applications, including existing CRM and ERP packaged applications, and older object-oriented system implementations, as well as business intelligence applications. The composite layered architecture of an SOA can leverage existing systems and integrate them using service-oriented integration techniques, which is a common case in most businesses.
Enterprise components layer: This is the layer of enterprise components that are responsible for realizing functionality and maintaining the QoS of the exposed services. This layer typically uses container-based technologies such as application servers to implement the components, workload management, high-availability, and load balancing. For example, components in these layers could be enterprise beans, asynch beans, message queues, thread pools, resource adapters, etc.,
Services layer: The services the business chooses to expose reside in this layer. They can be discovered or be statically bound and then invoked, or even, choreographed into a composite service. This service exposure layer also provides for the mechanism to take enterprise wide components, business unit specific components, and in some cases, project-specific components, and externalizes a subset of their interfaces in the form of service descriptions. Thus, the enterprise components provide service realization at runtime using the functionality provided by their interfaces. The interfaces get exported out as service descriptions in this layer, where they are exposed for use. They can exist in isolation or as a composite service.
Business process composition or choreography layer: Compositions and choreographies of services exposed in Service layer are defined in this layer. Services are bundled into a flow through orchestration or choreography, and thus act together as a single application. These applications support specific use cases and business processes.
Integration (ESB) Layer: This layer enables the integration of services through the introduction of a reliable set of capabilities, such as intelligent routing, protocol mediation, and other transformation mechanisms, often described as the ESB (the EDI equivalent). Web Services Description Language (WSDL) specifies a binding, which implies a location where the service is provided. On the other hand, an ESB provides a location independent mechanism for integration.
QoS Layer: This layer provides the capabilities required to monitor, manage, and maintain QoS such as performance, reliability, transaction, and availability. Some of these attributes are implemented at the Enterprise component layer (like transaction, security) whereas the others are monitored via background process that check the health of SOA applications.
As it can be seen now, design strategy for a SOA does not start from bottom-up as is often the case with a Web services-based approach. 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.
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