Constant MIDDLEWARE_HANDLER_INTERFACE 

pub const MIDDLEWARE_HANDLER_INTERFACE: &str = "wasi:http/handler";

The interface an HTTP middleware component both imports (to forward a request to the next component in the pipeline) and exports (to receive a request from the previous one).

Any semver-compatible 0.3.x version of this interface is accepted, and a component’s import and export may use different 0.3.x versions.