EDIT1: The example code for reproducing is at https://github.com/ElvinEfendi/envoy-wasm-regional-routing.

I have been experimenting with https://github.com/proxy-wasm/proxy-wasm-rust-sdk recently. I want to write a filter that for a given request, dynamically picks what Envoy cluster the request should be proxied to.

I tried using cluster_header: region routing functionality of Envoy and was hoping I could dynamically change the header value in the plugin. But that did not work as I expected. Turns out self.set_http_request_header(&"region", Some(&"us_central1")); changes/sets the header only for the upstream. Envoy's router does not see the updated value when making routing decision.

Is this an expected behaviour? If so, is there any plan to introduce a dedicated ABI for customizing Envoy's routing decision when it comes to upstream/cluster selection?

FWIW this seems to be possible with a Lua filter: https://medium.com/safetycultureengineering/edge-routing-with-envoy-and-lua-621f3d776c57