An Abstract SOA Reference Model
A fluffy draft on a fluffy topic - feedback and flames encouraged...
While a well planned and executed SOA undertaking can help organizations realize greater responsiveness in a changing marketplace, not all service-oriented efforts have been successful. SOA projects may experience limited success when they are driven from the bottom up by developers unfamiliar with the strategic needs of the organization. Building SOA for the sake of SOA without reference to the business context is a project without organizing principles and guidance. The result is a chaotic implementation that has no business relevance. On the other hand, taking a top-down enterprise-wide approach to SOA requires such enormous time investments that by the time the project is complete, the solution no longer maps to business needs. (This of course is one of the problems SOA is supposed to solve!)
By contrast, Microsoft advocates a “middle out” approach which combines both top-down and bottom-up methodologies. In this approach, SOA efforts are driven by strategic vision and business need, and are met through incremental, iterative SOA projects that are designed to deliver on business goals one business need at a time. Microsoft has been using this technique to assist customers with their SOA initiatives since the .NET framework was first released in 1999.
The concept of SOA can be viewed from several possible perspectives. While no single perspective or set of perspectives represents a definitive view of a SOA, from a holistic view these perspectives assist in understanding the underlying architectural requirements. Microsoft believes that there are three abstract capability layers exposed within a SOA:
An illustration of these categories and their relationships to one another appears below:
Figure 6: An Abstract Reference Model for SOA
Expose focuses on how existing IT investments are exposed as a set of broad, standards-based services, enabling these investments to be available to a broader set of consumers. Existing investments are likely to be based upon a set of heterogeneous platforms and vendors. If these applications are unable to natively support Web services a set of application or protocol-specific set of adapters may be required. Service creation can be fine grained (a single service that maps on to a single business process), or coarse grained (multiple services come together to perform a related set of business functions). Expose is also concerned with how the services are implemented. The functionality of underlying IT resources can be made available natively if they already speak Web services, or can be made available as Web services though use of an adapter. A Service Implementation Architecturedescribeshow services are developed, deployed and managed (we will cover this topic in greater detail in Chapter 2).
We will examine the Expose concept in greater detail in Chapter 2 (Service Orientation).
Once services are created, they can be combined into more complex services, applications or cross-functional business processes. Because services are exist independently of one another they can be combined (or “composed”) and reused with maximum flexibility. As business processes evolve, business rules and practices can be adjusted without constraint from the limitations of the underlying applications. Services compositions enable new cross-functional processes to emerge, allowing the enterprise to adopt new business processes, tune processes for greater efficiency, or improve service levels for customers and partners. A Service Integration Architecture describesa set of capabilities for composing services and other components into larger constructs such as business processes.
Composing services requires some sort of workflow or orchestration mechanism. Microsoft provides these capabilities via BizTalk Server 2006 (BTS) or Windows Workflow Foundation (WF). While BTS and WF may appear to serve similar needs, they are actually quite different. WF and BTS are complementary technologies designed to serve two very different needs:
- BTS is a licensed product designed to implement workflow (“orchestrations”) across disparate applications and platforms.
- WF is a developer framework designed to expose workflow capabilities within your application. There are no fees or licensing restrictions associated with using or deploying WF.
We will examine workflow, orchestration and the use of BizTalk/WF in Chapter 3 (Workflow and Business Processes).
When a new application or business process has been created that functionality must be made available for access (consumption) by IT systems, other services or by end-users. Consumption focuses on delivering new applications that enable increased productivity and enhanced insight into business performance. Users may consume “composed” services through a broad number of outlets including web portals, rich clients, Office business applications (OBA), and mobile devices. “Composed” services can be used to rapidly roll out applications that result in new business capabilities or improved productivity. These application roll-outs can be used to measure the return on investment (ROI) in an SOA. A Service Oriented Application Architecture describes how “composed services” are made available for consumption through as business processes, new services or new end-user applications. This concept is sometimes referred to as Composite Applications since it implies service consumption by end-user applications. Microsoft’s Office Business Applications (OBAs) support the Composite Application notion of transactional systems while expanding the scope of user interaction via the familiar Office environment.
We will examine consumption in greater detail in Chapter 5 (User Experiences).
While the architectures described within Expose / Compose / Consume may be interdependent, they are designed to be loosely coupled. This enables services to be managed, versioned and configured independently of how they are exposed,