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The Art of Unix Programming
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Unix Programming - Speaking of Complexity - Tradeoffs between Interface and Implementation Complexity

Tradeoffs between Interface and Implementation Complexity

One of the most perceptive observations ever made about the Unix tradition by someone standing outside it was contained in Richard Gabriel's paper called Lisp: Good News, Bad News, and How to Win Big [Gabriel]. Gabriel is a long-time leader of the Lisp community, and the paper was primarily an argument for a particular style of Lisp design, but the author himself acknowledges that it is now remembered primarily for the section called ‘The Rise of Worse Is Better .

The paper argued that Unix and C have the characteristics of viruses, and that in the evolutionary struggle among software designs traits like implementation simplicity and portability which lead to rapid propagation (infectiousness) are more effective than correctness and completeness of the design. Gabriel came so close to anticipating the ‘many-eyeballs’ effect on open-source software that the open-source community retrospectively adopted him as one of its theorists after 1997.

Less remembered is that the Gabriel's central argument was about a very specific tradeoff between implementation and interface complexity, one which rather exactly fits the categories we have examined in this chapter. Gabriel contrasts an ‘MIT’ philosophy most valuing interface simplicity with a ‘New Jersey’ philosophy most valuing implementation simplicity. He then proposes that although the MIT philosophy leads to software that is better in the abstract, the (worse) New Jersey model has better propagation characteristics. Over time, people pay more attention to software written in the New Jersey style, so it improves faster. Worse becomes better.

In fact, the MIT and New Jersey philosophies have analogs as conflicting tendencies within the Unix design tradition itself. One strain of Unix thinking emphasizes small sharp tools, starting designs from zero, and interfaces that are simple and consistent. This point of view has been most famously championed by Doug McIlroy. Another strain emphasizes doing simple implementations that work, and that ship quickly, even if the methods are brute-force and some edge cases have to be punted. Ken Thompson's code and his maxims about programming have often seemed to lean in this direction.

The tension between these approaches arises precisely because one can sometimes get a simpler interface if one is willing to pay implementation complexity for it, or vice versa. Gabriel's original example, about how system calls that do long operations handle interrupts they cannot hold or mask, is still one of the best. Under the MIT philosophy, the right thing to do would be to back out of the system call and automatically resume it once the interrupt has been handled; this is harder to implement but leads to a simpler interface. Under the New Jersey philosophy, the system call would return an error indicating that it has been interrupted and the user must re-execute; this can be implemented far more simply, but leads to a programming interface that is more difficult to use.

Both approaches have been tried. Old Unix hands will instantly think of System-V-style vs. BSD-style handling of software signals; the latter follows the MIT philosophy, while the former hails from New Jersey. Underlying the choice between them is a pressing question that has nothing directly to do with the software's infectiousness: if your goal is to hold down total global complexity, where are you most willing to pay to do that? Where should you be most willing to pay?

One epochal example not mentioned in Gabriel's paper is from distributed hypertext systems. Early distributed-hypertext projects such as NLS and Xanadu were severely constrained by the MIT-philosophy assumption that dangling links were an unacceptable breakdown in the user interface; this constrained the systems to either browsing only a controlled, closed set of documents (such as on a single CD-ROM) or implementing various increasingly elaborate replication, caching, and indexing methods in an attempt to prevent documents from randomly disappearing. Tim Berners-Lee cut through this Gordian knot by punting the problem in classic New Jersey style. The simplicity of implementation he bought by allowing “404: Not Found” as a response was what made the World Wide Web lightweight enough to propagate and succeed.

Gabriel himself, while sticking with the observation that ‘worse’ is more infectious and tends to win in the end, has publicly changed his mind several times about the underlying complexity-related question of whether or not this is actually a good thing. His uncertainty mirrors a lot of ongoing design debates within the Unix community.

We cannot offer a one-size-fits-all answer. As with most of the large questions in this chapter, good taste and engineering judgement will demand different answers in different situations. The important thing is to develop the habit of thinking carefully about this issue on each and every one of your designs. As we have observed before in discussing software modularity, complexity is a cost you must budget very carefully.


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The Art of Unix Programming
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