/brz/remove-bazaar

=============================

Bazaar Architectural Overview

=============================

This document describes the key classes and concepts within Bazaar.  It is

intended to be useful to be useful to people working on the Bazaar

codebase, or to people writing plugins.

If you have any questions or 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 <../../developers/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 like 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 io mechanisms.

Filenames vs URLs

=================

Transports work in 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 their URL to Unicode reliably.

Equally problematic if we're given a url-like string containing non-ascii

characters (such as the accented a) we can't be sure how to convert that

to the correct URL, because we don't know what encoding the server expects

for those characters.  (Although this is not totally reliable we might still

accept these and assume 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.  If we try to convert URLs to regular Unicode

paths this information will be lost.

This implies that Transports must natively deal with URLs; for simplicity

they *only* deal with URLs and conversion of other strings to URLs is done

elsewhere.  Information they 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

====================

Repositories 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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