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I think option e is our best bet. It's the *only* option if we want to efficiently separate our services (as Jason has pointed out).

Waldon


On Nov 29, 2011, at 10:52 AM, Devin Carlen wrote:

> Hey Soren,
> 
> On Nov 29, 2011, at 7:20 AM, Soren Hansen wrote:
> 
>> SQLAlchemy does support looking these things up on the fly. In fact,
>> AFAIK, this is its default behaviour. We just override it with
>> joinedload options, because we don't use scoped sessions.
>> 
>> My fake db driver looks stuff like this up on the fly (so the
>> assertEquals after the virtual_interface_create will fail with that db
>> driver).
>> 
>> So my question is this: Should this be
>> 
>> a) looked up on the fly,
>> b) looked up on first key access and then cached,
>> c) looked up when the parent object is loaded and then never again,
>> d) or up to the driver author?
>> 
>> Or should we do away with this stuff altogether? I.e. no more looking up
>> related objects by way of __getitem__ lookups, and instead only allow
>> lookups through db methods. So, instead of
>> network['virtual_interfaces'], you'd always do
>> db.virtual_interfaces_get_by_network(ctxt, network['id']).  Let's call
>> this option e).
> 
> I think a simpler expectation of what the data objects should be capable of enables a much wider variety of possible implementations.
> 
> The main advantage to option e) is that it is simple both from an implementation and from a debugging point of view.  You treat the entire db layer as though it's just dumb dictionaries and then you enable a wider support of implementation.  Sure sqlalchemy supports lookups on __get__item, but maybe other potential implementations won't.
> 
>> I'm pretty undecided myself. If we go with option e) it becomes clear to
>> consumers of the DB api when they're pulling out fresh stuff from the DB
>> and when they're reusing potentially old results.  Explicit is better
>> than implicit, but it'll take quite a bit of work to change this.
> 
> Well, this is the way nova *used* to work.  I'm not exactly sure when and if that changed.
> 
>> 
>> If we go with one of options a) through d), my order of preference is
>> (from most to least preferred): a), d), c), b).
>> 
>> Option e) is also easy to explain and do reviews for, btw.
>> 
>> It seems I've talked myself into preferring option e). It's too much
>> work to do on my own, though, and it's going to be disruptive, so we
>> need to do it real soon. I think it'll be worth it, though.
>> 
>> -- 
>> Soren Hansen        | http://linux2go.dk/
>> Ubuntu Developer    | http://www.ubuntu.com/
>> OpenStack Developer | http://www.openstack.org/
>> 
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