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I will build an Asp.net MVC 3 web page.

View: The view (web page) invoke about five Ajax(jQuery) calls against the methods, which return JsonResult, in a controller and render the results on the web page.

Control: The controller methods read a SQL Server 2008 database using EF4. Two of the SQL statements may take half a minute to execute depending on the server load.

I wish the users can at least see the contents returned from the quick controller/database calls as soon as possible. The page will not have a lot of users (maybe up to 15). Will the long run controller method calls block others if they are not asynchronous? Or is it irrelevant as long as the thread pool is big enough to handle the peak requests of the users?

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From the user's view, loading the initial web page is synchronous, i.e. he has to wait until the server delivers the page. The Ajax requests however look asynchronous to him because he can already see part of the page.

From the server's view, everything is synchronous. There is an HTTP request that needs to be processed and the answer is either HTML, JSON or whatever. The client will wait until it receives the answer. And several requests can be processed in parallel.

So unless you implement some special locking (either on the web server or in the database) that blocks some of the requests, nothing will be blocked.

The proposed approach seems just fine to me.

Update:

There's one thing I forgot: ASP.NET contains a locking mechanism to synchronize access to the session data that can get into the way if you have several concurrent requests from the same user. Have a look at the SessionState attribute for a way to work around that problem.

Update 2:

And for an asynchronous behavior from the user's point of view, there's no need to use the AsyncController class. They where built for something else, which is not relevant in your case since only have 15 users.

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Will the long run controller method calls block others if they are not asynchronous?

The first important thing to note is that all those controller actions should not have write access to the Session. If they write to the session whether they are sync or async they will always execute sequentially and never in parallel. That's due to the fact that ASP.NET Session is not thread safe and if multiple requests from the same session arrive they will be queued. If you are only reading from the Session it is OK.

Now, the slow controller actions will not block the fast controller actions. No matter whether they are synchronous or not. For long controller actions it could make sense to make them asynchronous only if you are using the asynchronous ADO.NET methods to access the database and thus benefit from the I/O Completion Ports. They allow you to not consume any threads during I/O operations such as database access. If you use standard blocking calls to the database you get no benefit from async actions.

I would recommend you the following article for getting deeper understanding of when asynchronous actions could be beneficial.

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