========================
Bazaar Design Principles
========================

We have learned or adopted a few general principles for code in Bazaar.
Generally we will try to follow them in future, either for consistency or
because they've been proven to work well, or both.  

We may need to depart from these principles in particular special cases,
or modify them as we learn more, or we might be diverging for them for no
very good reason but just because of bugs.  If in doubt, ask.  

See also: `Bazaar Developer Document Catalog <index.html>`_.


Testing
-------

Untested code is broken code.

So if a function is removed from the normal flow of execution (perhaps
because a new default format was introduced) we have to make sure we can
still execute and test the old code -- or remove it altogether.



Data formats
------------

Fixing code once it's released is easy; fixing a problematic data format
once people have started using it is more difficult.  We should document
and review formats separately from the code that implements them.

Data formats should have clear format markers that allow us to support new
formats in future.  It should be easy to read the format without reading
the whole object.

The format marker should be a string understandable by a user that names
the format and gives the bzr release that introduced it.  If the bzr
program doesn't understand that format, it can at least show that format
marker to the user.

Once we mark a format as supported, we'll continue supporting it for
several future releases, and support upgrading from it
forever.

Once we've released a format, we normally don't change it.  Adding new
optional elements can cause problems when older clients don't understand
those changes, or don't propagate them properly.

We clearly distinguish internal files from user files.  Files inside
``.bzr/`` are only written to by bzr and we discourage users from editing
them.  Within bzr, code addressing the abstract interface of the Branch,
BzrDir, etc shouldn't know where or how the internal files are stored.  If
anything else is written in there, it won't be propagated when pushing or
pulling, and won't be converted when upgrading.  (This is not quite true
though; there is a ``branch.conf``.)

User files within the tree, by contrast, we always store and return
verbatim.  It's OK for Bazaar to read and act on these files (as we do
with ``.bzrignore``), and to update them (as ``bzr ignore`` does), but
they remain clearly user files and can be directly edited.
