materializing a MaCommitLogRecord

[Chris Muller]

Hi Dave, thanks for the great questions.

On Fri, Jun 11, 2010 at 1:12 PM, Dave Woodward
<davewoodward at promedmedical.net> wrote:
> I'm wondering if it is possible to directly materialize the objects in a
> MaCommitLogRecord instance.

You can materialize the CommitPackage which was applied for that
commit.  The CommitPackage contains the serialized first-class Buffers
which could be used to materialize the domain objects.

So, the domain objects are three-materializations removed from
existence.  These levels of abstraction are necessary for Magma to
handle HA functionality generically; i.e., it never has to deal with
specific classes of domain objects.

> It seems that the commit log records are used exclusively for "advancing"
> the repository.

That is their primary purpose, yes.

>  However I'm wondering if I can use the commit log as a sort
> of poor-man's audit log and if I'd be able to manually extract objects from
> specific commit log records.

Interesting.  Now, you probably noticed already that you have
easy-access to a Magma-provided timestamp and commitNumber for each
record.  Those would seem to be useful for an "audit log".

As far as getting at the objects, it is not difficult.  You would just
need to open the repository to establish the proper class-map, and
then use that serializer to materialize the "objects" instVar of the
MaCommitPackage contained within the MaCommitLogRecord.

>  Right now when I try to materialize a record
> in Pharo 1.0, I get an error because the serializer tries to call basicNew:
> on the MagmaLocalLocation class.
> I'm not sure if this is even a good path to take, so fell free to tell me
> I'm crazy or that I shouldn't go there :)

Not sure..  :)  Why are you inclined to do this?

> I'm also wondering if the commits could be "rolled back" so as to be able to
> rewind the database back to a specific commit?  I see how one can restore up
> to a certain point in time, but I'm just wondering if there is any more
> specific way to "un-apply" the commit records.

Only a partial commit can be rolled back, the last one, and Magma does
it automatically only if necessary.

Beyond that, Magma only supports rolling forward to a point in time.
Domain objects that are updated in the client are replaced "in-place"
on the hard-disk by the server.

Even if you never made a fullBackup, but you have all of the commit
log files from the beginning of time, you can apply them to the "seed"
repository that was automatically created and resides inside the magma
directory.  It is just one of several things Magma does to go out of
its way to protect the users persistent model (another is the SHA
verification of every single commit-record ever applied to any db).

Regards,
  Chris


>
> --
> Dave Woodward
>
> _______________________________________________
> Magma mailing list
> Magma at lists.squeakfoundation.org
> http://lists.squeakfoundation.org/mailman/listinfo/magma
>