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Interview: David Roundy of Darcs Revision Control
comforteagle writes "In the aftermath of our last interview with Tom Lord, regardless of personalities, it became apparent that the idea of decentralizing CVS is a big deal. Many mentioned darcs as an alternative to Arch. Mark Stosberg has interviewed project head-hancho David Roundy about darcs, his 'theory of patches,' what's next, and on using Haskell for the project."
There was a good post on this on the mailing list a while ago.
Some people will learn a language because they want to know a language that has that specific set of features, regardless of what applications have already been written in that language.
It's a small group, but if you've the only game in town (in terms of OSS projects for them to work with)... well, that works out pretty well for you!
No, if Darcs has any major issues, it's the RAM and CPU time requirements, some of which the design makes inherently unresolvable.
Like CVS, you can get productive within minutes; the same cannot be said for Arch or even Subversion. Let's see:
You now have a Darcs repository! Let's do something with it:
Now your repository contains all your files. Let's look at the changelog:
Now, where's the server? You need a server to share your repository, right? Nearly -- every repository is a potential server, as long as it's accessible either through the file system, through SSH/SFTP, HTTP or email. Let's go to another machine and check out the repository we just made:
We now have a repository on Jane's box. Let's make a modification:
This last output, by the way, is Darcs' patch format. A "hunk" is a line-based diff. Other types of changes that may be contained in a changeset include renames, moves and binary changes. (Yes, you can also get a GNU-patch-compatible output similar to "cvs diff".)
Now let's commit and push the changes back to John's repository:
Now we can go back to John's machine and look:
(Note how Darcs generates a GNU-style changelog for you automatically.)
Where are the revision numbers, you ask? Well, they don't exist, because they're not needed. Darcs is changeset-oriented, not file-oriented. You can refer to a changeset by name, date, or a special hash identity.
Darcs changesets aren't just GNU patches; they have context, which means, for example, that someone can check out a repository, move a file "foo.c" into the directory "bar" and commit; meanwhile, another person, working on an older copy of the same repository, edits foo.c (which is still in its old location) and commits that. Darcs know that this edit should apply to foo.c in the new location -- and unlike CVS, you don't need to do anything similar to "cvs update" if you're committing files that have been changed on the server. In other words, people can freely commit changes, and the only kind of visible "conflict" will occur when you actually edit the exact same line.
Unlike CVS and Subversion, but like Arch and Monotone, Darcs is a distributed version control system. Repositories are islands which are constantly out of sync with each other, and Darcs' patch commutation system takes care of integration the changes that flow between them.
This system has several extremely useful effects:
For pilot-testing a migration to Darcs, there are scripts available that convert other repository formats (Subversion, CVS, possibly others) into Darcs (and back, actually), so you avoid losing history when making the transition.
no matter what the change set "theory" are implemented into the product if it's not easy to do things with it, it will languish. VSS and CVS are still used widely becuase there are lots of clients and tools that make it useful. the draw of SVN (loved or hated) is that it has a good client and the command line client is easy to drive with scripting tools
We are looking at something to replace our ageing CVS system. We have large OSS projects, worked on by teams of 3-10 people. CVS is very good for what it does but we are feeling its limitations. The biggest problems are that forks are too delicate to use, so we don't use them, and that in order to work you need access to the central archive.
Darcs looked like the best choice. We converted and imported some of our archives. Then we tried checking them out. With CVS, 2-3 minutes. With darcs, 30 minutes.
Our conclusion: darcs is not scalable. Admittedly our code base is large and has a huge history, but in order to use darcs we would have had to break our projects into many small pieces, each with their own repository.
Darcs looks good. But it needs to be made much, much faster if it's to work with large projects.
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1. It's actually hard to use the patch commutation code to do any good outside the concept of a darcs repository.
1.5 I've thought about creating a C library for manipulating/querying darcs repositories, but haven't gotten around to it. The hard part would be of course designing the API. Ideally I'd like the interface to be such that programs using the library couldn't accidentally corrupt the repository.
2. Darcs requires ghc, since it uses some library code only available in ghc to do more efficient IO, string manipulation and to access zlib. It turns out to be a pain on many systems to link with the necesary libaries when using the interpereted version of ghc. So probably accessing darcs from perl will have to go through the executable until a C library is written (which could of course have perl bindings).
3. Rewriting darcs in perl (or parts of it) would be possible, but would be a pain. In particular, the commutation of patches which have conflicts is pretty complicated.