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SOA Book Excerpt: A Methodology for Service Modeling and Design - Part 1

An SOA Reference Architecture

Layer 6: Integration Layer: This layer provides the capability for service consumers to locate service providers and initiate service invocations. Through the three basic capabilities of mediation, routing, and data and protocol transformation, this layer helps foster a service ecosystem wherein services can communicate with each other while being a part of a business process. The key nonfunctional requirements such as security, latency, and quality of service between adjacent layers in the reference architecture are implemented by the architecture building blocks in this layer. The functions of this layer are typically and increasingly being collectively defined as the enterprise service bus (ESB). An ESB is a collection of architecture patterns that uses open standards and protocols to implement the three basic capabilities of this layer and provide a layer of indirection between the service consumers and the service provider by exposing the services only through the ESB. ESB products usually add some specialized features to provide differentiated capabilities in the marketplace.

The integration capabilities are most commonly used by ABBs residing between Layer 2 through Layer 5. As an example, in Layer 5 there can be many consumers accessing enterprise services through different channel types. Each channel type can use different protocols - HTML, WML (for mobile users), and VoiceXML (for IVR users), to name a few. Each of these protocols and message formats may be passed through an Extensible Stylesheet Language Transformations (XSLT) engine before the actual service is invoked. This XSLT transform is usually an ESB-provided feature. The beauty of the ESB-based integration layer is that any feature or function that can be exposed in a manner that follows open standards and protocols for access can be plugged into the ESB so that it is enabled to take part in a service-based ecosystem. Figure 2 depicts a logical view of the ESB.

As Figure 2 suggests, the ESB provides capabilities for service consumers and providers to connect to each other, for services to be discovered using the registry, for services to be managed and for secure invocations, and provides application programming interfaces (API) to facilitate the development of service connectivity.

Layer 7: QoS Layer: This layer focuses on implementing and managing the nonfunctional requirements (NFR) that the services need to implement. Although SOA brings some real value proposition through the new architectural style, the programming model that supports the building of the first-class SOA constructs adds some inherent challenges that are nontrivial to address. The challenges arise while trying to comply with the essential tenets of SOA: abstraction, open standards and protocols, distributed computing, heterogeneous computing infrastructures, federated service ecosystem, and so on. Adherence to these compliance requirements often makes the implementation of the NFRs that much more complicated. This layer provides the infrastructure capabilities to realize the NFRs. It captures the data elements that provide the information around noncompliance to NFRs at each of the horizontal layers, standard NFRs that it monitors for noncompliance is security, availability, scalability, and reliability.

Layer 8: Information Architecture Layer: This layer ensures a proper representation of the data and information that is required in an SOA. The data architecture and the information architecture representation (along with its key considerations and guidelines for its design and usage) at each specific horizontal layer are the responsibilities of this layer.

Industry models (for example, ACORD, IAA, JXDD) and their usage to define the information architecture, along with business protocols used to exchange business data, are addressed in this layer. It also stores the metadata required for data mining and business intelligence. Refer to the "References" section for the details of some industry models.

Layer 9: Governance Layer: This layer ensures the proper management of the entire life cycle of the services. It is responsible for prioritizing which high-value services should be implemented, for each of the layers in the architecture, and for providing a rationalization based on how the service satisfies a business or IT goal of the enterprise. Enforcing both design-time and runtime policies that the services should implement and conform to is one of the key responsibilities of this layer. Essentially, this layer provides a framework that efficiently oversees the design and implementation of services so that they comply with the various business and IT regulatory policies and requirements.

It is worth noting that one of the primary reasons for the SOA solution stack representation is that it helps to communicate, to the business and IT stakeholders, the evolution and realization of the enterprises SOA vision and roadmap through iterative implementation. Communicating with the stakeholders is key to ensure that the business commitment is pervasive across the various phases of an SOA initiative.

The methodology that we discuss in this excerpt will help in identifying, specifying, and realizing the first-class constructs of an SOA and their placement in the various layers of the architecture stack. This logical view of the SOA reference architecture is also known as the SOA solution stack or just the solution stack. Therefore, the terms SOA reference architecture, SOA solution stack, and solution stack all refer to the same concept and hence can be used interchangeably.

This chapter is an excerpt from the new book, Executing SOA: A Practical Guide for the Service-Oriented Architect authored by Norbert Bieberstein, Robert Laird, Keith Jones and Tilak Mitra, published by IBM Press, May 2008, ISBN 0132353741, www.ibmpressbooks.com Copyright 2008 by International Business Machines Corporation. All rights reserved.

References

Links to developerWorks Articles

More Stories By Tilak Mitra

Tilak Mitra is a Certified Senior IT Architect at IBM. He specializes in mid- to large-range enterprise and application architectures based on J2EE, MQ, and other EAI technologies. You can reach him at [email protected]

More Stories By Norbert Bieberstein

Norbert Bieberstein, solution architect for IBM's Enterprise Integration team, has extensive first-hand experience with customers migrating to SOA-based On-Demand solutions.

More Stories By Keith Jones

Keith Jones, PhD, IT architect at IBM Enterprise Integration Solutions, focuses on helping customers define and implement service-oriented architectures.

More Stories By Robert Laird

Robert Laird is an IT architect in IBM's SOA Advanced Technologies group.

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