openstack team mailing list archive

[Jay Pipes <jaypipes@xxxxxxxxx>]

On Tue, Nov 29, 2011 at 2:58 PM, Soren Hansen <soren@xxxxxxxxxxx> wrote:
> 2011/11/29 Jay Pipes <jaypipes@xxxxxxxxx>:
>> There's a very good reason this hasn't happened so far: handling
>> highly relational datasets with a non-relational data store is a bad
>> idea. In fact, I seem to remember that is exactly how Nova's data
>> store started out life (*cough* Redis *cough*)
>
> To be fair, we're only barely making use of this in our DB
> implementation. I don't think we do any foreign key checking at all,
> and deletes (because we don't actually delete anything, we just mark
> it as deleted) don't cascade, so there are all sort of ways in which
> our data store could be inconsistent.

Because the database schema isn't properly protecting against
referential integrity failures does not mean the relational database
store is a failure itself.

> Besides, we don't really use transactions. I could easily read the
> same data from two separate nodes, make different (irreconcilable)
> changes on both nodes, and write them back, and the last one to write
> simply wins.

Sure, but using a KV store doesn't solve this problem...

> In short, it seems to me we're not really getting much out of having a
> relational data store?

We're getting out of it what we ask of it. We aren't using scoped
sessions properly, aren't using transactions properly, and we aren't
enforcing referential integrity. But those are choices we've made, not
some native deficiency in relational data stores.

As soon as someone can demonstrate the performance, scalability, and
robustness advantages of rewriting the data layer to use a
non-relational data store, I'm all ears. Until that point, I remain
unconvinced that the relational database is the source of major
bottlenecks.

Cheers,
-jay