VCS comparison table

[James Henstridge]

On 20/10/06, Jakub Narebski <jnareb at gmail.com> wrote:
> James Henstridge wrote:
> > With the above layout, I would just type:
> >     bzr branch http://server/repo/branch1
>
> With Cogito (you can think of it either as alternate Git UI, or as SCM
> built on top of Git) you would use
>
>    $ cg clone http://server/repo#branch
>
> for example
>
>    $ cg clone git://git.kernel.org/pub/scm/git/git.git#next
>
> to clone _single_ branch (in bzr terminology, "heavy checkout" of branch).

My understanding of git is that this would be equivalent to the "bzr
branch" command.  A checkout (heavy or lightweight) has the property
that commits are made to the original branch.

> But you can also clone _whole_ repository, _all_ published branches with
>
>    $ cg clone git://git.kernel.org/pub/scm/git/git.git

I suppose that'd be useful if you want a copy of all the branches at
once.  There is no builtin command in Bazaar to do that at present.


> With core Git it is the same, but we don't have the above shortcut
> for checking only one branch; branches to checkout are in separate
> arguments to git-clone.
>
> In bzr it seems that you cannot distinguish (at least not only
> from URL) where repository ends and branch begins.

I guess this highlights that the two tools optimise for different workflows.
> > This command behaves identically whether the repository data is in
> > /repo or in /repo/branch1.  Someone pulling from the branch doesn't
> > have to care what the repository structure is.  Having a separate
> > namespace for branch names only really makes sense if the user needs
> > to care about it.
> >
> > As for hierarchical names, there is nothing stopping you from using
> > deaper directory structures with Bazaar too.  Bazaar just checks each
> > successive parent directory til it finds a repository for the branch.
> >
> >> The idea of "branches (and tags) as directories" was if I understand
> >> it correctly introduced by Subversion, and from what can be seen from
> >> troubles with git-svn (stemming from the fact that division between
> >> project name and branch name is the matter of _convention_) at least
> >> slightly brain-damaged.
> >
> > I think you are a bit confused about how Bazaar works here.  A Bazaar
> > repository is a store of trees and revision metadata.  A Bazaar branch
> > is just a pointer to a head revision in the repository.  As you can
> > probably guess, the data for the branch is a lot smaller than the data
> > for the repository.
> >
> > You can store the repository and branch in the same directory to get a
> > standalone branch.  The layout I described above has a repository in a
> > parent directory, shared by multiple branches.
> >
> > If you are comparing Subversion and Bazaar, a Bazaar branch shares
> > more properties with a full Subversion repository rather than a
> > Subversion branch.
>
> Oh, that explained yet another difference between Bazaar-NG (and other
> SCM which uses similar model) and Git.
>
> In Git branch is just a pointer to head (top) commit (hence they are stored
> under .git/refs/heads/) in given line of development. Git also stores
> information (in .git/HEAD) about which branch we are currently on, which
> means on which branch git puts new commits. Nothing more (well, there
> can be log of changes to head in .git/logs/refs/heads/ but that is optional
> and purely local information). In Bazaar-NG you have to store (if I
> understand it correctly) mapping from revnos to revisions.
>
> By default (it means for example default behavior of git-clone, if we don't
> use --bare option) git repository is _embedded_ in working area. We have

Two points:
(1) if we are publishing branches, we wouldn't include working trees
-- they are not needed to pull or merge from such a branch.
(2) if we did have working trees, they'd be rooted at /repo/branch1
and /repo/branch2 -- not at /repo (since /repo is not a branch).

In case (2) there is a potential for conflicts if you nest branches,
but people don't generally trigger this problem with the way they use
Bazaar.

> So repo/branch wouldn't work, because 'branch' would conflict with working
> area files. GIT doesn't follow the CVS model of separate storage area
> (CVSROOT) and having only pointer to said area (files in CVS/
> subdirectories) in working directory.

That is fairly similar to the default mode of operation with Bazaar:
you have a repository, branch and working tree all rooted in the same
directory.  If you have separated working trees and branches, then
that is because you specifically asked for it.


> In GIT to work on some repository you don't (like from what I understand
> in Bazaar-NG) "checkout" some branch (which would automatically copy some
> data in case of "heavy checkout" or just save some pointer to repository
> in "lightweight checkout" case). You clone whole repository; well you can
> select which branches to clone. "Checkout" in GIT terminology means to
> populate working area with given version (and change in repository which
> branch is current, usually).

I think you have a slight misunderstanding of what a Bazaar checkout is.

>
> How checked out working area looks like in Bazaar-NG?

The layout of a standalone branch would be:
  .bzr/repository/ -- storage of trees and metadata
  .bzr/branch/ -- branch metadagta (e.g. pointer to the head revision)
  .bzr/checkout/ -- working tree book-keeping files
  source code

If we use a shared repository, the contained branches would lack the
.bzr/repository/ directory.  The parent directory would instead have a
.bzr/repository/, but usually wouldn't have .bzr/branch/ (unless there
is a branch rooted at the base of the repository).

if we are publishing a branch to a web server, we'd skip the working
tree, so the source code and .bzr/checkout/ directory would be
missing.

In the case of a checkout, the .bzr/branch/ directory has a special
format and acts as a pointer to the original branch.  If the checkout
is lightweight, the .bzr/repository/ directory would be missing, and
bzr would need to contact the original branch for the data.


> >>> For similar reasons, the cost of publishing 20 related Bazaar branches
> >>> on my web server is generally not 20 times the cost of publishing a
> >>> single branch.
> >>>
> >>> I understand that you get similar benefits by a GIT repository with
> >>> multiple head revisions.
> >>
> >> You can get similar benefits by a GIT repository with shared object
> >> database using alternates mechanism. And that is usually preferred
> >> over storing unrelated branches, i.e. branches pointing to disconnected
> >> DAG (separate trees in BK terminology) of revision, if that you mean by
> >> multiple head revisions (because in GIT there is no notion of "mainline"
> >> branch, only of current (HEAD) branch).
> >
> > I may have got the git terminology wrong. I was trying to draw
> > parallels between the .git/refs/... files in a git repository and the
> > way multiple branches can be stored in a Bazaar repository.
>
> Yes, but using Git that way has serious disadvantages. For example
> there is only one current branch pointer and only one index (dircache)
> per git repository.

Okay.  So using Bazaar terminology, this seems to be an issue of the
working tree being associated with the repository rather than the
branch?


[...]
> But I agree that saving "old fork" info as separate branch doesn't lead
> to that much inefficiency as might be thought.
>
> But after saving "old fork" as a branch revno based revision identifiers
> change from http://old.host/old/repo:127 to http://host/repo/old.fork:127
> That is maybe minimal change, but this is change!

Well, a branch can easily have multiple URLs even if there is only one
copy of it.  I might write to it via local file access or sftp (which
would be a file: or sftp: URL).

Mirrors of branches don't usually confuse users (and remember that the
revision numbers are primarily intended for users -- if I am writing a
Bazaar plugin, I'd work in terms of revision IDs).


James.