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

Chapter 3. Architectural Patterns 37Both presented definitions present architectural pattern as a pattern appliedon level of architecture. In turn definition of Design Pattern given in [35] saysthat:“Design Pattern describes a commonly–recurring structure of communicatingcomponents that solve a general design problem in a particularcontext.”Other definition [22] says that:“Design patterns are medium–scale patterns. They are smaller inscale than architectural patterns, but are at a higher level than the programminglanguage–specific idioms. The application of a design patternhas no effect on the fundamental structure of a software system,but may have a strong influence on the architecture of a subsystem.”Architectural patterns are the patterns that have an impact on architecture ofthe whole system while design patterns may have an impact only on subsystems.Even having the definitions, classification of a specified pattern to a particularcategory is not so obvious. In some specific circumstances a design pattern canbe implemented as an architectural pattern. This can happen when a systemis very specialized and subordinated to a specific task that imposes solutionslike design patterns. In this case, elements of a design pattern can grow to thesize of subsystems and determinate architecture. In order to provide a betterexplanation of this concept, an example imaginary system (“DataProvider”) willbe considered.The “DataProvider” system provides information about individuals to requestingand authorized institutions. Information provided by the system canbe general like personal data or detailed like pictures and fingerprints. Databaseof the system contains thousands of records and it is distributed over the country.One of the most important aspects of the system is availability of the system andits resources. The system is queried twenty-four hours, seven days a week. Thesystem will be designed as a distributed application. The system has to be stableand has to have high availability. The description of requirements of the systemmatches one of the design patterns, namely lazy acquisition design pattern. Lazyacquisition design pattern [45] has two very important properties what makes ituseful to apply here:1. Availability – all downloadable data are available, but the data is not acquiredin advance. This approach saves resources of the system what isespecially important when the system has to process a lot of data (especiallymultimedia).2. Stability – all resources are acquired only when needed so the system doesnot have to overuse memory and other resources. Overview of the designpattern is presented on the figure 3.3