Load balancers seems to be a natural first class extension point in Envoy, but we don't support this today. As load balancer behaviors become more complicated, e.g. witness issues such as #4685, and the need for custom locality handling in Istio (CC @rshriram @costinm), it would be great to allow for LB extensions and even CDS delivery of LB behaviors via Lua/WASM.

Complications that make this challenging include the tight integration between LB and various ClusterManager data structures such as host, priority and locality data structures. We would neat a tighter, more stable and better defined API here. Also, allowing reuse of existing LBs and just minor customization, without having to reimplement the entire LB, would be useful.

I'm opening this issue for discussion and long-term evolution of the LB implementation, help wanted.

CC @mattklein123 @snowp @cpakulski @rshriram @costinm