c# - Creating an async webservice method

Question

I've tried to read up on async methods and am now trying to create my own async method. The method is a webservice call that returns a list of error logs. I'm not sure that I've understood correctly so I thought I'd share my code to see if I should do anything different.

All I want the code to do is return a list of errorlogs by calling a method GetAllErrorLogs(), that is a synchronized method. Since it can take a second to fetch all the error logs I want to have the opportunity to do other stuff once I called the GetAllErrorLogs() method. Here is the code.

[WebMethod]
public async Task<List<ErrorLog>> GetAllErrorLogs()
{
    List<ErrorLog> errorLogs = new List<ErrorLog>();

    await System.Threading.Tasks.Task.Run(() => {
        errorLogs = ErrorLogRepository.GetAllErrorLogs();
    });


    if (errorLogs == null)
        return new List<ErrorLog>();

    return errorLogs;
}

Thanks!

Answer 1

I recently gave a talk at ThatConference on async on the server side, and I address this issue in the slides.

On the server side, you want to avoid the use of Task.Run and other constructs that queue work to the thread pool. As much as possible, keep thread pool threads available for handling requests.

So, ideally your repository would have an asynchronous method GetAllErrorLogsAsync, which would itself be asynchronous. If GetAllErrorLogs cannot be asynchronous, then you may as well just call it directly (removing the await Task.Run).

Since it can take a second to fetch all the error logs I want to have the opportunity to do other stuff once I called the GetAllErrorLogs() method.

If you have a GetAllErrorLogsAsync available, then this can easily be done using Task.WhenAll. However, if GetAllErrorLogs is synchronous, then you can only do this by doing parallel work in your request (e.g., multiple calls to Task.Run followed by Task.WhenAll).

Parallel code on the server must be approached with great trepidation. It is only acceptable in a very limited set of scenarios. The entire point of async on the server side is to use fewer threads per request, and when you start parallelizing, you're doing the opposite: multiple threads per request. This is only appropriate if you know your user base is very small; otherwise, you'll kill your server scalability.