On Jul 1, 1:02 am, Chouser <chou...@gmail.com> wrote:
> On Tue, Jun 30, 2009 at 8:37 AM, Rowdy Rednose<rowdy.redn...@gmx.net> wrote:
>
> > Would it be easy to implement an in-memory database in clojure that
> > allows concurrent access?
>
> > It sounds pretty easy, as most of the features are already provided by
> > Clojure. I'm not sure though about the "locking granularity".
> > Of course you don't pessimistically lock in Clojure, but if you have a
> > relation that is "protected" by a Ref, then concurrently adding two
> > rows to that relation will make one of the two transaction replay,
> > although the added rows could be completely independent (unless there
> > are unique constraints, for example, which, now that I think about it,
> > are present in every real-world table in the form of primary keys).
>
> You could wrap a Ref around every value, if you chose, to
> allow independent changes to different "rows" at the same
> time -- though this would not help when inserting rows.
>
> You could partition your key space somehow (perhaps by hash)
> to allow n groups of rows, and protect each of those with
> a Ref. This would allow insertions and deletions
> independently as long as the operations were being applied
> to different groups.
>
> I imagine there are other possible solutions as well...
>
> > Furthermore, if you have constraints that involve other relations, say
> > foreign key constraints, then the locking granularity gets even
> > coarser, spanning all relations involved, and thus affecting every
> > mutating access to any one of those relations.
>
> With either of the layouts I mentioned, this kind of
> multi-relation transaction ought to allow Clojure STM to
> really shine when compared to a more naive locking scheme.
I think the implementation of "ensure" will need to change for this
use case to shine -
http://www.mail-archive.com/clojure@googlegroups.com/msg14915.html
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