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

Chapter 5. Guidelines 105DescriptionOrder of application of the patterns presented in the “Map” section is determinedby impact of each pattern on the final architecture. The first patterns set theframe of the architecture; the next patterns add new elements to the patternlanguage.The pattern that has the largest impact on the architecture Service Layerspattern. This pattern defines two basic layers. The first (lower) layer containsbasic services while the second contains process services. At this point, a way ofcommunication between layers is needed, so the Canonical Protocol is applied.Those two patterns define together two layers of services and the mean of communicationbetween them. Canonical Schema along with Policy Centralisationdefine application wide documents: schemas and policies. Early introduction ofthose patterns is important because all the services will share the same documents.At this point, the main elements (layers,communication protocol, schema andpolicies) are established. Application of the next patterns introduce basic servicesinto architecture. Service belonging to Utility Abstraction are the firstto identify and migrate because they provide functionality that is used by otherservices. The next are services defined by Canonical Resources Those servicesenable usage of external resources. Next Entity Abstraction and Rules Centralisationshould be applied. Those two types of services provide entity–relatedoperations.Inventory Endpoints is a pattern that introduces an additional layer aboutbasic services. Endpoints are facades. They are introduced when basic servicesare defined.Process Abstraction introduces process services. This type of services requiresbasic services to execute processes. Process services are stateful, it meansthat they maintain their state for the whole live of the process. This time doesnot have to be short. State Repository can be introduced when it is knownwhat services need state deferring mechanism.The last elements that remained are GUI related. Frontends are the firstto be migrated. Accomplishment of migration of frontends makes possible identificationof all the places where continuous feedback to user is required andintroduction of UI–Mediator.Patterns1. Name Service LayersProblem Services delivered by different teams may contain inconsistenciesand redundant functionalities.