Understanding legacy architecture patterns

While often being the not very well liked stepchild of IT departments, legacy information systems are valuable assets for companies. After many years of development and maintenance, these systems often contain valuable business logic and implement business processes that are nowadays unknown even to the owner of the system. However, maintenance and further development is often costly and requires an increased effort compared to modern applications. Hence, developing sound strategies for gradually modernizing these applications and lowering the associated costs is of paramount importance. For carrying out such strategies, it is useful to understand why and how certain aspects of these systems are implemented. At itestra, we have collected architectural patterns in legacy information systems and use these to understand legacy information systems better and avoid mistakes in the analysis of behavior of such legacy systems. We present these patterns here in order to facilitate the decision-making process in modernization projects and increase their success probability.