Geo DNS based load balancing policy

[maleck13]

What

We currently only support Round Robin / Even weighted load balancing. We want to support GEO based load balancing. The idea here is that the DNS for a given listener will be setup based on geo metadata added to the managedcluster resource.

Initially we will focus on supporting continents but we should also consider what it means to support more fine grained options such as countries.

Example EU (Europe) NA (North America)

We will also need to support defining a default for when a request falls outside of a defined geo region

Use Cases

As a gateway administrator setting up multiple instances of a gateway that are deployed to clusters in different geographic regions, I want to be able to specify that traffic coming into these gateways should be load balanced based on where the request originated. Additionally I want to be able to specify  default / fall back location to cover the case where a request falls outside a specified geo location

Proposal

Only a single DNSPolicy can be applied and we should use the creation timestamp as the decider with the oldest winning
https://gateway-api.sigs.k8s.io/references/policy-attachment/#conflict-resolution

This work should build on top of what was done with GLBC in the past to support this. Work originally done by @MNairn

One option is to support a set of attributes on the ManagedCluster resource

kuadrant.io/attribute-geo-contienent: EU

kind: DNSPolicy
spec:
  targetRef:
     ....  Gateway
   routing:
     strategy: GEO # default even weighting
     default: EU

Add a reference to a DNSPolicy to the DNSRecord resource and DNSRecord reconciler follows that to figure out how to create the records. This reference should be a part of the status or an ownerref

using an owner ref may allow us to get events via the following mechanism

Watches(&source.Kind{Type: &v1alpha1.DNSRecord{}}, &handler.EnqueueRequestForOwner{
			IsController: false,
			OwnerType:    &v1alpha1.DNSPolicy{},
		}). 

• Unit / integration tests

This implementation is for AWS only currently and expand based on the below investigation with follow up issues

• Investigate geo and weighted routing policies on azure and google #220
• Move all DNS related responsibilities to DNSPolicy reconciler #221

[maleck13]

In a future iteration, we may also look to support "overriding" the gateway policy with a policy that directly targets a DNSRecord resource

[maleck13]

https://github.com/kcp-dev/kcp-glbc/pull/504

[mikenairn]

Dropped this out of the iteration for now since we really want the policy stuff to land before taking this on. #221