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soa学习第三篇

(2006-05-01 10:14:12)
分类: 吴定明

Service composition

One way to explore the value of Service-Oriented Architecture (SOA) is from the perspective of the flexibility. Business value is created when a set of recomposable business process elements are defined as the key capabilities of a business. Then the business model that is created as a result of composing those business elements into processes can more effectively be supported by a flexible IT infrastructure with services that support those specific processs elements.

Another way to look at flexibility is in terms of future extensibility. This is achieved by introducing a service layer into an enterprise architecture. Packaged applications and existing systems can be integrated through SOA by a loosely coupled set of service interfaces that collaborate to achieve business objectives.

This flexibility of SOA is the Strategy pattern. In this pattern, services are used to decouple an interface from its implementation and, using Remote Service Strategy.

In this seminal article, Parnas describes the criteria by which the partitioning of large and complex computer applications can be broken down into a set of constituent components or modules that can be interconnected in order to weave the fabric of the underlying functional requirements of the system.

Variation-Oriented Analysis and Design

In reality, the catalyst in the adoption of Web services and SOA is the flexibility it can provide. This flexibility manifests itself in multiple dimensions. The key element is the ability to request the invocation of the piece of functionality which today may be provided internally within the organization by a given business line, and yet reserve the right to choose an alternative service provider in a later period in time.

This flexibility in IT is an implementation of the strategy pattern in combination with the separation of interface from implementation and implementation from binding to communication protocols. The strategy pattern is usually implemented in a single address space using a single programming language. However, when faced with a kind of binding that is not bound to the same address space but manifested as a distributed system call or a remote procedure call, in order to fulfill the dynamic binding to the service provider either at compile time or at run-time, this pattern is called the Remote Service Strategy or Distributed Strategy.

Service composition and flexibility

let's explore the degrees of flexibility afforded through the notion of Service Composition.

Computations are the fine-grained activities performed by using general purpose programming languages (such as Java™ or C#) that typically include mathematical calculations, Boolean evaluations, and performance of functions based on these evaluations.

Composition languages, such as Piccola, aim at providing a small syntax with the goal of component assembly.

Collaborations, on the other hand, are defined by flows (Business Process Execution Language for Web Services (BPEL4WS) or DAML-S, for example) which may be workflows or other forms of purposeful integration of component behavior to achieve a collective goal. Others have stressed the need for integration services to lead this composition.

In the world of SOA, services reign supreme (in other words, the fundamental construct is the service, which can be composed into applications). Quality of Service (QoS) and flexibility are two of the most sought-after characteristics of applications developed with this architectural style. SOA provides flexibility primarily through its three first-class constructs: services, components (that realize the services), and flows (or processes).

Each of these three main SOA constructs provides a different kind of flexibility. For example, from a services angle, flexibility (and its "sister," reuse) is gained through separation of the interface from implementation and from binding to specific protocols. A special species of components, called enterprise components (see Resources), provide the realization of the service(s). They ensure QoS.

Composition and composability

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