Erlware and the Git Development Model
Recently the core committers for Erlware, mostly Martin and I, have decided to migrate Erlware’s source control system from the previous Google hosted subversion repository to self-hosted git. Linus’ Tech Talk on Git Convinced us that this was a good idea. This isn’t just a source control change, but a methodology change. It changes the way code gets from its source (the Erlware community) into Erlware itself. The main things we want to get out of this migration is
- Greater visibility into the code base for participants
- Increased code quality
- Faster, cleaner incremental development
It’s yet to be seen whether we will achieve our third goal, but we have already achieved the first two, which is encouraging.
We have had a few false starts. That is just to be expected. When we first started, we took the natural default and just replaced the central subversion repository with a git repository. We then treated that git repository as if it were just another subversion repo. That makes for a nasty commit history, and it doesn’t leverage git’s new development model very well at all. So I started searching for information about how to do a large project in git in a truly distributed fashion. Let me tell you; there just isn’t that much information out there about organizing a project around git. It’s just not there! Some people are working on that, I hear. In any case, I finally pulled aside a friend (hey Scott!), who was familiar with git, and asked them how this whole distributed development thing is supposed to work. Much to my amazement he knew! He could even explain it to a dim bulb like me!
What follows describes how we are implementing git in Erlware. It is based on what Scott told me, lurking on the git mailing list and widely varied sources out there on the interweb.
The Development Model
The model we are using is dirt-simple. Each person has one or more personal git repositories. This is where they do their hacking and keep track of their commit history. There is one official repository. However, this official repository never has commits pushed to it directly. To get some change into the canonical repository (and into the project) you have to send a patch representing your change to the firstname.lastname@example.org Mailing List. This means that every single change is pushed out and viewed by everyone on the mailing list. If we foster the community well, people will give good feedback and code reviews. Hopefully, we can build a community that is as engaged as the community that surrounds git itself.
In any case, once the patch arrives on the list, gets commented upon, changed as required, etc., it will be applied to the canonical repository and become part of Erlware. Think about this for a second, every change to Erlware goes through the Erlware dev list as a patch. Every member of the community has a chance to comment, critique and discuss it. The direction of the project is evident to anyone who has access to the mailing list, which is anyone that wants access. It makes it extraordinarily easy for anyone to contribute to the project without ever having to code. They can just subscribe to the list and provide their knowledge and insight on the code passing through to the implementers of the patch. Of course, the committers have the final say into what makes it into the official repository. However, the community has a tremendous amount of leeway in making sure that the code is correct and of the highest quality.
You may think that having to submit a patch would inevitably slow down development on the project. However, you would be wrong. Each developer has his personal repository with which he can do anything he wants. We encourage developers, and require committers, to make their repository publicly available. So those developers and anyone working with that developer can have easy access to each other’s code. Their development velocity can be anything they are comfortable with. When they finally have something that they think is ready for commit to the official repository, they can create patches and submit them. Of course, they will need to spend some time refactoring their changes into a nice set of small interrelated patches. This, though, is time well spent and will give them one final chance to refactor their code.
Overall, this should be a huge boost to our productivity as a project and our transparency as project leaders.
The Nuts and Bolts
Actually getting this entire thing set up isn’t a trivial project. You need to have some public place to put your git repo, http://git.erlware.org/git or git://git.erlware.org/git in our case). You need to set up that repo with, at the very least, an HTTP server fronting it. You should probably also set up the git-daemon. It makes cloning a git repository very, very fast. Much faster then cloning a repository over HTTP. git-daemon doesn’t really have any idea of, permissions or users though. So you should set this up for read access only. You do that by making sure that the user git-daemon is running as doesn’t have authority to write to any of the files in the repo.
Setting Up a Git Repo
These instructions work whether you are setting up your repository or the official repository.
1) Create a directory for your repos. You can just use
mkdir for this, though if you are going to have multiple submitters you probably want to set the sticky group bit on the directory.
$> mkdir repo_dir $> chown me:group_that_every_commiter_is_in repo_dir $> chmod g+s repo_dir $> cd repo_dir $> mkdir project_git_dir $> cd project_git_dir $> git --bare init
This is going to act as a public server, so we want to enable a bit of index generation. To do that we simply make the post_update hook executable. Git will do the right thing with that.
$> chmod a+x hooks/post_update
If you look in post-update, you will see the command ‘git-update-server-info’. This command allows git to update the indices that it needs when cloning over a ‘dumb’ protocol like HTTP.
Point your HTTP server at the repo and you are done! If you want to do some fancy stuff like send an email on every commit then you need to get then copy the post_receive_email script from the contrib/hooks directory of the git source tarball to hooks/post_receive in your Git repo. Then make that file executable. You need to fiddle with the config a bit, but the instructions for how to do that are in the file itself so there isn’t any need to repeat it here.
$> cp /contrib/hooks/post_receive_email ./hooks/post-receive $> chmod a+x ./hooks/post-receive
Working as a Member of The Project
Working with a git project means that there are three commands that you are going to be using quite a lot. These are git-format-patch, git-send-email, and git-am. Getting good with these commands will let you interact well with Erlware and any other git-based project.
git-format-patch takes some command line options to detail which commits you want to turn into patches and then writes the appropriate patches out a set of files, one patch per commit. It’s dirt simple, though you will need to learn how to structure your commits in a reasonable way. This isn’t hard, but it is a bit involved so I will save that process for another blog. In any case, you will learn how to do it pretty quick once you start supplying patches.
git-send-email Lets you take the patches created by git-format-patch and send them to a specified email address. The process is very well documented in the man page linked above so there isn’t any need to go into much detail. One note, though, if you are a Gmail user and don’t already have
procmail setup to use email, I suggest you use msmtp. There are detail instructions on how to do it on the GitTips page of the GitWiki. If you are as blunt as I am, following the instructions there will save you a huge amount of time.
Finally, git-am allows you to take patches from the mailing list and apply them to your local git repository. It understands both
mbox format and raw patches generated by git-format-patch. This makes it pretty damn useful. You can set up a pull from your mail server to a local mbox and then just run git-am on it. You can also just pull the patches manually and run git-am on those. The whole process is well thought out and very, very simple.
The only real problem that I have had so far is that git-send-email Doesn’t let you prepend any information to the subject line of the sent email. They all end up with [PATCH] . This would be fine for most projects, but we run several projects under Erlware, and it’d be good if we could do something so that the subject looked something like [PROJECT][PATCH] . Anything that would let us indicate the project would be great. When and if I get some time I intend to remedy this and send a patch back to the git community.
That’s it. No doubt I have missed a huge number of things. Hopefully, the commenters will be nitpicky and point them out so I can fix the problems. I am excited about this new model and expect it to do some awesome things for our project. If it doesn’t do anything but spread knowledge about the codebase to all our committers, I will be overjoyed.