Is there any difference using Asyn and Sync operation in Asp.net MVC (Web Development)

Question

In HTTP protocol, client anyway has to wait till server process the request and return HTML/JSON. What is significant/different between Async and sync operation? I mean there is no GUI which may freeze if sync operation used.

Async method

[HttpGet]
[Route("{id}")]
public async Task<ActionResult<Employee>> GetByID(int id)
{
    return await _employeeRepo.GetAsyncByID(id);
}

Sync method

[HttpGet]
[Route("{id}")]
public ActionResult<Employee> GetByID(int id)
{
    return _employeeRepo.GetByID(id);
}

Async will not freeze the main thread but then how does it make any difference when there is no UI

Answer 1

As you said, the choice to use async Tasks for your controller methods has nothing to do with the client, which will still wait for the HTTP response in the same way. The use of async controller methods allows the server to more easily use async methods internally to manage its performance.

In your first example, assuming your _emplyeeRepo uses async properly, the request thread is free while the DB access layer waits for the DB call to return. This means there's a thread free for other requests while waiting for the DB.

In your second example, though, the thread that serves the HTTP request is blocked until the DB returns.

Answer 2

If you blindly use async method the performance will be poor than a sync method. The reason is a async method is a parallel operation hence take extra time to create the parallel operation and time slice allocated to it by operating system.

Lots of developer blindly use async method while calling a web method or https request. But when exactly you need to implement async method? Suppose you are calling more than one APIs or web service which take significant amount of time to complete. In such a situation to save the amount of time you need to declare all these kind of methods as async so that all of them executes parallel and you can save time.

One clear example will be, suppose you have to call file RESTful services (apis) and each call takes 1 second. Hence a set of 5 sync calls will take exactly 5 seconds. But if you make them all async methods it will take 1 seconds and few milliseconds to creates the thread for each methods and time slice allocation. Hence all of them will be completed in 1 second and few milliseconds. This parallel execution also depends on number of processor and size of memory in you system as well.