What constitutes the "identity" of a changeset?

[Paul Moore]

On 29/03/2008, Stephen J. Turnbull <turnbull at sk.tsukuba.ac.jp> wrote:
> James Westby writes:
>
>   > However, you are neglecting the caveat I put in to try and make the
>   > discussion simpler, "assuming  certain rules along the way",
>
> Of course I neglected it, since I had no idea what rules you were
>  talking about.
>
>  This is a serious problem for people new to bzr.

This was my point, as well. There are certain rules which make it
possible to reason about revisions, and they aren't clearly states
anywhere. Indeed, from the discussions it seems that the rules can
differ between repositories/plugins/??? (bzr-svn seems to try to
respect identity based on svn repository UUID, whereas there is no
equivalent for Bazaar native repositories)

Saying that revision identity is based on revision ID, and that is
arbitrary, avoids the issue. It just means that the rules by which
revision ID is calculated need specifying. Aaron did this earlier,
presumably only for Bazaar core and not for bzr-svn. In that case IDs
are random, and so *are* unique. So the only revision ID preserving
operation is a copy. Which then leaves the question of what operations
copy, and what don't. For example, diff/patch clearly isn't revision
ID preserving, and bzr clone must be (otherwise what's the point?).
But what about greyer areas like creating a merge directive and
merging it? Upgrading a repository? The things loom does with
up-thread etc? I can assume, but I don't *know*.

Arguably, this is simply a case of documentation not being written yet
- internals documentation is lower priority than user documentation,
certainly. And "use the source" is always available as an answer. But
these sort of things arise fairly often, and affect people's mental
model, so I believe that more effort needs to be made to clarify them.
The biggest issue with the newer crop of DVCS is that they all *look*
pretty similar, but there are large but subtle differences in the
concepts involved, and it's easy to get confused when moving from one
to another, and transferring incorrect concepts across.

Paul