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The Modular Structure of Complex Systems. D.L. Parnas, P.C. Clements, D.M. Weiss. Why Was This Paper Written. The gap between academic software engineering and practicing software developers Practitioners tend to be unwilling to adopt work resulting from software engineering research
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The Modular Structure ofComplex Systems D.L. Parnas, P.C. Clements, D.M. Weiss
Why Was This Paper Written • The gap between academic software engineering and practicing software developers • Practitioners tend to be unwilling to adopt work resulting from software engineering research • Research is impractical for “real” software • Managers are not risk takers – research approaches are not proven • The research examples were not like the real problems practitioners encounter • Research ideas require refinement before they can be applied in practice • Practitioners are not intellectually capable of working with ideas formulated in the research lab • This paper provides an example – case study – showing that Parnas’ information hiding approach could be used on a “real” and “hard” software system
The Domain • Reimplement the onboard flight program for the A-7E aircraft • Use information hiding, documentation, etc • Can compare the results to the original program which was considered to be a good software system • Compare via functional implementation, the software's ability to adopt to change and the ease of fixing defects
Important Software Structures • Module Structure • Decomposition of the system into modules • Hides secrets and provides the assumptions that the implementers of one module are allowed to make of the other modules • Uses Structure • A form of the “requires the presence of relation” • Process Structure • Decomposition of the system into runtime processes – the system has many concurrent processes
Information Hiding (Again) • The module structure is formulated using information hiding • Secrets of modules are based on things that are likely to change • Interfaces are designed based on things that are unlikely to change • Goals • The modules should be simple and comprehensible • Enables design for change • A well defined system concern should be quickly mappable to a set of modules • Minimize module coupling (examine fan-out/fan-in)
Documentation – Module Guide • The Module Guide is a document • Lead a reader to the module that implements a particular aspect of the system • States the criteria used to assign a particular responsibility to a module • Ways to describe a module • Roles played by the individual modules in the overall system • Secrets associated with each module • Facilities provided by each module
The A-7E Modules • During the design of the A-7E modules it was apparent that there would be many modules • Especially compared to an “academically-sized” system • Needed a way to help the users navigate through all of the modules • Solution: Create a module hierarchy based on the type of secret
The A-7E Module Structure • High-Level modules • Hardware Hiding Module • Abstracts hardware from the software – allows for plug and play hardware • Behavior Hiding Module • Abstracts requirements related to the system behavior that are likely to change • Software Decision Module • Hides algorithms and implementation decisions
The A-7E Module Structure • Design then proceeded to define the second and third-level modules • The third-level modules seem to be at the level that can be directly implemented • Examples: Spatial Relations Module, Aircraft Motion Module, etc. • The higher levels were for organization (they were abstract), while the lower levels were suitable for implementation
Conclusions • An information hiding approach towards design is appropriate for “real world” systems • Parnas suggests that the Module Guide was a key artifact for guiding the design, and keeping it on track • The new system never was implemented in the A-7E aircraft, but it was used in a simulator