Complete Systems

Top-down and bottom-up are strategies of information processing and knowledge ordering, mostly involving software, and by extension other humanistic and scientific system theories (see systemics). more...

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In the top-down model an overview of the system is formulated, without going into detail for any part of it. Each part of the system is then refined by designing it in more detail. Each new part may then be refined again, defining it in yet more detail until the entire specification is detailed enough to validate the model. The top-down model is often designed with the assistance of "dark boxes" that make it easier to bring to fulfillment but insufficient and irrelevant in understanding the elementary mechanisms.

In bottom-up design, first the individual parts of the system are specified in great detail. The parts are then linked together to form larger components, which are in turn linked until a complete system is formed. This strategy often resembles a "seed" model, whereby the beginnings are small, but eventually grow in complexity and completeness.

Computer science

Software development

Part of this section is from the Perl Design Patterns Book.

In the software development process, the top-down and bottom-up approaches play a key role.

Top-down approaches emphasise planning and a complete understanding of the system. It is inherent that no coding can begin until a sufficient level of detail has been reached in the design of at least some part of the system. This, however, delays testing of the ultimate functional units of a system until significant design is complete. Bottom-up emphasizes coding and early testing, which can begin as soon as the first module has been specified. This approach, however, runs the risk that modules may be coded without having a clear idea of how they link to other parts of the system, and that such linking may not be as easy as first thought. Re-usability of code is one of the main benefits of the bottom-up approach.

Top-down design was promoted in the 1970s by IBM researcher Harlan Mills and Niklaus Wirth. Mills developed structured programming concepts for practical use and tested them in a 1969 project to automate the New York Times morgue index. The engineering and management success of this project led to the spread of the top-down approach through IBM and the rest of the computer industry. Niklaus Wirth, among other achievements the developer of Pascal programming language, wrote the influential paper Program Development by Stepwise Refinement. Top-down methods were favored in software engineering until the rise of object-oriented programming in the late 1980s.

Modern software design approaches usually combine both top-down and bottom-up approaches. Although an understanding of the complete system is usually considered necessary for good design, leading theoretically to a top-down approach, most software projects attempt to make use of existing code to some degree. Pre-existing modules give designs a bottom-up flavour. Some design approaches also use an approach where a partially-functional system is designed and coded to completion, and this system is then expanded to fulfill all the requirements for the project.

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