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

Chapter 4. Service Oriented Architecture 81implementation details, because the consumer is interested only in the result ofinvoked operations. There are also providers who have to design services as technicallyindependent as possible. This independency allows consuming service’soperations by any potential consumer. Each service should have possible highestencapsulation.Coupling – ddescribes dependencies between services. This term in case ofservices is much broader then coupling between components. Coupling refersalso to [13]:1. localisation –the localization of a service should not determinate behaviorand accessibility of a service. To achieve low localization coupling, servicesshould be accessible via configurable component like Gateway [59], EnterpriseService Bus (ESB)[59] or Repository [31]2. technology – the implementation technology of the service cannot determinepossible consumers. To eliminate technology coupling open -standardsfor both transport and messaging like [13]SOAP,XML, TCP/IP are recommended.There is also coupling between [13], message formats, languages, network protocolsand interface contracts but they are consequences and special cases of previouslydescribed cases. The aim for architects and designers is to keep servicesloosely coupled.Cohesion – cohesion refers to relations between operations offered by a service.If operations relate to each other by sharing the same data and/or references toobject then the cohesion is high. A low cohesion means that operations are notrelated to each other or the types of arguments of operations are primitives. Thehigher cohesion a service has, the more flexible and reusable is. Consequently, agood service is characterised by a high cohesion.Granularity – granularity describes size of a service. The size is consideredin two dimensions [13].The firs perspective is the size that can be fine or coarse.The second is a purpose, which can be business or technical. Literature pointssize of a service as factor determining granularity but it also does not provide anymeasure which can conclude that a service characterized by a particular size isfine/coarse grained. In practise [13], coarse-grained services are mainly invokers,while fine grained are invoked . Other definition says that a coarse-grained serviceis a service that provides significant business capability [49]. This definition alsodoes not define how to measure this signification.