constraints

[Gustavo Niemeyer]

On Wed, Feb 6, 2013 at 5:29 PM, roger peppe <rogpeppe at gmail.com> wrote:
> There seems to be a desire to be able to specify constraints both in a
> portable way (for example by specifying a required amount of RAM), and
> in a provider-specific way (for example by specifying an instance type).
>
> Part of the complexity of getting this to work well comes, I think,
> from the fact that we're trying to allow both kinds of constraint in
> the same constraint set.  That means that we can get arbitrary semantic
> overlap between provider-specific constraints and portable constraints -
> the constraint attributes are not orthogonal.
>
> I *think* that by explicitly separating portable constraints from
> provider-specific constraints, the implementation can become simpler
(...)
> The combination rules are as follows:

That sounds pretty involved, and a bit problematic.
instance-type=m1.whatever is an EC2 setting. What happens when we want
to specify instance types for both EC2 and Google Compute?

I suggest something more trivial:

- Introduce an ec2-type constraint
- Non-EC2 providers ignore it
- The EC2 provider handles it in whatever way it pleases
- Allow mixing of constraints. If ec2-type is seen, the EC2 provider
will respect it, as documented.

That seems easy to implement, and to understand, and doesn't suffer
from the problem mentioned.


gustavo @ http://niemeyer.net