Horizon has a config options which says which version of the Keystone
API it should work against: V2 or V3. I am not certain that there is
still any reason for Horizon to go against V2. However, If we defer the
decision to Keystone, we come up against the problem of discovery.
On the surface it is easy, as the Keystone client supports version
discovery. The problem is that discovery must be run for each new
client creation, and Horizon uses a new client per request. That would
mean that every request to Horizon that talks to Keystone would generate
at least one additional request. Is this significant?
It gets a little worse when you start thinking about all of the other
services out there. If each new request that has to talk to multiple
services needs to run discovery, you can image that soon the majority of
network chatter would be discovery based.
It seems to me that Horizon should somehow cache this data, and share it
among clients. Note that I am not talking about user specific data like
the endpoints from the service catalog for a specific project. But the
overall service catalog, as well as the supported versions of the API,
should be cacheable. We can use the standard HTTP cache management API
on the Keystone side to specify how long Horizon can trust the data to
be current.
I think this actually goes for the rest of the endpoints as well: we
want to get to a much smaller service catalog, and we can do that by
making the catalog holds on IDs. The constraints spec for endpoint
binding will be endpoint only anyway, and so having the rest of the
endpoint data cached will be valuable there as well.
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