Editing Software versioning (section)

==== Degree of compatibility ====
Some projects use the major version number to indicate incompatible releases. Two examples are [[Apache Portable Runtime]] (APR)<ref name="apr">{{cite web|url=http://apr.apache.org/versioning.html|title=Versioning Numbering Concepts – The Apache Portable Runtime Project|access-date=April 11, 2009}}</ref> and the FarCry CMS.<ref name="farcry">{{cite web|url=http://blog.daemon.com.au/archives/000276.html|title=Daemonite: The science of version numbering|date=September 14, 2004|access-date=April 11, 2009}}</ref>

Often programmers write new software to be [[backward compatible]], i.e., the new software is designed to interact correctly with older versions of the software (using old protocols and file formats) and the most recent version (using the latest protocols and file formats). For example, IBM [[z/OS]] is designed to work properly with 3 consecutive major versions of the operating system running in the same sysplex.
This enables people who run a [[high availability]] computer cluster to keep most of the computers up and running while one machine at a time is shut down, upgraded, and restored to service.<ref>
Frank Kyne, Bert de Beer, Luis Martinez, Harriet Morril, Miha Petric, David Viguers, Suzi Wendler.
[https://books.google.com/books?id=UxjCAgAAQBAJ "System z Parallel Sysplex Best Practices"].
2011.
p. 6.
</ref>

Often [[packet header]]s and [[file format#Internal metadata|file format]] include a version number – sometimes the same as the version number of the software that wrote it; other times a "protocol version number" independent of the software version number.
The code to handle old [[deprecated]] protocols and file formats is often seen as [[cruft]].