Re: Dual cache key calculation and build planning modes

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

On Thu, 2017-07-06 at 07:15 +0100, Paul Sherwood wrote:
> Hi Tristan,
> On 2017-07-05 10:05, Tristan Van Berkom wrote:
> > Premise
> > ~~~~~~~
> > The premise is simply that every element has 2 cache keys generated at
> > all times.
>
> I haven't had chance to follow all of the detail here, but isn't your 
> 'weak' key just a hash of a subset of the same factors in for your 
> 'strong' key?
>
> In which case, would it be simpler and better to have one key which 
> combines both?
>
> foo.<hash(weak-factors)>.<hash(strong-factors - weak-factors)>
>
> Then tooling can decide whether it cares about the whole key or just the 
> 'weak' part?

Interesting idea, but I'm not sure what it would add.

What I think it comes down to, is the ability to perform lookups in an
artifact cache for one or the other.

We would want to be able to:

  o Lookup the artifact for a given 'weak' key
  o Lookup the artifact for a given 'strong' key
  o Given an artifact, be able to know what is the weak
    and strong key (we can do this part anyway with the
    metadata encoded in the artifact).

So I think, it's pretty arbitrary whether we do a <weak>.<strong>
notation for the strong key; in any case we'll want to revision both
the '<weak>' and the '<weak>.<strong>' keys for lookup purposes.

Cheers,
    -Tristan