Launchpad offers many things to developers, and open source software developers in particular. One of these things is the ability to host Bazaar branches. For those that have looked a little deeper, they will have noticed that there are four types of branches in Launchpad: Hosted; Mirrored; Remote; and Imported. Hmm, this isn't really what I was intending to talk about at all, but I'm going to go with the flow.
Hosted branches are those where Launchpad is the primary public location of the branch. Hosted branches are normally created by pushing a branch directly to Launchpad. Before you do that though, you need to have registered on Launchpad, and supplied an SSH key. This is how Launchpad knows who you are. There are two ways you can push a branch to Launchpad: one is via SFTP; and the other using the Bazaar smart server (bzr+ssh).
As an example I'm going to use my alias-command bzr branch. The complete SFTP location would be sftp://firstname.lastname@example.org/~thumper/bzr/alias-command, and the smart server one bzr+ssh://email@example.com/~thumper/bzr/alias-command. These are a bit unwieldy, so we extended the lp type urls for bzr to support writing if the launchpad plug-in knows who you are. In order for you to do this you use the lp-login command. bzr lp-login will tell you the username that is currently set. If you have not done this yet, you'll see a message like "No Launchpad user ID configured." I set mine by saying bzr lp-login thumper. This stores thumper as the launchpad_username in the bazaar.conf file. This also means I can use bzr push lp:~thumper/bzr/alias-command to push to my hosted Launchpad branch.
Mirrored branches allow you to have your branches stored publicly in some location that you control, and you let Launchpad know where this is. Launchpad will then update its copy of your branch every six hours. This is handy if you don't have an SSH key, or you have a slow network connection, or you just like having your branches available on your own server.
Remote branches are a bit different. Remote branches were sort of created out of necessity. Some people were registering mirrored branches with unreachable locations. Some of these were possibly by mistake, but quite a few were obviously inaccessible. But more strange is that those branches were linked to bugs or blueprints. There was obviously a desire to have branch meta-data there, but not actually allow Launchpad to get access to the branches. So we have remote branches. You cannot get a copy of a remote branch from Launchpad as Launchpad does not have a copy of it.
Imported branches are those branches where Launchpad get the code from either CVS or Subversion, and puts it into a Bazaar branch. I was really wanting to talk about this as I saw two projects recently where we are importing code that I didn't know about. One is my favourite music player, Amarok, and the other was MPlayer. Just out of curiosity I looked at both of these branches on Launchpad. The Amarok one has 12195 revisions as I'm writing this, and the last revision was 11 hours old, and MPlayer had even more revisions, at 26761. However that isn't even the cool bit. What is really nifty is you can go bzr branch lp:amarok or bzr branch lp:mplayer to get the code. Just to check I did just that, and got a copy of the amarok source. It was the first bit of C++ I had looked at in a long time (it used to be all I did).
Anyway, that was what I really wanted to say. Oh yeah, and bzr rocks.