Big Design Up Front (BDUF) is a software development approach in which the program's design is to be completed and perfected before that program's implementation is started. It is often associated with the waterfall model of software development.
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Proponents of BDUF argue that time spent in designing is a worthwhile investment, and reference numerous studies which have concluded that less time and effort is spent fixing a bug in the early stages of a software products lifecycle than when that same bug is found and must be fixed later. That is, it is much easier to fix a requirements bug in the requirements phase than to fix that same bug in the implementation phase, as to fix a requirements bug in the implementation phase requires scrapping at least some implementation and design work which has already been completed.
Joel Spolsky, a popular online commentator on software development, has argued strongly in favor of Big Design Up Front:[1]
"Many times, thinking things out in advance saved us serious development headaches later on. ... [on making a particular specification change] ... Making this change in the spec took an hour or two. If we had made this change in code, it would have added weeks to the schedule. I can’t tell you how strongly I believe in Big Design Up Front, which the proponents of Extreme Programming consider anathema. I have consistently saved time and made better products by using BDUF and I’m proud to use it, no matter what the XP fanatics claim. They’re just wrong on this point and I can’t be any clearer than that."
However, some argue that what Joel has called Big Design Up Front doesn't resemble the BDUF criticized by advocates of XP and other agile software development methodologies.[2][3]
Critics (notably those who practice agile software development) argue that BDUF is poorly adaptable to changing requirements and that BDUF assumes that designers are able to foresee problem areas without extensive prototyping and at least some investment into implementation.
They also assert that there is an overhead to be balanced between the time spent planning and the time that fixing a defect would actually cost. This is sometimes termed analysis paralysis.
If the cost of planning is greater than the cost of fixing then time spent planning is wasted.
Continuous Deployment, Automatic Updates, Fault Tolerance, Lisp's Read-eval-print loop and related ideas seek to substantially reduce the cost of defects in production so that they become cheaper to fix at run-time than to plan out at the beginning.
Also, in most projects there is a significant lack of comprehensive written (or even well known) requirements. So in BDUF a lot of assumptions are made that later prove to be false but are designed and possibly already coded.
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