Multi-CDN steering and failover

Design the multi-CDN steering and failover system for a streaming service that delivers VOD + live across three commercial CDNs plus its own edge. You must route each viewer's segment requests to the CDN that's currently fastest/cheapest for them, detect a CDN degrading (a region browning out, error rate climbing) within seconds, and shift traffic away, all without breaking in-flight playback sessions or stampeding a healthy CDN. Scale: tens of millions of concurrent streams, a single live event can spike one region 10x.