Rate Limiting configuration - policy validation

Background and Motivation

The ASP.NET Core rate limiting middleware is great, but “limited” in terms of policy validation. Let’s start with some code that you can write today in .NET 7:

builder.Services.AddRateLimiter(options =>
{
    options.AddFixedWindowLimiter("customPolicy", opt =>
    {
        opt.PermitLimit = 4;
        opt.Window = TimeSpan.FromSeconds(12);
        opt.QueueProcessingOrder = QueueProcessingOrder.OldestFirst;
        opt.QueueLimit = 2;
    });
    // ...
});

There is no way to validate that customPolicy actually exists. This is useful when configuring multiple routes from configuration such as is the case for YARP. See https://github.com/microsoft/reverse-proxy/pull/1967

Proposed API

It would be preferred to something similar to IAuthorizationPolicyProvider implemented via DefaultAuthorizationPolicyProvider and ICorsPolicyProvider implemented via DefaultCorsPolicyProvider

namespace Microsoft.AspNetCore.RateLimiting;

-  internal struct DefaultKeyType 
+  public struct DefaultKeyType 
{
// omitted ...
}
+
+ public interface IRateLimiterPolicyProvider
+ {
+     ValueTask<IRateLimiterPolicy<DefaultKeyType>?> GetDefaultPolicyAsync();
+     ValueTask<IRateLimiterPolicy<DefaultKeyType>?> GetPolicyAsync(string policyName);
+ }
+
+ public class DefaultRateLimiterPolicyProvider : IRateLimiterPolicyProvider
+ {
+     private readonly RateLimiterOptions _options;
+     
+     public DefaultRateLimiterPolicyProvider(IOptions<RateLimiterOptions> options)
+     {
+     
+     }
+     
+     public ValueTask<IRateLimiterPolicy<DefaultKeyType>?> GetPolicyAsync(string policyName)
+     {
+         options.PolicyMap[policyName] ?? options.UnactivatedPolicyMap[policyName];
+     }
+ }

RateLimiterOptions.PolicyMap is internal hence this feature cannot be added in another library or the final application.

Usage Examples

See YARP: https://github.com/microsoft/reverse-proxy/blob/26ce1d15f868cb8da1891d65db1e59a20fd6ecbf/src/ReverseProxy/Configuration/ConfigValidator.cs#L312-L318

Alternative Designs

None

Risks

None

[adityamandaleeka]

This proposal will be discussed by our team in an upcoming API review meeting, after which we’ll provide feedback/suggestions.

Once a proposal gets to the api-approved state, we’ll be ready to take a PR to implement the change.

[mburumaxwell]

@adityamandaleeka this is done