Asynchrony in (ASP).NET Aliaksandr

Developer and a Kettle

1.Wait 2.Make someone to wait 3.Check periodically 4.Make someone to check 5.Put a whistle on a kettle 6.Add a web-interface to a kettle 1.Sync 2.Sync with threads 3.Async-sync 4.Async with threads 5.Async with callbacks 6.Async with events

Comparison Approach Transferring 1K, rps Transferring 1M, rps Sync2844,44100%32,5726% Async with callbacks 2547,2690%125,97100% Async-sync2327,2882%33,0426% Sync with threads861,2330%83,8967% Async with threads604,8421%80,3664%

Evolution: WinAPI struct OVERLAPPED (IAsyncResult) OVERLAPPED_COMPLETION_ROUTINE (AsyncCallback)

6 Evolution:.NET 1.0 – 1.1 IAsyncResult AsyncCallback callback, userState) EndXXX(asyncResult)

7 Evolution:.NET 2.0 – … event XXXCompleted XXXCompletedEventArgs EventHandler userState) CancelAsync() AsyncOperationManager AsyncOperation SyncronizationContext

Evolution:.NET 4.0 Task Parallel Library

IAsyncResult vs Events IAsyncResult 1.Callbacks are not thread safe 2.No context in callbacks (i.e. no HttpContext) 3.Complexity of chaining (custom IAsyncResult, etc.) 4.Poor support of components Events 1.Event handlers are under lock 2.HttpContext available 3.Chaining is simple 4.Component-oriented ContextAwareResult, but no guarantee

SyncronizationContext Event-based async pattern Thread #1 XAsyncYAsync Thread #3YCompleted Thread #2 ZAsync Thread #4 ZCompleted PreRenderComplete PreRender AsyncOperationManager XY XCompleted Z All operations are completed

TTT #1: Use closures var asyncResult = component.BeginSomeOperation(parameter, aresult => TrickyCallback(aresult, component), state); void TrickyCallback(IAsyncResult state, Component component) { bool result = component.EndOperation(state); }

TTT#2: Use userState carefully static void Main(string[] args) { var asyncResult = component.BeginSomeOperation( parameter, Callback, state); } void Callback(IAsyncResult aresult) { bool result = component.EndOperation(aresult); object state = asyncResult.AsyncState; }

TTT#3: Unsubscribe from events static void Main(string[] args) { component.OperationCompleted += component_OperationCompleted; component.OperationAsync("data", null); } static void component_OperationCompleted(object sender, EventArgs e) { component.OperationCompleted -= component_OperationCompleted; }

TTT#5: There is no timeout support in both async patterns

TTT#6 AsyncResult callbacks don’t have HttpContext It could be, but there is no guarantee

TTT#7 Do not convert event-based pattern to IAsyncResult one Event-based pattern uses locks, that could be not desirable

TTT #8: Do not use [ThreadStatic] Do not forget about ThreadPool

TTT#9: Do not throw exceptions in callbacks Even WSE 3.0 has had a bug we had spotted.

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