~bzr-pqm/bzr/bzr.dev

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

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 available online 

within the developer documentation, <http://doc.bazaar-vcs.org/developers/>.

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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