The mainframe centered IT environment is still a common architecture in many large organizations, especially in the financial industry. But the increasing demand for new applications and integration of modern communication technology forces a paradigm change. The paper deals with the challenging task of defining and controlling a disciplined evolution of the existing centralized IT systems into a new integrated architecture. First an introduction into the problem domain of large scale IT systems and into the basic characteristics of mainframe computing is provided. Analyzing the characteristics of large scale systems leads to the concept of a managed evolution and a service architecture. The description of requirements for this new architecture is followed by a discussion of design guidelines and a detailed explanation of the mainframe implementation. The paper concludes with up-to-date testing results which prove the scalability of the implementation up to several thousand distributed clients.
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