[okfn-help] vdm: primary keys question

Fri Aug 20 19:15:05 BST 2010

On 19 August 2010 16:18, Martijn Faassen <faassen at startifact.com> wrote:
[...]
> Alternatively:
>
> * the user makes 3 changes
>
> * the user wants to revert one of them
>
> The way I have this in the UI of an app is that you can revert a
> particular operation on a particular record. This fits vdm less well,
> as vdm works on whole revisions. I guess with vdm you could model this

You could just have separate revisions for each of these changes and
then it would be fine. vdm nests the one-revision-per-record model as
one special case -- you can do it in vdm but you can also do multiple
object changes (e.g. altering a package and adding new tags to it in
one revision)

> as a more general undo facility. But I imagine you can't undo a
> revision that has later revisions stacked on top of it, correct?

Yes you can undo (purge) a revision at any time but it won't have any
effect if further revisions to the same object have happened since
then since we do copy on write. (In any case those kind of reverts are
*very* hard to do in a coherent, sensible, way).

> I'm going to actively play with vdm in a project to see how far I can
> get it, and I'll keep an eye out for documentation issues.

Great.

Rufus
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