Aspect-oriented evolution of legacy information systems
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A legacy information system is an old system that typically has been developed several years ago, and remains in operation within an organization. Since the software requirements change, legacy systems must be evolved accordingly. Various approaches such as wrapping, migration and redevelopment have been proposed to maintain legacy information systems. Unfortunately, these approaches have not explicitly considered the concerns that are difficult to capture in single components, and tend to crosscut many components. Examples of such crosscutting concerns include distribution, synchronization, persistence, security, logging and real-time behavior. The crosscutting property of concerns seriously complicates the maintenance of legacy systems because the code of the system needs to be changed at multiple places, and conventional maintenance techniques fall short to do this effectively. Aspect-Oriented Software Development (AOSD) provides explicit mechanisms for coping with these crosscutting concerns. However, current AOSD approaches have primarily focused on coping with crosscutting concerns in software systems that are developed from scratch. Hereby, the crosscutting concerns are implemented as aspects at the beginning, hence localized in single modules. In this way the implementation and maintenance of crosscutting concerns can be prepared to a large extent so that the maintenance of these systems will be easier later on. Unfortunately, legacy systems impose harsher requirements, because crosscutting concerns in legacy systems are neither explicitly identified nor have been prepared before. We provide a systematic process for analyzing the impact of crosscutting concerns on legacy systems. The process, which is called Aspectual Legacy Analysis Process (ALAP), consists of three sub-processes, Feasibility Analysis, Aspectual Analysis and Maintenance Analysis. All the three sub-processes consist of a set of heuristic rules and the corresponding control. Feasibility Analysis, which consists of two phases, describes rules for categorizing legacy systems, in the first phase; and describes the rules for evaluating legacy systems with respect to the ability to implement static crosscutting and ability to implement dynamic crosscutting, in the second phase. The rules of the first phase are based on the categories of legacy systems that we have defined after a thorough study to legacy information systems, and the rules of the second phase are based on our discussion of these categories with respect to crosscutting implementation. Once the legacy system has been categorized and evaluated with respect to crosscutting implementation, the Aspectual Analysis sub-process describes rules for identifying and specifying aspects in legacy systems. Based on the results of the Feasibility Analysis and Aspectual Analysis sub-processes, the Maintenance Analysis describes the rules for the selection of the appropriate legacy maintenance approach. ALAP has been implemented in the Aspectual Legacy Analysis Tool (ALAT), which implements the rules of the three sub-processes and as such helps to support the legacy maintainer in analyzing the legacy system and identifying the appropriate maintenance approach.