Conditional Request Forwarding

[noh626]

Hi,

I’m currently exploring ways to selectively forward requests either to local controllers or downstream services based on a custom flag. While utilizing the Order property seems to be the most straightforward approach for routes with the same pattern, it only works effectively when the route configuration in YARP and the route for the local controller are identically specific, as the most specific match wins.

Here’s the scenario I’m dealing with:

• I have a local endpoint defined for "controller/action1".

• YARP is configured to proxy "controller/{**remainder}" to a cluster.

With this setup, a request matching "controller/action1" naturally hits the local endpoint. However, I want to have the flexibility to selectively proxy this request based on a custom flag, even when it matches the local route.

Is there a way I can manually control whether a request gets proxied or not within a middleware component? Alternatively, if there’s another approach or strategy that could achieve this selective forwarding, I’m open to suggestions and would appreciate any insights.

[Tratcher]

Yes, in the middleware you can check the HttpContext to see if you want to forward. If so, you call IHttpForwarder, else call the next middleware.

This will show you how to get and call the IHttpForwarder.
https://microsoft.github.io/reverse-proxy/articles/direct-forwarding.html

[noh626]

I understand the approach you've outlined. However, I'd like to know if there's a way to still take advantage of YARP's routing and transformation configuration within my custom middleware. My intention is to potentially use the metadata property in YARP's configuration to inform the logic of my middleware. This way, I could manage selective forwarding while also making use of the sophisticated routing and request transformation setup that YARP provides. How might I go about accessing and utilizing YARP's configured metadata for this purpose?

[noh626]

I've made an interesting observation regarding the Order field in YARP's configuration that seems to contradict my understanding.

Initially, I believed that the Order field would only influence the routing when that path specified in YARP's RouteConfig had the same level of specificity as a local route. However, during my testing with the following setup:

• YARP's RouteConfig:
  "Route1": { "Order": -1, "Match": { "Path": "abc/123/{**remainder}" } }
• Local action route: abc/123/action/{id}

I expected that a request matching abc/123/action/{id} would always be handled by the local action, regardless of the Order value in RouteConfig. Contrary to this, Route1 with Order set to -1 took precedence over the local route.

The reason I wanted to use the Metadata property in YARP's configuration was under the assumption that the Order property did not affect routes with varying levels of path specification.

Did I misunderstand the role of Order in YARP's routing?

All I wanted are to leverage YARP's cofig model and selectively route traffic to either local controllers or remote APIs. I didn't want to enumerate every single endpoint in YARP's config using the Order property. I assumed that Order wouldn't apply across routes of different path specificity which led me to consider the Metadata property as a potential solution.

[Tratcher]

Right, Order is actually checked first, then the path. Note this is not YARP specific, YARP is just plugging into AspNetCore's routing.
https://learn.microsoft.com/en-us/aspnet/core/fundamentals/routing?view=aspnetcore-7.0#url-matching

That's also why it's hard to do dynamically choose between routes, that's not a feature supported by AspNetCore. There are some advanced route constraint extensions, but they're not easy to use. That's why I suggested middleware instead.

[noh626]

I thought ASP.NET Core routing mechanism doesn't check the Order before the route template specificity and the default behavior is for the routing to match the most specific route template first.

However, it appears to function in the reverse order, with Order taking precedence.

This actually simplifies my scenario. By dynamically adjusting the Order in YARP's config as needed, I can manage the routing to prefer local endpoints or proxy forwarding accordingly. For example, if a specific endpoint requires a unique forward flag, I could introduce a dedicated route config with an Order that takes precedence over a more generic route configuration and YARP's config can be updated during the runtime.

Do you think this is a viable solution?

[Tratcher]

If that's dynamic enough for you, great.