[GSoC] migration

[Umesh Singla]

On Thu, Aug 10, 2017 at 2:54 AM, Joshua Root <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".

- Umesh
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