Zeroconf Branch Sharing with Bazaar

At Canonical, one of the approaches taken to accelerate development is to hold coding sprints (otherwise known as hackathons, hackfests or similar). Certain things get done a lot quicker face to face compared to mailing lists, IRC or VoIP.

When collaborating with someone at one of these sprints the usual way to let others look at my work would be to commit the changes so that they could be pulled or merged by others. With legacy version control systems like CVS or Subversion, this would generally result in me uploading all my changes to a server in another country only for them to be downloaded back to the sprint location by others.

In contrast, with a modern VCS like Bazaar we should be able to avoid this since the full history of the branch is available locally – enough information to let others pull or merge the changes. That said, we've often ended up using a server on the internet to exchange changes despite this. This is the same work flow we use when working from home, so I guess the pain of switching to a new work flow outweighs the potential productivity gains.

The Solution

Bazaar makes it easy to run a read only server locally:

bzr serve [--directory=DIR]

However, there is still the issue of others finding the branch. They'd need to know the IP address assigned to my computer at the sprint, and the path to the branch on the server. Ideally they'd just need to know the name of the my branch. As it happens, we've got the technology to fix this.

Avahi makes it trivial to advertise and browse for services on the local network without having to worry about what IP addresses have been assigned or what people name their computer. So the solution is to hook Avahi and Bazaar together. This was fairly easy due to Avahi's DBus interface and the dbus-python bindings.

The result is my bzr-avahi plugin. You can either download tarballs or install the latest version directly with from Bazaar:

bzr branch lp:bzr-avahi ~/.bazaar/plugins/avahi

To use the plugin, you must have at least version 1.1 of Bazaar, the Python bindings for DBus and Avahi, and a working Avahi setup. Once the plugin is installed, it hooks into the standard "bzr serve" command to do the following:

  • scan the directory being served for branches that the user has asked to advertise.
  • ask Avahi to advertise said branches

You can ask to advertise a branch using the new "bzr advertise" command:

bzr advertise [BRANCH-NAME]

If no name is specified, the branch's nickname is used. The advertise command sends a signal over the session bus to tell any running servers about the change, so there is no need to restart "bzr serve" to see the change.

At this point, the advertised branches should be visible with a service browser like avahi-discover, so that's half the problem solved. From the client side two things are provided: a special redirecting transport and a command to list all advertised branches on the local network.

The transport allows you to access the branch by its advertised name with most Bazaar commands. For example, merging a branch is as simple as:

$ bzr merge local:BRANCH-NAME
local:BRANCH-NAME is redirected to bzr://hostname.local:4155/path/to/branch
...
All changes applied successfully.
$

If you want to get a list of all advertised branches on the network, the "bzr browse" command will print out a list of branch names and the URLs they translate to.

I believe using these tools together should offer a low enough overhead for direct sharing of branches at sprints that people would actually bother using it. It should be quite useful at the next sprint I go to.

Comments:

Stuart Colville -

This is such a brilliant way of taking full advantage of de-centralisation and perfect for sharing branches between localised teams. The only downside is that Avahi is Linux only and a cross platform solution would be even better!

Mathieu Cadet -

Woa! Just tested it. That's really awesome. Great job here! This should be included by default in bazaar.

James Henstridge -

Stuart: Avahi is not Linux only, but I catch your meaning: this isn't going to work with the mDNS responder on MacOS X, and Avahi has not been ported to Windows.

As there is no standard mDNS API for Python, I programmed to the API of the responder I could test things on. I am sure that the ideas from my plugin could be used to write a similar plugin for Apple's Bonjour client library, but I am not sure how much code it would share (bzr-avahi is almost entirely glue code).

Antono Vasiljev -

Awesome idea! I'll try to make something similar for git.

Stuart Metcalfe -

This is an great use of Zeroconf - well done! It's worth noting that Apple's mDNS responder has both Mac OSX and Windows versions which share a common API.