I’ve long had an interest in version control systems (VCS), also known
as source code management (SCM) systems — beginning with RCS, SCCS and
CVS. CVS
was already showing it’s age when I started using it in 1998. When the
company I worked for, Axent, was acquired by Symantec in 2000, we switched to
using Perforce. At first, I
thought Perforce was a step backwards from CVS. After using it heavily
for a few months, it was clear that CVS and WinCVS
didn’t come close to the ease-of-use and features of Perforce and
p4win. CVS was dreadfully slow compared to Perforce, which was
lightning fast (and still is).
Perforce encourages third-party developers to develop add-ons for use
with their software, which is almost as good as what you get with an
active open-source project. Alough Perforce is proprietary, it’s about as open
as I’ve ever seen a commercial project. It runs on many platforms, has
conversion scripts to migrate CVS repositories to Perforce, etc. It’s not
cheap, unless you’re working on an open-source project, in which case, you can
get free licenses to use it.
At some point, I heard about the Subversion
project, which aimed to correct many of the deficiencies of CVS. Those
were the pre-1.0 days, and it was interesting to watch the development
of Subversion.
About the same time, Bitkeeper was in the news. It was different than CVS, Subversion and Perforce because it was a distributed
version control system. The idea appealed to me because of the
idea that a developer could have version control for his/her private
changes without having to check-in to the main repository until they
were ready. At that time, there weren’t any mature open-source
distributed version control systems to investigate.
I switched jobs late in 2004, and my new company was using Subversion.
Overall, I have been very pleased with Subversion in day-to-day use.
It’s much better than CVS. We had some reliability problems with the
Subversion server. It was running on Windows with the BDB database
storage back-end. When it was switched to a Linux server with the FSFS
back-end, it became much more reliable. My team uses TortiseSVN — an
excellent user interface that integrates with Windows Explorer.
I’ve periodically kept tabs on version control systems. Many open-source variants have sprung up over the last few years: Mercurial, Bazaar-NG, Git/Cogito, Darcs, SVK, Arch and Monotone.
Lately though, I haven’t seen any great reviews on which ones are the
most mature, or what the pros and cons are of each. So, I’ve done some
google research to figure it out, focusing primarily on the distributed
variants.
The conclusion I’ve come to is that the developers of each version
control system are learning from the developers of the other version
control systems, and each project is improving. The Subversion developers are
learning from the distributed version control developers. Recently, there was
an SVN developer summit and they tried out Mercurial, which tells me that there’s merit to the distributed approach.
If you’re already using a modern version control system, the cost to
switch may outweight the benefit. Organizations seem to be able to
cope with legacy tools like Visual SourceSafe and CVS, although better tools
can make developer’s lives easier.
Here’s my own highly subjective comparison table. I’ve marked, in red, some of the things
I think are noteworthy. I focused my efforts on the compeitors that
seem to have garnered the most community adoption. I’ve included one
commercial system, Perforce. Each item is rated on a scale of 1 to 10, 10 being the best. (Update: There’s a better table than mine at http://bazaar-vcs.org/RcsComparisons and various comparisons at Wikipedia)
Comparison of Source Code Management systems
| January 31, 2007 | Subversion | SVK | Git/Cogito | Mercurial | Bazaar-NG | Darcs | Perforce | Notes |
| Command-line name | svn | svk | git / cg | hg | bzr | darcs | p4 | |
| Cross-Platform | 10 | 9 | 6 | 10 | 10 | 9 | 10 | Windows, Linux, Mac, Solaris, etc. |
| Maturity | 9 | 6 | 8 | 7 | 5 | 8 | 10 | Maturity based on lifetime, and project flux in code |
| Maturity: GUI | 9 | 0 | 5 | 4 | 3 | 1 | 10 | |
| Disconnected/offline operation | 2 | 10 | 10 | 10 | 10 | 10 | 0 | Disconnected 1. editing of files, 2. branching, 3. merging, 4. history, etc. Especially handy when there’s no network connectivity, such as when on an airplane. |
| Community Adoption | 10 | 2 | 8 | 7 | 5 | 2 | 1 | |
| Documentation Quality | 10 | 7 | 7 | 8 | 6 | 8 | 10 | |
| Storage Format: Robustness | 5 | 5 | 10 | 8 | 7 | 5 | 5 | Storage format least susceptible to corruption. |
| Storage Format: Not in flux | 1 | 1 | 10 | 8 | 1 | 1 | ? | |
| (re)Merging support | 0 | 9 | 9 | 9 | 9 | 10 | 4 | Remembers prior merges, cherry-picking, etc. |
| Repository Size | 1 | 9 | 10 | 9 | ? | ? | ? | |
| Speed | 2 | 7 | 10 | 8 | 6 | 10 | ||
| Scalability | 9 | 9 | 10 | 9 | 5 | 5 | 9 | |
| Commercial Backing | 10 | 5 | 10 | 10 | 10 | 5 | 10 | |
| Subversion Integration | 10 | 8 | 6 | 5 | 4 | 4 | ? | Tailor can be used to migrate changes between all systems |
| Totals: | 88 | 87 | 119 | 112 | 81 | 68 | 79 |
If I were to pick a VCS system today, it would probably be Git, followed by Mercurial. What follows are my unpolished notes and ideas.
Git/Cogito
Git is very scalable, and is
the fastest
open-source version control system available. Git has a wide community
of professional engineers supporting it, and it has a bright
future. There are graphical user interfaces available for Git such as
gitk and qgit, although none of them are as mature as the user interfaces available for Subversion. Cogito is the easy-to-use command-line wrapper around git. See also the Cogito Wiki.
According to Keith Packard of xorg fame, Git has the most
robust/reliable repository storage format. Advantages of git and all distributed VCSes include 1. offline repository access, 2.
private branches, 3. distributed backups including change history.
For those wishing to use Git/Cogito on Windows, use Cygwin and select the git and/or cogito packages and read the information at http://git.or.cz/gitwiki/WindowsInstall. For those organizations wishing for excellent Windows-Explorer integration, use git-cvsserver in combination with TortiseCVS.
To install git and cogito on Fedora, run the following as root:
yum install git cogito qgit
I’ve reluctantly decided that Git isn’t as mature as Subversion, which
shouldn’t be surprising because Subversion has been around for longer. Git
isn’t the right fit for all projects. Git was designed for monolithic code
bases, not for modular code bases, although work is in progress to allow it to
support sub projects
(similar to svn:externals).
“Such flexibility is an implicit feature of centralized SCMs, but is much more
difficult to implement in a distributed system like git. As a result, git
currently lacks built-in subproject support, although gitweb does have a notion
of subprojects.”
There’s a document that describes Common Mistakes made when using Git. Unfortunately, most of it isn’t written yet — there’s only a loose outline.
Tutorials:
- http://wellington.pm.org/archive/200510/git/
- Crash course for CVS users: http://git.or.cz/course/cvs.html
- Crash course for Subversion users: http://git.or.cz/course/svn.html
- Howto use git to hack on sources coming from a Subversion repository.
http://tw.apinc.org/weblog/2007/01/03#subverting-git
Tools — See http://git.or.cz/gitwiki/InterfacesFrontendsAndTools
Mercurial
The OpenSolaris project decided between Bazaar-NG, Git and Mercurial.
Mercurial was chosen primarily because 1. it was fast (although Git is
faster), 2. the Mercurial developers were very responsive to the
OpenSolaris developers and 3. OpenSolaris developers felt like they
could hack Python code, and 4. the repository format works well with ZFS &
NetApp filesystem snapshotting. Their evaluation of Git is here,
and it looks like the listed downsides are now out-of-date or superficial. The Mozilla project had a “version control shootout“, and although they haven’t yet made a decision, Mercurial and Bazaar-NG sounded the best to them.
The following has diagrams to illustrate distributed merging:
http://www.selenic.com/mercurial/wiki/index.cgi/UnderstandingMercurial
Mercurial is more mature than Bazaar-NG, and Mercurial is faster:
http://sayspy.blogspot.com/2006/11/bazaar-vs-mercurial-unscientific.html
“Technologically, centralized systems are a single point of failure–
any problems with the central server are problems for all people using
it.” — http://bazaar-vcs.org/WhyUseBzr
Mercurial supports access control, email notify, line-ending conversion,
etc.:
http://www.selenic.com/mercurial/wiki/index.cgi/UsingExtensions
SVK
SVK is built on top of Subversion, so it should, in theory, integrate
well with an existing Subversion repository, allowing developers to use
a distributed tool even if the master server remains a Subversion
server. Community adoption is high enough to have some confidence
in the future of the project, although adoption isn’t nearly as high as with Git, Mercurial or Bazaar-NG.
It used to be difficult to install, but you can now get a prebuilt
installer for Windows and probably for Linux as well. Working copies
(sandboxes) have no extra meta data (no .svn directory which interfere
with find, etc.) The repository format is significantly smaller than
with Subversion. I’ve found that SVK is much faster than Subversion,
although I haven’t used it much. There is not yet a graphical
user interface — a must for many organizations/communities.
The good, the bad and the ugly about SVK (Sept 2006):
http://kitenet.net/~joey/blog/entry/svk.html
Darcs
Users of darcs, including myself, appreciate its simplicity and
ease-of-use (note: Cogito, Mercurial and Bazaar-NG are also easy to
use). Downsides of darcs are that 1. Darcs is implemented in Haskel,
which limits the contributing developer community (perhaps it will inspire
people to learn Haskel), 2. depends on having Haskel libraries installed and
3. there’s no graphical user interface, unless you consider darcsweb. Still, I like darcs, and I use it on my
home linux box. Like Perforce and SVK, darcs doesn’t clutter up directories
with .darcs metadata. It used to be that Darcs wasn’t very scalable, but
I hear that it’s become much more scalable as of mid-2006. I’ve read that
Mercurial and Darcs feel somewhat similiar in their command-line user
interface.
Mirroring Subversion with Darcs and Tailor (Sept 2006):
http://fiatdev.com/articles/2006/09/10/mirroring-subversion-with-darcs-and-tailor
Subversion
Subversion has a bright future, I think, and we may yet see some of the
advantages of distributed systems appear. For those who need merge
history tracking, which makes future merges from the same branch
easier, there’s svnmerge.py. In a future release, Subversion will have this feature built-in.
The Subversion 1.4 release brought impressive speedups for working copy operations.
Control/Power
Changing information flow by switching from a centralized system to a
distributed system will empower or disempower different sets of people.
I wouldn’t be surprised if one encounters resistance in switching.
In the centralized model, developers are empowered to make any change
they want, which may affect everyone, without consulting others. Of course,
if they abuse that power, they may lose commit access. With a distributed system, an integrator pulls in people’s changes based on what and whom they trust. If
you’re aiming for quality code that doesn’t destabilize a system, it
sounds like a good approach, and it works well for Linux kernel development. Most distributed systems can be used similiar to a
centralized system, so that no integrator is required — individuals can push their changes to the master repository.
