This is my rule of thumb on evaluating software - it must be flexible. The flexibillity can come from many places. The lowest level is source code, there you can do a long jump, but it can be too time consuming to choose precisely the landing point, it should be the task of some higher level layer to adjust the jump more accurately⁽¹⁾. So the ideal is to have multiple flexibillity layers. The purpose of this meditation is an attempt to categorise them. Here is my list - starting from the lowest level.

• source code - possibillity to change the core of the program and amend it to your needs is flexibillity at the lowest level (it too can have sublevels in heterogenues systems like perl with C XS code).
• API - were you can write some additionall code to the program, but only in some places previously identified by the author
• templates - used in web environement
• the externall interface of the program (like protocols, input/output formats)
• configuration files
• GUI

How do you plan the evolution of your programs? What preparations do you make for future amendments?

Update: Added protocols (thanks to chip).

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1) I could make the model more precise - but it would never be precise enough - so I leave the vague metric space, long and short jumps model.

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