[Python-Dev] Fwd: Distributed RCS

Guido van Rossum gvanrossum at gmail.com
Sat Aug 13 23:27:22 CEST 2005

With permission, I'm forwarding an email from Mark Shuttleworth about
Bazaar-2 (aka Bazaar-NG), a distributed source control system (not
entirely unlike bitkeeper, I presume) written in Python and in use by
the Ubuntu system. What do people think of using this for Python? Is
it the right model? Do we want to encourage widespread experimentation
with the Python source code?

--Guido van Rossum (home page: http://www.python.org/~guido/)

---------- Forwarded message ----------
From: Mark Shuttleworth <mark at canonical.com>
Date: Aug 11, 2005 12:13 PM
Subject: Distributed RCS
To: Guido van Rossum <gvanrossum at gmail.com>
Cc: Steve Alexander <steve at canonical.com>, Martin Pool
<mbp at canonical.com>, Fredrik Lundh <fredrik at pythonware.com>

Hi Guido

Steve forwarded your mail to me regarding distributed revision control,
so I thought I'd follow up with some thoughts on why I agree with
Frederick Lundh that it's important, and where we are going with the
Bazaar project.

First, distributed RCS systems reduce the barrier to participation.
Anybody can create their own branches, and begin work on features, with
full revision control support, without having to coordinate with the
core RCS server sysadmin. So, for example, if someone gets an idea to
work on PEP X, they can simply create a branch, and start hacking on it
locally, with full RCS functionality like commit and undo, and logs of
their changes over time. They can easily merge continually from the
trunk, to keep their branch up to date. And they can publish their
branch using only a web server.

With Bazaar, these branches can be personal or shared group branches.

The net effect of this is to make branching a core part of the
development process. Each feature gets developed on a branch, and then
merged when its ready. Instead of passing patches around in email, you
find yourself passing out branch references, which are much easier to
deal with since they are always "up to date". In Launchpad, we have
evolved to work around this branch-per-feature approach, and built a
review process so that each branch gets a review before the code is
merged to the trunk.

It also has a positive social impact, because groups that are interested
in a feautre can begin to collaborate on it immediately rather than
waiting to get consensus from everybody else, they just start their
branch and get more testing when it is reaching a reasonable state of
maturity - then the project decides whether or not it lands. That
results in less argument about whether or not a feature is a good idea
before anybody really knows what it's going to look like. Those who are
interested, participate, and those who aren't reserve judgement till
it's done.

As for Bazaar, we have just wrapped up our latest sprint, where we
decided that bazaar-ng (bzr), which is being written in Python by Martin
Pool, will become Bazaar 2.x, in the first quarter of 2006. The current
1.x line of development has served us well, but the ideas we developed
and which have been implemented as a working bazaar-ng reference by
Martin are now proven enough that I'm committing the project (Ubuntu,
and all of Launchpad) to it. Martin will continue to work on it full
time, and will be joined by the current Bazaar 1.x team, Robert Collins,
David Allouche and James Blackwell. That makes for a substantial chunk
of resources but I think it's worth it because we need a truly superb
free revision control system when dealing with something as large and
complex as an entire distribution.

The whole of Ubuntu will be in Bazaar in due course. Currently, we have
about 500 upstreams published in the Bazaar 1.x format (see
http://bazaar.ubuntu.com/ for the list), all of those will be converted
to Bazaar 2.x and in addition we will continue to publish more and more
upstreams in the 2.x archive format. We actively convert CVS and SVN
upstreams and publish them in the Bazaar format to allow us to use a
single, distributed revision control system across all of those
packages. So there's a lot of real-world data and real-world coding
going on with Bazaar as the RCS holding it all together.

Perhaps more importantly, we are integrating Bazaar tightly with the
other Launchpad applications, Rosetta and Malone. This means that bug
tracking and translation will be "branch aware". You will be able to
close a bug by noting that a commit in one of your branches fixes the
bug, then merging it into the relevant mainline branch, and have the
launchpad bug tracker automatically mark the bug as closed, if you wish.
Similarly you will be able to get the latest translations just by
merging from the branch published by Rosetta that has the latest
translations in it for your application.

The combination of distributed revision control, and ultimately
integrated bug tracking and translations, will I think be a very
efficient platform for collaborative development.

Bazaar is free, and the use of Launchpad is free though we have not yet
released the code to the web services for bug tracking and translation.

I hope that puts bazaar into perspective for you. Give it a spin - the
development 2.x codebase is robust enough now to handle a line of
development and do basic merging, we are switching our own development
to the pre-release 2.x line in October, and we will switch over all the
public archives we maintain in around March next year.


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