When the programming model shifted from the traditional procedural model to that of object-orientation, a major paradigm shift occurred in the world of IT development. The focus was on encapsulating the state and behavior of entities and calling that encapsulation a class. Instances of a class were called objects, which occupied some space in the memory. Object orientation (OO) brought in concepts of inheritance, encapsulation, and polymorphism that could be applied to define relationships between classes. With the prevalence of the use of OO in the programming world, developers and architects started noticing some patterns that can be applied to the usage of OO principles to solve similar types of problems. The patterns depicted the deconstruction of a problem into multiple class entities, together with their interrelationships using the basic concepts of OO, to provide a solution to the problem. The seminal work in this field was done by the Gang of Four authors in the book called Design Patterns: Elements of Reusable Object-Oriented Software. (See the "References" section.) Whereas in OO the first-class constructs were objects and classes, the next-generation methodology for building software applications was called component-based development (CBD). In CBD, the first-class constructs were components, where a component was defined by its external specification, which could be used without any knowledge of its internal implementation. As such, the same external specification could be implemented in a different programming language (for example, Java, C#). The internal implementation of a component may use multiple classes that collectively provide the implementation of the external specification. The classes could use one or more design patterns, thereby leveraging the advantages of OO principles.

In SOA, the main emphasis is on the identification of the right services followed by their specification and realization. Although some might argue that object-oriented analysis and design (OOAD) techniques can be used as a good starting point for services, its main emphasis is on microlevel abstractions. Services, on the other hand, are business-aligned entities and therefore are at a much higher level of abstraction than are objects and components.

The main first-class constructs in an SOA are services, service components, and process flows. For the sake of brevity, we refer to process flows as just flows. These are at a level of abstraction that is higher than that of objects, classes, and components. Hence, there needs to be a higher level of modeling and design principles that deal with the first-class constructs of an SOA. Service-oriented modeling and design is a discipline that provides prescriptive guidance about how to effectively design an SOA using services, service components, and flows. Rational Software, now a part of IBM, has provided an extension to Rational Unified Process (RUP) called RUP-SOMA (see the "References" section), which is built on a service-oriented analysis and design technique developed by IBM called Service Oriented Modeling and Architecture (SOMA). The rest of this excerpt takes you through the SOMA technique and explains how it helps in the identification, specification, and realization of services, service components, and flows.

• SOA Book Excerpt: Service Oriented Modeling and Architecture - Part 2
• SOA Book Excerpt: The First-Class Constructs of SOA - Part 3