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XUnit - Simulate concurrency and load against an API endpoint

Today I was working on an ASP.NET Core Web API that implements rate limiting. The ASP.NET Core Web API should still accept the incoming requests but the handling of the requests should be buffered when the rate limits where exceeded. Maybe I’ll share how I implemented this in another blog post, but in this post I want to focus on how I tested this functionality.

My first idea was to use Parallel.For() to generate a lot of concurrent requests. You would think it would generate a number of request using the specified or calculated degree of parallelism.  Unfortunately that is not the case. Instead, all the tasks are started at the same time.

In the example above all 10 tasks are started at the same time.

Another issue is that Parallel.For() doesn’t wait for the started tasks to finish which would result in the completion of the test without waiting for the results.

So we need to find another way to achieve our goal. I got what I want by using a combination of Task.WhenAll() and SemaphoreSlim to limit the concurrency.

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