HTTPHandler : IRequiresSessionState halts execution of pages - asp.net

Question

I have ASHX HTTPHandler which implements IRequiresSessionState. When I call this handler, and call aspx page of same application in another window, aspx page does not start processing until ashx page finishes processing. Even call to same ashx page from two different windows show that page which gets called first executes and then the next one.
When I do not implement IRequiresSessionState, pages load asynchronously without waiting for other page to complete.
This could be serious bottleneck for end user, who would like to work in multiple windows.
Session have user specific data. If above approach does not work, need alternative way to store user specific data for session, which can be used in HTTPHandler.
Other Info:
- Am using ashx handler to process and send file which needs Session internally.

Answer 1

session state access requires that the aspx pages, and handler, run sequentially. Each page/handler may read from and write to the session state. To handle this without creating any errors in the session state .net will only run one at a time.
As Darin suggested, running with the read only session state will improve the situation. Asp.net will allow multiple handlers to run at the same time if they require only read access. However, the page will require write access to the session, so no handlers will run when the aspx page itself is running. This is the standard multi-reader single-writer locking pattern. You can have multiple readers at the same time, but only a single writer.
The only way to make this work is to have the handlers not require session state so that they run at the same time that t he aspx page is running.
You can work around this, by finding another way to store this data instead of using session state. You can implement your own shared cache/data structure, but you will need to provide similar locking (multi-reader/single writer) for that structure as well.

Answer 2

In addition to the points made by #Mike about IRequiresSessionState forcing each "user" (session) to only process one request at a time, there is good for another reason to want this.
Suppose you have an ajax service (implemented with a Generic Handler - *.ashx) which performs an "update" for a specific user based on the posted data. Let's say the update is heavy (around 250ms) and involves a "read/merge/write" process to the database (again data is isolated to an individual user). The client (app, website etc.) however can perform multiple calls in quick succession which the server has no control of.
By implementing IRequiresSessionState (simply including it as an inteface to your Generic Handler) ASP.NET will ensure that for a given user/session requests are handled serially. Multiple users however can still make calls to the update in parallel. This can allow us to avoid the "lost update problem" as the 2nd request won't retrieve a copy of the data until the 1st request has finished updating it.
public class SaveDataHandler : IHttpHandler, IRequiresSessionState
{
public void ProcessRequest(HttpContext context)
{
// Read request parameters
// Read database (user specific data)
// Process
// Write back to database (user specific data)
}
public bool IsReusable => false;
}