Compatibility Rules

Mercurial has been used in production by major software projects since a couple months after its initial release. Thus, Mercurial has always made a serious effort to be backward compatible from release to release with a minimum of surprises. Here's an attempt to distill what our rules are:

0.1. File formats and layout:

• New Mercurial should always be able to read old Mercurial repositories
• Old Mercurial should always be able to pull from new Mercurial servers
• Old Mercurial should break with a meaningful error message if it can't
  • read a new Mercurial repository
• New requirements are listed in the requirements file
• Revlog files have a revision flag and per-revision feature flags
• User-visible changes are mentioned in the release notes

0.2. Commands:

• Changes to existing command behavior is minimal
• Removing a feature requires a deprecation period of at least one major release
• Output formats for commands with output likely to be parsed (especially log and status) are intended to be
  • stable enough to be parsable by dumb scripts and tools
• Other commands may occasionally add or change output
• Changes likely to affect parsers are documented in the release notes
• Meaningless return values may become sensible

0.3. Config Options:

• Config options are long-lived and should not change behavior
• We may deprecate some options, giving their replacements different names
• Undocumented config options may quietly disappear if they've outlived their usefulness

0.4. Hooks:

• The hook calling convention is intended to be extremely stable
• New hooks and environment variables may be added, but old ones will be preserved

0.5. Extensions:

• Extensions that are shipped in hgext/ follow the same compatibility rules as core code
• Bear in mind that extensions are not "canonical", and their behavior may change or break core
  • Mercurial functionality

0.6. Wire protocol:

• The wire protocol can check for individual features and use them if available
• The basic wire protocol is extremely stable

0.7. Web interface:

• URLs from old Mercurial should continue to work with new Mercurial
• HTML output from the web interface is moderately stable, but screen-scraping it may not be reliable
• However, parsing of output from raw-style URLs should be stable

0.8. Internal API:

• Significant improvements to internal APIs are still being made
• Writers of extensions can expect a small amount of porting work between releases
• Extensions included in hgext/ and contrib/ will be updated by the Mercurial team