The capability to manage complexity is one of the key competencies of system engineers for large IT-solutions. We call a technical system "complex" (in contrast to "complicated") if it is impossible (due to the networked interaction of its components) to predict the behavior of the whole system, even if you know exactly how each of the system components behave. On the other hand, customers expect increasingly high reliability of IT systems as their business is more and more dependent on the proper operation and interoperation of the IT systems. First we show how a network of interactions increases the complexity of the overall-system. Then we analyze the complexity management strategies of our system engineers and present generalized strategies based on examples of large customer projects. The examples demonstrate that a high maturity in managing complexity enables to provide IT solutions of ultra-high reliability even if they are complex solutions in the above defined sense.

1. Management of Complexity in System Design for Large IT-Solutions Dr. Michael Heiss Global Vice President for Knowledge, Innovation & Technology Dipl.-Ing. Stefan Huber Senior Architekt Siemens IT Solutions and Services © Siemens AG Austria 2009. All rights reserved.

11. Requirements Engineering – a cycle of detecting and reducing complexity Page Detecting complexity (problem space) Info for the requirements engineer from various sources (requirements documents, interviews with stake holders, discussing prototypes, market studies,...)  The world is more complex than it seems to be at first sight Reducing complexity (solution space) Distilling abstractions out of multiple input, finding out which functions are really needed by a customer (and not everything that is stated as requirement)  Make the solution as simple as possible , as complex as needed

18. Managing Software Complexity by Patterns Page Software Architecture Patterns Patterns at Software architecture level Design & Architecture Patterns Training in our Software Architect Curriculum * UI...User Interface Business Layer Data Layer Web / UI* Layer

20. Architectural Qualities and Tactics Page Page Tactics Patterns guides selection of implements a collection of Qualities are achieved by using Modifiability Localize modifications Layers Prevent rippling effects Explicit Interface Testability Manage I/O

21. Organizational patterns – experience based practices to act successfully in a specific context Page Source: Siemens IT Solutions and Services SDE Support Center PM