Dave Abrahams <dave@boostpro.com> writes:
on Thu Mar 22 2012, Martin Geisler <mg-AT-aragost.com> wrote:
This kind of mental model is stimulated by git. It explains why git users make a fuss about amending, rebasing and efficient branching and merging.
I'm afraid I don't agree with this. The version control systems that came after CVS switched to storing repository-wide snapshots. CVS was a collection of RCS files and so completely file-centric. SVN, Mercurial, Git, ... are all repository-centric. Conceptually they work by storing a series (linear or not) of working copy snapshots.
Darcs is a possible exception to this: I think it might fit more closely to your unit-of-work model since it models the repository state as a result of a number of patches (units-of-work).
People get hung up on this all the time, but whether the storage format is fundamentally snapshots or diffs is not really important. They're isomorphic to one another.
Yes, that's mostly true. Both Git and Mercurial conceptually stores snapshots of your working copy. These snapshots are of course heavily delta compressed -- otherwise we couldn't do DVCS in the first place. But I say that they conceptually store snapshots since that's what commands like 'hg diff' operate on. When you 'hg diff -r 1:2', then Mercurial has to go out and re-compute the patch that brings you from revision 1 to 2. People often think that changeset 2 "contains" this diff, but it's more complicated that this: the deltas we store on disk don't correspond directly to this diff. This especially true for a merge changeset where the deltas on disk are made on a per-file basis against the parent that produces the smallest delta.
Git, like any other modern tool, provides commands that support both views of the history. Rebase, for example, treats your commits as units-of-work and "replays" that work on a new base commit. Many other elements of the interface treat commits like snapshots.
It actually don't replay anything: it does a series of three-way merges, and three-way merges are an inherently snapshot based thing. -- Martin Geisler Mercurial links: http://mercurial.ch/