March 23rd, 2007
I’ve talked a lot lately about different VCSs.
I got some interesting comments in reply to my most recent post. One person took issue with my complaint that nobody really understood how to specify a revision to git format-patch, and proceeded to issue an incorrect suggestion. And a couple of people complained about my comments about bzr, which generally came down to the released version of bzr didn’t have anything compelling and also didn’t support tags.
So I went into #bzr, asked them what bzr has that git, Mercurial, and darcs don’t. And gave bzr the benefit of the doubt that 0.15 will be out soon and will be stable. What I got back were these general items:
- Renaming of directories (not in hg, git)
- 2-way sync with Subversion (not in hg, darcs)
- Checkouts (not in any others by default)
- No server-side push requirement
Let’s look at these in more detail.
1. Renaming of directories
All of them can rename files and (excepting git) completely accurately track back the file’s history. But consider this: if person A commits a change to branch A that adds a file, and person B then renames the directory that the file is in on his branch, will a merge cause person A’s file to appear in the new directory name? In darcs and bzr, yes. In Mercurial and git, no.
So yes, this is a nice thing. But I have never actually had this situation crop up in practice, and even if it did, could be trivially remedied. I would say that for me, I don’t really care.
[Update: Current stable releases of Mercurial can do this too. I'm not quite sure how, but it does work. So git is the only one that can't do that.]
2. 2-way sync with Subversion
This is a really nice feature and is present in both git and bzr. I haven’t tested it either place, but if it works as advertised — and properly supports tracking multiple related svn branches and merges — would be slick. That was enough to make me consider using git, but in retrospect, I so rarely interact with people using svn that it is not that big a deal to me.
Still, for those that have to work with svn users, this feature in bzr and git could be a big one.
Better yet would be to get all those svn holdouts over to DVCS.
A bzr checkout is basically a way to make local commits be pushed to the remote repo immediately, as with svn. This is of no utility to me, though I can see some may have a use for it. But it can be done with hg hooks and probably approximated with scripting in others.
4. No server-side process necessary for pushing repos
bzr has built-in support to push to a server that has sftp only, and doesn’t require a copy of itself on the server. While I believe that none of the other three have that, it is possible to rsync (and probably ftp) darcs and Mercurial repos to a server in a safe fashion by moving repo files in a defined order. Probably also possible with git. All four can pull repos using nothing but regular HTTP.
What bzr still doesn’t have
Integrated patch emailing. The big thing is that it has no built-in emailing of patches support. darcs is extremely strong in this area, followed by hg, and git is probably third. “darcs send” is all it takes to have darcs look at the remote repo, figure out what you have that they don’t, and e-mail a bundle of changesets to them. I posted an extension and later a patchset that does all this for Mercurial except for automatically figuring out what default email address to do (that’ll come in a few days, I think). One feature Mercurial has had for awhile that Darcs hasn’t is sending multiple textual diffs as a thread, with one message per changeset. bzr doesn’t have any support for emailing patches yet, which is disappointing. Because of the strong support for this in darcs and Mercurial, people running those systems feel less of a need to publish their repos.
[Update: There is a plugin for bzr that seems to address some of this. I haven't tested it, and it's not in bzr core (so doesn't quite meet my very friendly for a newbie requirement), but this does exist, though apparently not as advanced as Mercurial]
Performance. Supposedly 0.15 is supposed to be better on this, but even if bzr achieves the claimed doubling of performance, most benchmarks I have seen would rate it as still being significantly behind git and Mercurial, though it may overtake darcs in some tests.
Extensive documentation. I would say that bzr’s docs are better in some ways than git’s (its tutorials especially), but lack depth. If you want to know some detail about how the repository works on-disk, it’s not really documented. Darcs still has David’s excellent manual, and Mercurial has the hg book which is still great as well.
Merging not as advanced. darcs is pretty obviously way on top here, but of the others, Mercurial does a pretty good job with its automatic handling of renames and automatic resolving of different branches that commit the same change (even if that same change is a rename, or an add of the same content). bzr can’t resolve as much automatically.
Well, I’ll say that bzr still doesn’t look compelling enough for my use cases to use, and the lack of an easy-for-a-newbie-to-use automated email submission feature is a pretty big disappointment. Though I did appreciate the time those on #bzr spent with me, and if I needed to sync with svn users frequently, I’d probably choose bzr over git.
For now, I’m happy with sticking with darcs for my code and hg for my Debian work.
But all four communities are aggressively working on their weaknesses, and this landscape may look very different in a year.