Ghosts of Unix Past: a historical search for design patterns
The exploration of design patterns is importantly a historical search. It is possible to tell in the present that a particular approach to design or coding works adequately in a particular situation, but to identify patterns which repeatedly work, or repeatedly fail to work, a longer term or historical perspective is needed. We benefit primarily from hindsight.
The previous series of articles on design patterns took advantage of the development history of the Linux Kernel only implicitly, looking at the patterns that could be found it the kernel at the time with little reference to how they got there. Perspective was provided by looking at the results of multiple long-term development efforts, all included in the one code base.
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For this series we try to look for patterns which become visible only over an extended time period. As development of a system proceeds, early decisions can have consequences that were not fully appreciated when they were made. If we can find patterns relating these decisions to their outcomes, it might be hoped that a review of these patterns while making new decisions will help to avoid old mistakes or to leverage established successes.
Full exploitation
A very appropriate starting point for this exploration is the Ritchie and Thompson paper, published in Communications of the ACM, which introduced "The Unix Time-Sharing System". In that paper the authors claimed that the success of Unix was not in "new inventions but rather in the full exploitation of a carefully selected set of fertile ideas." The importance of "careful selection" implies a historical perspective much like the one here proposed for exploring design patterns. A selection can only be made if previous experience is available which demonstrates a number of design avenues to choose between. It is to be hoped that identifying patterns would be one aspect of the care taken in that selection.
Over four weeks we will explore four design patterns which can be traced back to that early Unix of which Ritchie and Thompson wrote, but which can be seen much more clearly from the current perspective. Unfortunately they are not all good, but both good and bad can provide valuable lessons for guiding subsequent design.
"Full exploitation" is essentially a pattern in itself, and one we will come back to repeatedly. Whether it is applied to software development, architecture, or music composition, exploiting a good idea repeatedly can enhance the integrity and cohesion of the result and is - hopefully - a pattern that does not need further justification. That said, "full exploitation" can benefit from detailed illumination. We will gain such illumination for this, as for the other three patterns, by examining two specific examples.
Ritchie and Thompson identified in their abstract several features of Unix which they felt were noteworthy. The first two of these will be our first two examples. Using their words:
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