Async-Await in rest api scalability behavior

Question

I have following request pipeline

Here,
A - Is a user who is accessing a MVC web application and clicks a button
B - Is a MVC controller (without any async/await) and makes a call to a GetDataAsync method of a rest api(C)
C - Is a rest api with async method named GetDataAsync
D - Is a database method call using entity framework and database calling method is async type.

With reference to the explanation on this link - https://learn.microsoft.com/en-us/archive/msdn-magazine/2014/october/async-programming-introduction-to-async-await-on-asp-net, when the threads are moved back to ThreadPool when the call is made for any I/O based operation, my questions are

1. Is it the call at the point D in above pipeline which is resulting in thread being returned to the ThreadPool?
   
2. If yes, it means the scalability of the rest-api is increased by making an async call to a I/O based operation, which in this case is database call i.e. D ?
   
3. When the MVC controller makes an async call at point B, it is NOT contributing to scalability/performance of rest api, even if it is calling the async method of the api?

Answer 1

Assuming that your rest API is running as a separate app, something like this happens:

public class YourController : ControllerBase
{
    private readonly ISomeRepo _someRepo;

    public YourController(ISomeRepo someRepo)
    {
        _someRepo = someRepo;
    }

    [HttpGet("ExampleUrl")]
    public async ... GetDataAsync()
    {
       var data = await _someRepo.GetDataFromDbAsync(); // this is where async starts and thread may be returned to pool

       ...
    }
}

So I believe the answer to your first question should be: C

Question #2: Quote from link below, "For server applications, the primary benefit of async is scalability."

Question #3: If there is both an async and a sync version of same method in rest API, use of async method will improve scalability for rest API app. How client app makes the call, true async or with .GetAwaiter().GetResult(), does not matter for rest API app.

It is all about using a limited number of threads the best way.

There is a lot of good reading to choose from. Maybe start with this one: https://learn.microsoft.com/en-us/archive/msdn-magazine/2014/october/async-programming-introduction-to-async-await-on-asp-net

We are playing a little with parallel processing using async code in this question: https://stackoverflow.com/a/63692953/14072498