API Rate Limiting

Introduction

API rate limiting is essential for maintaining the stability and reliability of your API. By limiting the number of requests a user can make within a specified period, you can prevent excessive usage, protect your resources, and ensure fair access for all users.

In this tutorial, we will cover the following topics:

1. Configuring rate limiting in ASP.NET Core using middleware
2. Customizing rate limiting rules
3. Implementing a sliding window rate limiting strategy
4. Handling rate limiting violations

Configuring Rate Limiting in ASP.NET Core using Middleware

To configure rate limiting in ASP.NET Core, we’ll use the popular AspNetCoreRateLimit library. First, install the library using the following command:

dotnet add package AspNetCoreRateLimit

Next, add the necessary configuration settings to your appsettings.json:

{
  "RateLimiting": {
    "EnableEndpointRateLimiting": true,
    "StackBlockedRequests": false,
    "Rules": [
      {
        "Endpoint": "*",
        "Period": "1m",
        "Limit": 10
      }
    ]
  },
  // ...
}

Then, register the rate limiting services and middleware in your Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMemoryCache();
    services.Configure<IpRateLimitOptions>(Configuration.GetSection("RateLimiting"));
    services.AddSingleton<IIpPolicyStore, MemoryCacheIpPolicyStore>();
    services.AddSingleton<IRateLimitCounterStore, MemoryCacheRateLimitCounterStore>();
    services.AddSingleton<IRateLimitConfiguration, RateLimitConfiguration>();
    // ...
}

public void Configure(IApplicationBuilder app)
{
    app.UseIpRateLimiting();
    // ...
}

With this configuration, we’ve set a global rate limit of 10 requests per minute for all endpoints.

Customizing Rate Limiting Rules

You can customize rate limiting rules by modifying the Rules section in the appsettings.json file. For example, you can create separate rules for different endpoints, HTTP methods, or user roles.

Here’s an example of a more complex set of rules:

{
  "RateLimiting": {
    "Rules": [
      {
        "Endpoint": "GET:/api/values",
        "Period": "1m",
        "Limit": 5
      },
      {
        "Endpoint": "POST:/api/values",
        "Period": "1m",
        "Limit": 2
      },
      {
        "Endpoint": "*:/api/admin/*",
        "Period": "1m",
        "Limit": 30
      }
    ]
  },
  // ...
}

In this example, we’ve set specific rate limits for GET and POST requests to the /api/values endpoint, and a higher limit for all endpoints under /api/admin.

Implementing a Sliding Window Rate Limiting Strategy

The default rate limiting strategy used by AspNetCoreRateLimit is the fixed window strategy, which counts requests within fixed periods (e.g., per minute or per hour). This can lead to “bursts” of requests at the beginning or end of each period.

To implement a sliding window strategy, which counts requests within a moving time window, you can use the AspNetCoreRateLimit.SlidingWindow library.

First, install the library using the following command:

dotnet add package AspNetCoreRateLimit.SlidingWindow

Next, update your Startup.cs to use the sliding window rate limiting strategy:

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.AddSingleton<IRateLimitStrategy, SlidingWindowRateLimitStrategy>();
    // ...
}

Now, your application will use the sliding window rate limiting strategy, which provides a smoother distribution of requests over time.

Handling Rate Limiting Violations

When a client exceeds the allowed request rate, the middleware will return a 429 Too Many Requests HTTP response. You can customize the response by adding a custom error handler in your Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.Configure<IpRateLimitOptions>(options =>
    {
        options.QuotaExceededResponse = async (context, retryAfter) =>
        {
            context.Response.StatusCode = (int)HttpStatusCode.TooManyRequests;
            context.Response.Headers["Retry-After"] = retryAfter;

            var message = new
            {
                Error = "Rate limit exceeded",
                RetryAfterSeconds = retryAfter
            };

            var json = JsonSerializer.Serialize(message);
            await context.Response.WriteAsync(json);
        };
    });
    // ...
}

This custom error handler will return a JSON response with a helpful message and the number of seconds the client should wait before making another request.

Conclusion

In this post, we’ve discussed how to implement rate limiting in ASP.NET Core using middleware and custom rules. By applying rate limiting to your API, you can protect your resources and ensure fair access for all users. Remember to customize your rate limiting rules to suit your specific use case and to handle rate limiting violations gracefully.

Further Reading

Here are some resources that may help you dive deeper into rate limiting and troubleshooting issues with your implementation:

1. AspNetCoreRateLimit GitHub repository
2. Official ASP.NET Core Middleware documentation
3. Sliding Window Rate Limiting Algorithm

Feedback and Questions

We’d love to hear your feedback on this tutorial! If you have any questions or suggestions for improvement, please don’t hesitate to reach out. You can leave a comment below, or you can contact us through the following channels:

1. Email: shady@shadynagy.com
2. Twitter: @ShadyNagy_
3. LinkedIn: Shady Nagy
4. GitHub: ShadyNagy

We’ll do our best to address any questions or concerns you may have. We look forward to hearing from you and helping you make the most of Asp Net Core Rate Limit!