Loading of new page blocked by api call

Question

I have a website with two pages (A and B). Both have some content which is loaded via renderings. A has a button which opens B in a new tab. A also has some frontend code which calls one of my APIs, which again calls an external API to fetch some data. This data will be displayed on page A. Page B does not rely on that data. All of that happens asynchronously and enables the user to click on the button which opens page B in a new tab.

The problem is although the new tab opens, page B doesn't load until page A has loaded all of the data.

While investigating this problem I figured out that the rendering on page B is not invoking the assigned controller action until the API on page A is done.

Does anyone have an idea on how to solve it?

I'm using sitecore 9.1.1 and I'm happy to provide any additional information if needed.

---

EDIT:

This is the controller action for page B which doesn't do a whole lot except reading some fields of the item. This action doesn't get called until the API call is done.

public ActionResult Dashboard()
{
    Item datasource = RenderingContext.Current.Rendering.Item;
    var viewModel = _clientOverviewService.GetPageBViewModel(datasource);
    return View(viewModel);
}

The API call basically comes down to this:

switch (httpType)
{
    case HttpType.Post:
        return await httpClient.PostAsync(uri.ToString(), new StringContent(jsonData, Encoding.UTF8, "application/json"));

    case HttpType.Get:
        return await httpClient.GetAsync(uri.ToString());

    ... 
}

This is the JS code where the API call is made. The BigApiCallURL is being transformed to the uri variable in the code block above:

const axiosInstance = await axiosInstanceGenerator('BigApiCallURL');
const response = await axiosInstance.post('', requestPayload);

---

UPDATE:

A similar behaviour can be found in Sitecore itself. When you open the Content Editor and open an item in the Experience Editor, it usually takes a couple of seconds for the Experience Editor to load. While it loads, click on a different item in the Content Editor to view it and you will notice the process actually being blocked. It will resume once the Experience Editor is done loading.

Maybe both issues are related to each other?

Answer 1

It looks like a problem with session state behavior. I have faced similar issue in Sitecore 8, when API calls from frontend came to queue and block one another. It was fixed by adding [SessionStateBehavior.ReadOnly] attribute on controller that I used for fetching data (to prevent blocking upcoming requests while actions are executed). Try to do the same for your controller.

[SessionState(SessionStateBehavior.ReadOnly)]
public class FetchDataController
{
 ...
}

Answer 2

It sound like the ASP.NET request queue: https://github.com/microsoft/dotnet/blob/master/Documentation/compatibility/throttle-concurrent-requests-per-session.md

Sitecore uses <add key="aspnet:RequestQueueLimitPerSession" value="25" /> out of the box, so I guess this forces the requests with the same Session ID to be queued and executed sequentially.