create a asp.net rate limiter at 20 requests per second in csharp

To create a rate limiter in ASP.NET, we can use middleware. Middleware is a way to inject behavior into the HTTP request/response pipeline in ASP.NET. We will be using a middleware class to implement the rate limiter. Here's the code to implement a rate limiter at 20 requests per second:


main.cs
using System;
using System.Collections.Concurrent;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;

public class RateLimitMiddleware
{
    private readonly RequestDelegate next;
    private readonly ConcurrentDictionary<string, DateTime> requests = new ConcurrentDictionary<string, DateTime>();
    private readonly int requestLimit = 20; // maximum number of requests per second.
    private readonly int requestLimitPeriodInSeconds = 1; // the window of time in which the limit applies.

    public RateLimitMiddleware(RequestDelegate next)
    {
        this.next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        var ipAddress = context.Connection.RemoteIpAddress.ToString();
        var currentTime = DateTime.UtcNow;

        if (requests.TryGetValue(ipAddress, out var lastRequestTime))
        {
            var timeElapsed = currentTime - lastRequestTime;
            var timeUntilReset = TimeSpan.FromSeconds(requestLimitPeriodInSeconds) - timeElapsed;
            if (timeUntilReset > TimeSpan.Zero)
            {
                // We have exceeded the request limit, so return a 429 error.
                context.Response.Headers.Add("Retry-After", timeUntilReset.ToString());
                context.Response.StatusCode = 429;
                return;
            }
        }

        requests.AddOrUpdate(ipAddress, currentTime, (key, existingVal) => currentTime);

        await next(context);
    }
}
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We define a middleware class called RateLimitMiddleware, which takes a RequestDelegate as a constructor parameter. We use a ConcurrentDictionary<string, DateTime> to keep track of the last request time for each IP address. In the Invoke method, we retrieve the IP address of the current request, and check if the ConcurrentDictionary contains a previous request time for that IP address. If it does, we compute how much time has elapsed since the last request, and check if it is less than the requestLimitPeriodInSeconds interval. If it is less, we return a 429 error. If it is greater, we update the last request time for the IP address in the dictionary and call the next middleware in the pipeline.

To use the middleware, we need to add it to the HTTP pipeline using the UseMiddleware extension method. We can do this in the Configure method of the Startup class:


main.cs
public void Configure(IApplicationBuilder app)
{
    app.UseMiddleware<RateLimitMiddleware>();
    // Other middleware goes here.
}
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This will apply the rate limiter to all requests that pass through the middleware pipeline.

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