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Excerpts from David Stanek's message of 2015-03-08 11:18:05 -0700:
> On Sun, Mar 8, 2015 at 1:37 PM, Mike Bayer <mbayer at redhat.com> wrote:
> 
> > can you elaborate on your reasoning that FK constraints should be used less
> > overall?  or do you just mean that the client side should be mirroring the
> > same
> > rules that would be enforced by the FKs?
> >
> 
> I don't think he means that we will use them less.  Our SQL backends are
> full of them.  What Keystone can't do is rely on them because not all
> implementations of our backends support FKs.
> 

Note that they're also a huge waste of SQL performance. It's _far_ cheaper
to scale out application servers and garbage-collect using background jobs
like pt-archiver than it will ever be to scale out a consistent data-store
and do every single little bit of house keeping in real time.  So even
on SQL backends, I'd recommend just disabling and dropping FK constraints
if you expect any more than the bare minimum usage of Keystone.