UI thread waits for "await client.SendAsync(request).ConfigureAwait(false)" to finish before moving to another url

Question

Client starts some action via button. In controller its async method: async public Task<JsonResult> CreatePresentation that contains line using (var response = await client.SendAsync(request).ConfigureAwait(false)). Lets say this line takes 10s to finish. While that is happenning client wants to go to another url inside website and clicks on that url (that view is also async meaning its async public Task<ActionResult> BRDetails(int id)). But asp .net does not serve him view immidiatly. Instead it awaits for await line to finish before serving him the view (10s).

Why does asp framework waits for await line to finish before it starts producing view for next url that client requested?

I would thought that deliving the next url should start immidiatly in second thread and waits in first for await to finishes because of .ConfigureAwait(false).

Environment:

• .net framework v4.5,
• iiexpress that comes with visual studio

Here is part of code (since it's a software product I am not able to share full code):

        async public Task<JsonResult> CreateReportFile(int id, string email)
    {
        try
        {
            var fileName = "Report " + DateTime.Now.ToString("dd/MM/yyyy HH:mm:ss") + ".txt";
            fileName = fileName.Replace('/', '_');
            fileName = fileName.Replace(':', '_');
            var fullPath = "some path";
            var presentationTask = new Creator().CreateSomeFile("some values to pass", fileName);
            var ms = await presentationTask.ConfigureAwait(false);
            using (FileStream file = new FileStream(fullPath, FileMode.Create, System.IO.FileAccess.Write))
            {
                ms.WriteTo(file);
                ms.Close();
            }

            var emailAttachment = BusinessReportsEmail.savePresentation(fullPath, fileName);
            await BusinessReportsEmail.sendEmail(emailAttachment, email).ConfigureAwait(false);
            return Json(new
            {
                Status = "ok",
                Data = email
            });
        }
        catch (Exception ex)
        {
            return Json(new
            {
                Status = "error",
                Data = ex.Message
            });
        }
    }

public async Task<MemoryStream> CreateSomeFile(string programData, string outputFileName)
    {
        // Use this in production
        var url = string.Format(System.Configuration.ConfigurationManager.AppSettings["workFileUrl"]);
        var appCode = string.Format(System.Configuration.ConfigurationManager.AppSettings["appCode"]);
        var appKey = string.Format(System.Configuration.ConfigurationManager.AppSettings["appKey"]);
        using (var client = new HttpClient())
        {
            client.Timeout = TimeSpan.FromMinutes(20);
            using (var request = new HttpRequestMessage(HttpMethod.Post, url))
            {
                request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                request.Headers.Add("AppCode", appCode);
                request.Headers.Add("AppKey", appKey);
                var jsonString = JsonConvert.SerializeObject(new
                {
                    InputType = "json",
                    Content = programData
                });
                request.Content = new StringContent(jsonString, Encoding.UTF8, "application/json");
                using (var response = await client.SendAsync(request).ConfigureAwait(false))
                {
                    var responseContent = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
                    if (response.StatusCode != HttpStatusCode.OK)
                        throw new Exception($"Failed to create {outputFileName} file.");

                    var reponseObject = await response.Content.ReadAsAsync<FileResponse>().ConfigureAwait(false);
                    return new MemoryStream(Convert.FromBase64String(reponseObject.Content));
                }
            }
        }
    }

and than client goes to this view but needs to wait for previous call to finish.

    async public Task<ActionResult> ReviewDetails(int id)
    {
        return View();
    }

Answer 1

Why does asp framework waits for await line to finish before it starts producing view for next url that client requested?

In ASP.NET, await yields to the thread pool, not the HTTP client.

Remember, your web app is serving HTTP responses to HTTP requests. Generally speaking, a single HTTP request has a single HTTP response. Your web app can't return a response and then sometime later return another response. You can think of an await as an "asynchronous wait", and your method will asynchronously wait there, so it won't even get to the return Json(...) until after the await completes.

If you want to kick off an independent background process, then I recommend using asynchronous messaging, as described on my blog. It's complex, but it's necessary for reliable work. Alternatively, you could use something like a SPA and not return early at all - allow the user to change pages while the HTTP request is still in progress.