Application load balancers only see connections that already arrived. DNS is the first chance to send 10% of answers to a new pool. It will not be perfectly even. It will be good enough to canary a region.
Coarse on purpose
Resolvers cache and represent many users. Use weights for region splits and canaries, not per-request fairness. Inside a pool, keep an application load balancer.
Start conservative
Publish the new pool at a low weight with health checks attached. Watch origin and error metrics, then raise the weight. A one-second TTL makes that raise real.
Subnet splits
When you know resolver space, subnet load balancing is finer than continent maps. Use it sparingly; it is easy to overfit.
CDN in the mix
DNS can steer to more than one CDN or origin hostname. Cloud CDN origin groups handle delivery-side failover after the client has arrived.