Post-Transformed Path from Middleware

[pinkfloydx33]

How would you get the proxied/post-transformed path (not necessarily the full URL) from within custom middleware?

For example, suppose the following configuration:

{
	"Routes": {
		"my-route": {
			"ClusterId": "my-cluster",
			"Match": {
				"Path": "/path/{**catch-all}"
			},
			"Transforms": [
				{
					"PathRemovePrefix": "/path"
				},
				{
					"PathPrefix": "/api"
				}
			]
		}
	},
	"Clusters": {
		"my-cluster": {
			"Destinations": {
				"destination": {
					"Address": "http://my.backend.svc/"
				}
			}
		}
	}
}

An incoming path of the form /path/something will be translated to /api/something after the transforms are applied. With a final destination URL of http://my.backend.svc/api/something. From within middleware, I can inspect the Request.Path to get the original path (ie. /path/something), but what would I do to get the post-transformed path, /api/something (with or without the full destination host):

app.UseEndpoints(endpoints => 
{
   endpoints.MapReverseProxy(proxy => 
   {
      proxy.Use(async (ctx, next) => 
      {
         // How to get the transformed path here?
         var tranformedPath = ...;
         await next();
      });
      proxy.UseSessionAffinity();
      proxy.UseLoadBalancing();
      proxy.UsePassiveHealthChecks();
   });
});

[Tratcher]

Asp.Net middleware don't have access to that, the transformed request path only exists on the outbound HttpRequestMessage. You can capture it in a custom transform or in a DelegatingHandler

[Tratcher]

What did you plan on doing with the value?

[pinkfloydx33]

I'm not sure what I was thinking at the time. I guess I don't really need it. I ended up getting what I did need with custom endpoint Metadata and didn't need the path at all.

Though I suppose one might still want to know the actual post-transformed path for logging/debug purposes, especially if using a transform such as PathPattern whose results may not be obvious.

Purely educational at this point--If it were part of a transform, I suppose it would need to be the final one and you'd just grab it from RequestTransformContext.Path?

[Tratcher]

Purely educational at this point--If it were part of a transform, I suppose it would need to be the final one and you'd just grab it from RequestTransformContext.Path?

Right, though I think YARP already logs it.
https://github.com/microsoft/reverse-proxy/blob/a323abe22131818853ae523420d5aa72bf9d9d93/src/ReverseProxy/Forwarder/HttpForwarder.cs#L302-L304