Agile Design

Problems with software designs Rigidity Fragility Immobility Viscosity Needless Complexity Needless Repetition Opacity

Rigidity Rigid software is difficult change One change requires mods in multiple modules Changes are more difficult than estimated

Fragility The degree to which it is easy to break the program Small changes generate unexpected side-effects

Immobility The code is not modular enough for reuse

Viscosity Software Viscosity Environment Viscosity When design-preserving methods of extending the software are less easily achieved than changes that are not in concert with the overall design (i.e., hacks are easier than following design) the design has “high viscosity.” Environment Viscosity The process of making changes (e.g., compiling, getting approval, configuration management) can also encourage a “quick-and-dirty” fix, rather than a fix better suited to the long-term maintainability of the system.

Needless Complexity Preparing for unknown extensions Bad Algorithms Overly flexible Bad Algorithms Bad/Out-dated assumptions about system priorities

Needless Repetition Cut and paste is evil in software engineering. How many places does the same processing occur? That is the number of places that changes/fixes must be made Do people understand what the cut and paste? This is how software the literally no one understands enters the system. The same code, with minor changes, makes the minor changes easy to miss.

Opacity How clear is the code Code tends to get less clear as it is modified Code tends to get less clear when you think about something else for a while Code that is clear to you, might not be clear to others