Migrating to Distributed Version Control

A few weeks ago I migrated two major projects to distributed version control systems (DVCS), leaving only one project in Subversion, the one hosted on Savannah. As you can read in my prior posts, I have resisted switching over to DVCS. However, recently I’ve understood the benefits propounded by DVCS adherents, and I’ve found that it has more features than most tutorials let on.

Why Did I Resist?

I resisted DVCS so strongly for a few reasons:

1. Most arguments for DVCS I encountered were actually anti-Subversion arguments; much of them based on incorrect information about Subversion and CVS
2. Much of what I read sounded like knee-jerk trendiness: it sounded like people were doing it just because Linus Torvalds says Subversion is stupid
3. I had an important project (my dissertation!) in Subversion, managed with Trac. I didn’t want to lose all that history by doing a crappy conversion.

When the anti-Subversion arguments didn’t hold up, I ignored them. I thought maybe my working conditions were just different or other people just weren’t reading the manual. Those are still possibilities, but the harder thing to examine was my second reason for dismissal: I assumed that anyone who said these things was a total newbie, who had just been told that DVCS was better. I’ve talked about object-oriented programming proponents often just sound inexperienced with programming. I figured the same was true of DVCS proponents.

However, two things happened that really changed my mind. The first was that I’ve realized that the most annoying thing about somebody questioning my decisions is the feeling that they think my decision is poorly considered when it is deliberate, careful and took me weeks of preparation. It’s very easy to take that attitude with people online: when I don’t hear or see people, I don’t have that mirror held up to me. It’s very easy to just brush something off and say that the other person “just isn’t thinking about it.” Realizing how much that pisses me off when people take that attitude with me, I’ve thought a little more about how I consider peoples’ attitudes online.

Many experience hackers have switched

The second thing was realizing that people whose opinions I know I can value, people who definitely have done their homework, have switched major projects to DVCS. Emacs, my favorite piece of software that I am using right now to right this, is kept in Bazaar now. I know the people who made that decision were doing their homework, not going by knee-jerk reaction, certainly not just to copy Linus Torvalds. Bazaar is also part of the GNU Project.

What about my revisions?

svn2bzr answered my third concern. svn2bzr is a featureful-enough tool that will create Bazaar branches or repositories from SVN repository dumps. It’s really freakin’ easy to create whatever configuration you want:

 > python ~/.bazaar/plugins/svn2bzr/svn2bzr.py --prefix=subdir svndump newrepo

This will create a new Bazaar repository in the directory `newrepo’ that contains all the revisions in the subdirectory `subdir’ of the svn repository. This is where Bazaar’s concept of repositories shows its difference.

In a Bazaar repository you can have many branches beneath the repository in the filesystem, and you import a branch by branching into a subdirectory. I did’t get this for a few weeks, so let me give you an example. Suppose I have a branch called `branch’ located at `~/Public/src/branch’ and a repository called `repo’:

  > cd repo
  > bzr branch ~/Public/src/branch here

That creates a branch within the repository called `here’. Now I can create other branches, merge them, etc. The only tricky thing about getting my revisions into a place where Trac could use them was that I needed a repository hosted on HTTP. Then I used the TracBzr plugin to add the repository to Trac. I realized that changeset links are only used in Trac tickets, and since I had so few of those referencing current revisions, changes in the revision numbers wouldn’t matter that much.

Features of DVCS

I heard many, many anti-Subversion arguments and some really bogus arguments for DVCS. People have said “you can’t merge,” “you can’t make branches,” “Subversion causes brain damage” and on and on. The bogus pro-arguments I heard were that you can commit without a network connection, “forking is fundamental,” and that DVCS is “modern.” Answering these arguments is simple: committing without a network connection is not a big deal. On the other hand updating without a network connection is impossible, and it’s a situation I’ve found myself in more often, especially working with a laptop, instead of just two workstations. This is where DVCS was nice. Updating is a bigger problem than committing.

As to “you can’t merge” and “you can’t make branches,” we all know that’s bologna. However, what you can do much better with DVCS systems like git and Bazaar is edit directory structure and rename files. This is a huge advantage of DVCS systems. Bazaar, for instance, totally keeps track of all renames and copies in its history. Subversion, on the other hand, does renames with a DELETE operation and an ADD operation. Not so smooth. A good way to do get something better than CVS, but not the best.

Furthermore, DVCS systems are very good at merging. That doesn’t mean you can’t merge with Subversion — I’ve been doing that for years. However, merging between two branches in Bazaar is much simpler than merging in Subversion. I don’t have to read the help when I’m merging with Bazaar; merging with Subversion is not hard, but it’s not as simple. Simplicity is the name of the game, baby.

A Stupid Git Realization

I had tried using git before and didn’t enjoy it. I’m glad to say I was using it wrong. I had tried using it to manage my webpages, but whenever I pushed my local changes to my remote webpage tree on UNC’s servers, I would get messages about not updating the local tree and stuff like that. It was just confusing. It didn’t really make sense. I wasn’t interested in trying git again, hence using Bazaar for some new projects.

I had a weird realization one night: I was working with the git tree of Guile, and someone on irc had told me that the most updated git source had a known problem. I didn’t want to go get the tarball for Guile 1.9-13, so I thought “Wait, I have the git tree, so I should be able to generate whatever release version I want. How do I do that?”

  > git tag -l
  > git checkout release_1-9-13

and there I had it. Wow! That is cool.

I also followed a simple tutorial to get my webpages working with a hook that would update the local tree (the one served as my homepage) every time.
It seems a simple idea: make a repository in a different directory,
and check out to it every time I push to that repository. Why hadn’t
that occurred to me before? Conversion from SVN to git was insanely simple:

> sudo yum install git-svn
> git svn clone http://path/to/repo webgit

Conclusions

I think I’m done with Subversion. DVCS, at least git and Bazaar, can do a hell of a lot and I really like their features. I wouldn’t mind using Subversion for an existing project, but I think I’m not going to start any new projects with it. I’m also going to take it easy on people who disagree with me online. I’ve seen that at least some of them were speaking from the same position I hope to.