Wednesday, June 08, 2011

Asynchronous ASP.NET / SQL Server

We all agree that waiting is evil, right? Well, threads are evil too! Ok, we need a few threads to get the job done, but they (ideally) shouldn't ever block.

Achieving high concurrency has never been only about running operations on multiple threads. In fact, the best performance is when the thread count is a number very close to the hardware thread count[1].

.NET provides us with the Asynchronous Programming Model (APM) that gives us the ability to use non-blocking operations that might otherwise have blocked up a thread waiting for them to complete. It allows us to create as few threads as are absolutely necessary and to use them efficiently.

Let's say we have an ASP.NET web application (ignoring MVC until Part Two). The page will display a piece of data that takes 30 seconds to calculate, based on some input data. The calculation doesn't happen locally; in fact it's running on a cluster of big SQL Server boxes. We have 1000 users, and the database query that takes 30 seconds scales like magic (even at 2000 users, it still takes less than 45 seconds on average). Can we put the ASP.NET part of this application onto just one box? The answer is yes - I'll show you.

First of all, we want ASP.NET to treat our page differently to a regular ASP.NET page; when we begin our asynchronous operation, we want to signal ASP.NET that the thread we started with is now free for the next request. We also want ASP.NET to call us back when our result is ready. This is done by setting the following page attribute:
<%@ Page Async="true" ...

Our request will now be handled by an IHttpAsyncHandler and will now process in 4 phases instead of 1.

  1. PreInit,Init,InitComplete,PreLoad,LoadComplete,PreRender,PreRenderComplete,SaveState,SaveStateComplete,Render


  1. PreInit,Init,InitComplete,PreLoad,LoadComplete,PreRender

  2. Begin

  3. End

  4. PreRenderComplete,SaveState,SaveStateComplete,Render

See what's happened here? Instead of synchronously performing phases 1 and 4 as a single operation (on the same thread) like we would do in a regular ASP.NET page, we break the long running task into 4 chunks, running each independently, and allowing .NET to efficiently allocate tasks to physical threads. That's correct: now we have no guarantee that Render will be called on the same thread as Init. And why should it? No reason, that's why! (However, there's every possibility it *might* be more efficient to use the same physical thread - yes, just one - if the server in question had only a single core CPU. The point is we shouldn't write code that expects this to have been the case). Also note that two new phases have been sandwiched in between 1 and 4: Begin (2) and End (3).
To use Begin and End, we need only to (write and) register our callbacks, and ASP.NET will ensure they're called.
protected void Page_Load(object sender, EventArgs e)

We register (as many as we need) pairs of Begin and End methods that match the following signatures (both the Begin and End methods are called asynchronously, hence the async look and feel of the parameter lists):
public delegate IAsyncResult BeginEventHandler(object sender, EventArgs e, AsyncCallback cb, object extraData);

public delegate void EndEventHandler(IAsyncResult ar);

So far so good (hopefully), but all this would be pointless if SQL Server didn't also give us asynchronous connections that can call back into the thread pool when the command completes. To use it, set
Asynchronous Processing=true;
in the connection string and code up your operation to the APM pattern:
void command_BeginExecuteReader_AsyncCallback(IAsyncResult asyncResult)
SqlCommand command = (SqlCommand)asyncResult.AsyncState;
SqlDataReader reader = command.EndExecuteReader(asyncResult);
while (reader.Read())
} while (reader.NextResult());

IAsyncResult command_BeginExecuteReader(object sender, EventArgs e, AsyncCallback cb, object extraData)
SqlConnection connection = new SqlConnection(@"Server=Scandium;Trusted_Connection=true;Asynchronous Processing=true;");
SqlCommand command = new SqlCommand(@"/*hectique-code*/", connection);
return command.BeginExecuteReader(cb, command);

Did you spot the ASP.NET trick that allows this magic to work? See how our Begin method takes an AsyncCallback from the caller (ASP.NET)... well, we pass that callback to SQL Server instead of directly wiring up our End method as we might in a Windows application. That way, as soon as SQL Server is ready, ASP.NET gets notified (not us directly), which gives control to our page for us to handle the response, then when we're done (i.e. the End method call stack is unwound) ASP.NET moves onto the final phase to do it's PreRenderComplete etc.

This is how it's done. Simples. For the sake of clarity I've not bothered with error handling in the example. Resource management across threads makes C#'s using statement impossible, so implementers of IDisposable are dealt with in a slightly tricky way. Thrown exceptions would need special care too.

[1] I made this figure up, but it does seem to prove itself anecdotally quite a lot.

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