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Ideally, Yeah. The CEL could be used to evaluate the header and claims directly.
I believe subset lb could archieve your target.
https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/subsets
You need a way to remove the `:8081` from the endpoint list or mark it as unhelthy (outlier detection/health checker, eds, etc.) then the fallback policy will works.
FWIW, now, the multiple endpoints could (and should) be placed in same header/metadata entry in the latest EPP proposal.
> How could a DR provide a custom LB? I am thinking could the DR provide an extendable field with type TypedExtensionConfig in the xds/envoy-api (or similar things), then we...
Okay, although in practices there are too much features that require to use EnvoyFilter.