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

Chapter 4. Service Oriented Architecture 94provided by other services.This pattern introduces enterprise wide processservices as an additional layer. The ser- vices are mainly stateful and manipulateservices from all the underlying layers.Rationale: See Utility Abstraction rationale.7. Policy Centralisation–Services may require to process individual polices.Policies may refer to security, transaction requirements and Quality-of-Services. Replication of policies across inventories brings redundancy andproblems with synchronization of changes to the policies. Application ofthe pattern separates policies from services.Rationale: Forces services to use predefined WS–Policy files. WS–Policydescribes service polices like security.8. Rules Centralisation –The same business rules are applied in many differentservices. This may lead to redundancy and problem with maintenanceof the changes. Business rules are stored and accessible through dedicatedentities like business rules services.Rationale: However the pattern simplifies maintainability, the structureof process–centric services become more complex due to a need of additionalservices invocation in order to validate data against business rulesand constrains.9. Canonical Resources –Modern systems require access to external resourceslike databases, central directories or transaction management frameworks.The resources are accessed by many subsystems of the system. The accesslogic becomes a part of the subsystem. Canonical Resources identifiesand standardises resources across the company. A term resource refers todatabases, activity management systems, state deferral mechanism or otherresource like for instance public services. Separation of access to externalresources grows in importance when an application is not small, because insmall application access to external resources is rather centralised.Rationale: The pattern has significant impact on the architecture becauseeach service requiring access to external resources has to invoke those dedicatedservices.10. State Repository –Long living processes occupy system resources for a longtime. The pattern introduces a state repository that is meant to maintainstate of stateful services. Stateful services delegate their state to specialservices in order to free system resources. The state is not transferred automatically.The transfer occurs when special criteria are met. The criteriacan be for instance an extending waiting for a response. This solution occursto be very useful when an asynchronous communication is applied.Rationale: Application of the pattern requires additional effort related to