[GSoC] migration

[Joshua Root]

On 2017-8-11 02:31 , Umesh Singla wrote:
> 
> 
> On Thu, Aug 10, 2017 at 2:54 AM, Joshua Root <jmr at macports.org 
> <mailto:jmr at macports.org>> wrote:
> 
>     On 2017-8-10 04:59 , Umesh Singla wrote:
> 
>         Hi
> 
>         I was trying to streamline the whole process and I felt the need
>         to have the snapshot as a separate entity just like a reg_entry
>         or a reg_portgroup is, that is, "registry::snapshot" with a
>         bunch of functions like create, get, list_all etc. I think this
>         might help in writing the whole thing from a higher Tcl level.
> 
>         But then again, a snapshot is not something coming directly from
>         the database, so I'm not sure how to keep it. Can I get your
>         view on this?
> 
> 
>     Well, you already know that my view is that it would be far simpler
>     to store the snapshots as a text-based format, rather than write and
>     modify a large amount of non-trivial C.
> 
> 
> I think what Josh means here, is a complete text-based dump of the 
> current state in a file and then using restore, something like what we 
> currently expect our users to do themselves.
> 
> It could be useful in a case when a user wants to completely re-install 
> the OS from scratch and then, a text-formatted file can easily be backed 
> up for use by restore later. We can argue that keeping in registry.db is 
> also similar to keeping in a file, but the interface we are providing a 
> user help migrate are bit different. Because it is not intuitive to take 
> a dump of registry.db manually but running a take_snapshot command is.
> 
>     But if you are set on doing this in the sqlite database, the
>     relational way of doing it would be to add:
> 
>     1. A table of snapshots, consisting minimally of names and ids
>     2. A table associating snapshot ids with port ids on a many-to-many
>     basis
> 
>     Under this model, the set of currently installed ports would be just
>     another snapshot, albeit one with a special meaning. The activation
>     state of each port would have to move from the ports table and
>     become per-snapshot (maybe stored in the second table?).
> 
>     And yes, you would need procedures to manipulate this stuff from Tcl
>     as you suggested above. And the existing code would need to be
>     updated to only remove ports when they are no longer referenced by
>     any snapshot.
> 
> 
> And then again, I am sensing a confusion with the idea of snapshot with 
> Josh, like when he says "remove ports when they are no longer referenced 
> by any snapshot".

What confusion exactly? A snapshot is simply a set of ports (by which I 
mean rows in the 'ports' table, with a unique combination of 
name,version,revision,variants). When nothing references a row any more, 
it needs to be deleted.

- Josh