Asynchronous programming in ASP.NET
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Presentation for SDN Conference 2014: Async programming in ASP.NET and Visual Studio 2015 and best practices.
![Timeouts in MVC and WebAPI
• Use AsyncTimeoutAttribute on async actions
• When timeout occurs TimeoutException is thrown
from action
• Default timeout is AsyncManager’s 45000 milliseconds
• Might want to catch errors
[AsyncTimeout(2000)]
[HandleError(ExceptionType=typeof(TimeoutException))]
public async Task<ActionResult> SomeMethodAsync(CancellationToken token)
{
// Pass down CancellationToken to other async method calls
…
}](https://image.slidesharecdn.com/asynchronousprogramminginasp-160724223206/85/Asynchronous-programming-in-ASP-NET-26-320.jpg)
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Asynchronous programming in ASP.NET
- 2. Name: Role: https://nl.linkedin.com/in/alexthissen @alexthissen http://blog.alexthissen.nl Alex Thissen Lead Consultant, Xpirit Introducing myself
- 3. Agenda • Introduction on synchronicity • Threading and async programming in .NET • Details for • ASP.NET WebForms • ASP.NET MVC • ASP.NET WebAPI • ASP.NET SignalR • Gotchas • Questions and Answers
- 4. Talking about synchronicity Primer on async and multi-threading in Windows and .NET
- 5. (A)Synchronous in a web world Message Exchange Patterns Parallelization vs. multi- threading High Latency vs high throughput Asynchronicity is easy now Blocking operations Asynchronous is faster
- 6. A tale of fast food restaurants
- 7. Threading in Windows and .NET • Two concurrency types in Windows Operating System 1. Worker threads Physical units of work 2. IO Completion Ports Special construct for async I/O bound operations • Threads incur overhead • But threads waiting for IO Completion Port are efficient
- 8. .NET Threading primitives • System.Threading namespace • Threads: Thread and ThreadPool • Locks: Mutex, WaitHandle, Semaphore, Monitor, Interlocked • ThreadPool cannot scale hard (2 extra threads/second) • Each .NET logical thread adds overhead (1MB of managed memory) .NET 4.5 Worker Threads Completion Port Threads Minimum 4 4 Maximum 5000 (4095) 1000
- 9. Threading in ASP.NET Application Pool (w3wp.exe) CLR Threadpool Request Queue AppDomain Website http.sys IIS Unmanaged execution Kernel level … … Global Queue connectionManagement maxconnection httpRuntime/ minFreeThreads minLocalRequestFreeThreads processModel/ maxWorkerThreads minWorkerThreads maxIoThreads Outgoing connections applicationPool/ maxConcurrentRequestsPerCpu Worker Threads Completion Port Threads
- 10. Making a choice for (a)sync Synchronous • Operations are simple or short-running • Simplicity over efficiency • CPU-bound operations Asynchronous • Ability to cancel long- running tasks • Parallelism over simplicity • Blocking operations are bottleneck for performance • Network or I/O bound operations
- 11. Worker thread #1 Worker thread #2Worker thread #3 IO Completion Port Thread #3 Threads types and context switches ASP.NET Runtime Store Customers Async Windows IO Completion Port db.Save ChangesAsync Must support async pattern in some way As an example, Entity Framework 6 has support for async operations Unnecessary additional threads only occur overhead. Underlying SqlClient uses IO Completion Port for async I/O operation
- 12. Asynchronous programming .NET Framework support for async
- 13. History of .NET async programming Asynchronous Programming ModelAPM • Pairs of Begin/End methods • Example: FileStream.BeginWrite and FileStream.EndWrite • Convert using TaskFactory and TaskFactory<TResult> Event-based Asynchronous PatternEAP • Pairs of OperationAsync method and OperationCompleted event • Example: WebClient.DownloadStringAsync and WebClient.DownloadStringCompleted • TaskCompletionSource<T> to the rescue Task-based Asynchronous PatternTAP • Task and Task<T> • Preferred model
- 14. Async in .NET BCL classes • .NET Framework classes show each async style • Sometimes even mixed • Example: System.Net.WebClient • TPL (Task-based) APIs are preferred • Find and use new classes that support TAP natively
- 15. Async constructs in ASP.NET • ASP.NET runtime • Async Modules • Async Handlers • ASP.NET WebForms • AddOnPreRenderComplete • PageAsyncTask • ASP.NET MVC and WebAPI • Async actions
- 16. ASP.NET WebForms ASP.NET specifics part 1
- 17. Asynchronous programming in WebForms Your options: 1. Asynchronous pages 2. AsyncTasks It all comes down to hooking into async page lifecycle
- 18. Normal synchronous page lifecycle for ASP.NET WebForms … … LoadComplete PreRender PreRenderComplete SaveViewState Client Request page Send response Render … Thread 1
- 19. Switch to asynchronous handling … … LoadComplete PreRender PreRenderComplete SaveViewState Client Send response Render … Thread 1 Thread 2 IAsyncResult Request page
- 20. Async pages in WebForms Recipe for async pages • Add async="true" to @Page directive or <pages> element in web.config • Pass delegates for start and completion of asynchronous operation in AddOnPreRenderCompleteAsync method • Register event handler for PreRenderComplete private void Page_Load(object sender, EventArgs e) { this.AddOnPreRenderCompleteAsync( new BeginEventHandler(BeginAsynchronousOperation), new EndEventHandler(EndAsynchronousOperation)); this.PreRenderComplete += new EventHandler(LongRunningAsync_PreRenderComplete); }
- 21. PageAsyncTasks • Single unit of work • Encapsulated by PageAsyncTask class • Support for APM and TPL (new in ASP.NET 4.5) • Preferred way over async void event handlers • Can run multiple tasks in parallel // TAP async delegate as Page task RegisterAsyncTask(new PageAsyncTask(async (token) => { await Task.Delay(3000, token); }));
- 22. ASP.NET MVC and WebAPI ASP.NET specifics part 2
- 23. Async support in MVC • AsyncController (MVC3+) • Split actions in two parts 1. Starting async: void IndexAsync() 2. Completing async: ActionResult IndexCompleted(…) • AsyncManager • OutstandingOperations Increment and Decrement • Task and Task<ActionResult> (MVC 4+) • Async and await (C# 5+)
- 24. Async actions in ASP.NET MVC From synchronous public ActionResult Index() { // Call synchronous operations return View("Index", GetResults()); } To asynchronous public async Task<ActionResult> IndexAsync() { // Call operations asynchronously return View("Index", await GetResultsAsync()); }
- 25. Timeouts • Timeouts apply to synchronous handlers • Default timeout is 110 seconds (90 for ASP.NET 1.0 and 1.1) • MVC and WebAPI are always asynchronous • Even if you only use synchronous handlers • (Server script) timeouts do not apply <system.web> <compilation debug="true" targetFramework="4.5"/> <httpRuntime targetFramework="4.5" executionTimeout="5000" /> </system.web>
- 26. Timeouts in MVC and WebAPI • Use AsyncTimeoutAttribute on async actions • When timeout occurs TimeoutException is thrown from action • Default timeout is AsyncManager’s 45000 milliseconds • Might want to catch errors [AsyncTimeout(2000)] [HandleError(ExceptionType=typeof(TimeoutException))] public async Task<ActionResult> SomeMethodAsync(CancellationToken token) { // Pass down CancellationToken to other async method calls … }
- 27. ASP.NET SignalR async notes • In-memory message bus is very fast • Team decided not to make it async • Sending to clients is • always asynchronous • on different call-stack
- 28. Beware of the async gotchas • Cannot catch exceptions in async void methods • Mixing sync/async can deadlock threads in ASP.NET • Suboptimal performance for regular awaits Best practices for async: • Avoid async void • Async all the way • Configure your wait
- 29. Tips • Don’t do 3 gotcha’s • Avoid blocking threads and thread starvation • Remember I/O bound (await) vs. CPU bound (ThreadPool, Task.Run, Parallel.For) • Thread management: • Avoid creating too many • Create where needed and reuse if possible • Try to switch to IO Completion Port threads • Look for BCL support
- 30. Summary • Make sure you are comfortable with • Multi-threading, parallelism, concurrency, async and await • ASP.NET fully supports TPL and async/await • WebForms • MVC • WebAPI • Great performance and scalability comes from good thread management • Be aware of specific ASP.NET behavior
Editor's Notes
- #10: http://support.microsoft.com/kb/821268
- #11: http://www.asp.net/web-forms/overview/performance-and-caching/using-asynchronous-methods-in-aspnet-45
- #22: http://blogs.msdn.com/b/pfxteam/archive/2012/05/31/what-s-new-for-parallelism-in-visual-studio-2012-rc.aspx
- #28: http://stackoverflow.com/questions/19193451/should-signalr-server-side-methods-be-async-when-calling-clients
- #29: http://msdn.microsoft.com/en-us/magazine/jj991977.aspx