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Several software engineers {{who|date=September 2012}} have warned against total rewrites, especially under schedule constraints or competitive pressures. While developers may initially welcome the chance to correct historical design mistakes, a rewrite also discards those parts of the design that work as required. A rewrite commits the development team to deliver not just new features, but all those that exist in the previous code, while potentially introducing new bugs or [[Software regression|regressions]] of previously fixed bugs.<ref>{{cite web | url=http://www.ronkes.nl/blog/?2005-04-15-neverrewritecode | title=Never Rewrite Code From Scratch | first=Joost | last=Ronkes Agerbeek | date=April 15, 2005 | accessdate=2008-09-11}}</ref><ref name="spolsky">{{cite web | url=http://www.joelonsoftware.com/articles/fog0000000069.html | title=Things You Should Never Do | first=Joel | last=Spolsky | authorlink=Joel Spolsky | date=April 6, 2000 | accessdate=2008-09-11}}</ref> A rewrite also interferes with the tracking of unfixed bugs in the old version.<ref>{{cite web | url=http://www.jwz.org/doc/cadt.html | title=Cascade of Attention-Deficit Teenagers | first=Jamie | last=Zawinski | authorlink=Jamie Zawinski | accessdate=2008-09-11}}</ref>
The incremental rewrite is an alternative approach, in which developers gradually replace the existing code with calls into a new implementation, expanding that implementation until it fully replaces the old one. This approach avoids a broad loss of functionality during the rewrite. [[Cleanroom software engineering]] is another approach, which requires the team to work from an exhaustive written specification of the software's functionality, without access to its code.<ref>{{cite web | url=http://www.perlmonks.org/?node_id=115511 | title=Rewriting, from scratch, a huge code base | first=Ben | last=Tilly | date=September 29, 2001
==Notable examples==
[[Netscape]]'s project to improve HTML layout in [[Netscape Navigator|Navigator]] 4 has been cited as an example of a failed rewrite. The new layout engine ([[Gecko (layout engine)|Gecko]]) had developed independently from Navigator and did not integrate readily with Navigator's code; hence Navigator itself was rewritten around the new engine, breaking many existing features and delaying release by several months. Meanwhile [[Microsoft]] focused on incremental improvements to [[Internet Explorer]] and did not face the same obstacles.<ref name="spolsky"/><ref>{{cite web | url=http://www.jwz.org/gruntle/nomo.html | title=resignation and postmortem | first=Jamie | last=Zawinski | authorlink=Jamie Zawinski | date=March 31, 1999 | accessdate=2008-09-11}}</ref> Ironically, Navigator itself was a successful cleanroom rewrite of [[NCSA Mosaic]] overseen by that program's developers. See [[Browser wars]].
==See also==
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