Migration of a Chosen Architectural Pattern to Service Oriented ...

Chapter 4. Service Oriented Architecture 84Service Realisation – When services are identified and specified, they have tobe realized. However, realization of a particular service is the final activity; it hasto be avoided when it is possible. This may sound strange or irrational, but it isfully justified. Realisation should be avoided because of reusability. Reusabilityprohibits development of a new service where an existing service can be reused.A new service is needed only when there is no existing service that can performrequired operation. This new service can be build from previously separatedreusable components if there is such possibility or it can be implemented fromscratch.4.4 SOA – architectural point of viewOverview of Architecture of SOA can be presented on three levels of complexity.(see figure 4.2).Each level consists of several layers of abstraction [46].4.4.1 Fundamental SOAFundamental SOA is the first and the most abstract level. It distinguishes onlyEnterprise and Basic layers [46]. Those two layers are sufficient to provide basicSOA functionality. Enterprise layer contains application frontends (clients)(see 4.2.3 for more information about frontends ). Frontends provide access tothe application itself. Basic layer contains basic services (see 4.2.3 for more informationabout basic services ). The structure built on only two layers mayappear to be insufficient, but it is sufficient to provide basic functionality and itis a good starting point for further development of more elaborated and complexstructure. Due to clear separation of clients from basic services, the architecturesupports maintain activities and, in a long run prevents data redundancy. Theredundancy may exist when a company converts existing application into ServiceOriented Architecture.4.4.2 Networked SOAFundamental SOA may be sufficient for small projects but it is hard to maintainwhen a project grows in scope or when an application has to be integrated withanother system. Networked SOAis an upgraded version of Fundamental SOA[46]. The upgrade relies on introduction of one additional layer-Intermediarylayer. This additional layer is placed between Basic and Enterprise layer and ismeant to provide mechanisms facilitating technical and conceptual integrationby introducing Intermediary Services. Those services serve as gateways, facadesand adapters. The names of services of Intermediary Layer derive from designpatterns and behavior of those services correspond the patterns. The role ofparticular types of services is following: