While the importance of multi-tier architectures for enterprise information systems is widely accepted and their benefits are well published, the systematic migration from monolithic legacy systems toward multi-tier architectures is known to a much lesser extent. In this paper we present a pattern on how to re-use elements of legacy systems within multi-tier architectures, which also allows for a smooth migration path. We report on experience we made with migrating existing municipal information systems towards a multitier architecture. The experience is generalized by describing the underlying pattern such that it can be re-used for similar architectural migration tasks. The emerged Dublo pattern is based on the partial duplication of business logic among legacy system and newly deployed application server. While this somehow contradicts the separation-of-concerns principle, it offers a high degree of flexibility in the migration process and allows for a smooth transition.

Experience with the combination of outdated database technology with modern server-side component and web services technologies is discussed. In this context, we also report on technology and architecture selection processes.