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Rate limiting in Azure Function in .NET 7 is achieved using the "Azure App Service" middleware, which provides an easy way to configure and apply rate limits to incoming requests. The middleware intercepts the incoming request, and checks whether the request meets the specified criteria for rate limiting.

To implement rate limiting, you need to first add the "Azure App Service" middleware to your function app. To do this, add the following code in the Startup.cs file of your function app:

using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.FeatureManagement;

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        // Add feature management services
        services.AddFeatureManagement();

        // Add the Azure App Service middleware
        services.AddAzureAppService();
    }

    public void Configure(IApplicationBuilder app, IFeatureManager featureManager)
    {
        // Use the Azure App Service middleware to apply rate limits
        app.UseAzureAppServices();
    }
}

Once you have added the middleware, you can configure the rate limit using the appsettings.json file of your function app. You can specify the maximum number of requests allowed within a given time window, as well as the duration of the time window. For example:

{
  "AzureAppService": {
    "RateLimiting": {
      "MaxRequests": 100,
      "TimeWindow": "00:01:00"
    }
  }
}

In this example, the rate limit is set to 100 requests per minute. If a client exceeds this rate, they will receive a 429 "Too Many Requests" response.

Overall rate limiting is an important component of managing Azure Function applications as it helps you to control resource usage and ensure that your functions remain available and responsive.