=============================
Bazaar Architectural Overview
=============================

This document describes the key classes and concepts within Bazaar.  It is
intended to be useful to people working on the Bazaar codebase, or to
people writing plugins.

If you have any questions, or if something seems to be incorrect, unclear
or missing, please talk to us in ``irc://irc.freenode.net/#bzr``, or write
to the Bazaar mailing list.  To propose a correction or addition to this
document, send a merge request or new text to the mailing list.

The current version of this document is available in the file
``doc/developers/overview.txt`` in the source tree, and from 
<http://doc.bazaar-vcs.org/bzr.dev/>.

See also:

 * `Bazaar Developer Documentation Catalog <index.html>`_.
 * `Bazaar Developer Guide <../en/developer-guide/HACKING.html>`_
   (particularly the *Coding Style Guidelines* section.)

.. contents::

Essential Domain Classes
########################

The core domain objects within the bazaar model are:

* Transport

* Branch

* Repository

* WorkingTree

Transports are explained below. See http://bazaar-vcs.org/Classes/
for an introduction to the other key classes.

Transport
#########

The ``Transport`` layer handles access to local or remote directories.
Each Transport object acts as a logical connection to a particular
directory, and it allows various operations on files within it.  You can
*clone* a transport to get a new Transport connected to a subdirectory or
parent directory.

Transports are not used for access to the working tree.  At present
working trees are always local and they are accessed through the regular
Python file I/O mechanisms.

Filenames vs URLs
=================

Transports work in terms of URLs.  Take note that URLs are by definition
only ASCII - the decision of how to encode a Unicode string into a URL
must be taken at a higher level, typically in the Store.  (Note that
Stores also escape filenames which cannot be safely stored on all
filesystems, but this is a different level.)

The main reason for this is that it's not possible to safely roundtrip a
URL into Unicode and then back into the same URL.  The URL standard
gives a way to represent non-ASCII bytes in ASCII (as %-escapes), but
doesn't say how those bytes represent non-ASCII characters.  (They're not
guaranteed to be UTF-8 -- that is common but doesn't happen everywhere.)

For example, if the user enters the URL ``http://example/%e0``, there's no
way to tell whether that character represents "latin small letter a with
grave" in iso-8859-1, or "latin small letter r with acute" in iso-8859-2,
or malformed UTF-8.  So we can't convert the URL to Unicode reliably.

Equally problematic is if we're given a URL-like string containing
(unescaped) non-ASCII characters (such as the accented a).  We can't be
sure how to convert that to a valid (i.e. ASCII-only) URL, because we
don't know what encoding the server expects for those characters.
(Although it is not totally reliable, we might still accept these and
assume that they should be put into UTF-8.)

A similar edge case is that the URL ``http://foo/sweet%2Fsour`` contains
one directory component whose name is "sweet/sour".  The escaped slash is
not a directory separator, but if we try to convert the URL to a regular
Unicode path, this information will be lost.

This implies that Transports must natively deal with URLs.  For simplicity
they *only* deal with URLs; conversion of other strings to URLs is done
elsewhere.  Information that Transports return, such as from ``list_dir``,
is also in the form of URL components.


Repository
##########

Repositories store committed history: file texts, revisions, inventories,
and graph relationships between them.

Stacked Repositories
====================

A repository can be configured to refer to a list of "fallback"
repositories.  If a particular revision is not present in the original
repository, it refers the query to the fallbacks.

Compression deltas don't span physical repository boundaries.  So the
first commit to a new, empty repository with fallback repositories will
store a full text of the inventory, and of every new file text.

At runtime, repository stacking is actually configured by the branch, not
the repository.  So doing ``a_bzrdir.open_repository()`` 
gets you just the single physical repository, while 
``a_bzrdir.open_branch().repository`` gets one configured with a stacking. 
Therefore, to permanently change the fallback repository stored on disk,
you must use ``Branch.set_stacked_on_url``.  

Changing away from an existing stacked-on URL will copy across any
necessary history so that the repository remains usable.

A repository opened from an HPSS server is never stacked on the server
side, because this could cause complexity or security problems with the
server acting as a proxy for the client.  Instead, the branch on the
server exposes the stacked-on URL and the client can open that.


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