VCS comparison table

[Linus Torvalds]

On Wed, 18 Oct 2006, Carl Worth wrote:
> 
> I understand that bzr also has unique identifiers, but it sounds like
> the tools try to hide them, and people aren't in the habit of using
> them for things like this. Do bzr developers put revision numbers in
> their bug trackers? Is there a guarantee they will always be valid?

bzr seems to use the classic UUID format, and it's funny how much it looks 
like a real BK ChangeSet revision number ("key").

Here's the quoted bzr "true" revision ID:

	Matthieu.Moy at imag.fr-20061017152029-4c5a2861bcf23b7d

and here's a BK "ChangeSet Key":

	adi at zaphod.bitmover.com|ChangeSet|20031031183805|57296

(I don't have BK installed anywhere, so I had to google for changeset 
keys, and this was just some random key in the BK bugzilla ;)

Looks very similar, don't they? And yes, the true revision ID is stable 
over time (at least it was in BK, and I assume it is in bzr too).

The biggest difference seems to be that in bzr, the final checksum is 
64-bit, while for BK, it was just a 16-bit checksum/unique number (the 
rest is just user-name/machine-name and date: I assume that the bzr commit 
was done at 10/17/2006 3:20:29PM, and the example BK ChangeSet was created 
10/31/2003 6:38:50PM - it looks like _exactly_ the same date format).

With BK, you can also use a "md5 key", and I don't actually know how they 
work. They may just be the md5 hash of the ChangeSet key, I think that may 
be how those things are indexed. So in bkcvs, you'll see a line like this:

	BKrev: 42516681VmgTWL0bkLcltPGiI6Yk5Q

which is the BK md5 key for my last kernel revision in BK (2.6.12-rc2). 
Again, these numbers are stable, unlike the simple revisions.

Note that from a usability standpoint, the UUID's look more readable to a 
human, but are actually much worse than the md5 keys (or the SHA1's that 
git uses). At least with a hash, the first few digits are likely to be 
unique, so you can do things like auto-completion (or just short names). 
With the email+date+random number kind of UUID, you don't have that.

(Pure hashes obviously also tend to just all have the same length, and are 
easier to parse automatically, so from a programmatic standpoint they are 
a lot easier too - but the surprising thing is how they are actually 
easier on humans too, even if the UUID's look more readable).

			Linus