Asynchronous Programming in .NET

Asynchronous Programming in .NET

• 1.
  Demystifying Asynchronous Programming in.NET
• 2.
  What is asynchronousprogramming - Unit of work running separately from its caller. - Common implementations: promises and callbacks.
• 3.
  Asynchronous == Parallelism? Yes.No. Well, sort of. Parallelism can happen with the TPL although it’s not the goal. Parallelism -> splitting a task into smaller pieces to execute them concurrently.
• 4.
  Task Parallel Library(TPL) ● Includes asynchronous and parallelism APIs (Task, Parallel LINQ). ● Gotcha: although being in the .NET CLR, F# uses a completely distinct implementation. ● Mostly about Task and the underlying plumbing. ● async/await is the language mechanism to generate the code around Tasks for continuations
• 5.
  In C# async Task<String>GetContent(String url) { return await _httpClient.GetAsync(url); } C# 5 .NET 4.5
• 6.
  F# Async Distinct implementation,predates the TPL. let sleep milliseconds message: WebPart = fun (x : HttpContext) -> async { do! Async.Sleep milliseconds return! OK message x }
• 7.
  Non blocking I/O -Non blocking I/O is commonly implied in .NET when talking about async/await and the TPL. - Similar to NodeJs, same purpose to run many requests on few threads - Many of the .NET async APIs use async IO, such as ADO.NET
• 8.
  Non blocking I/Oin Operating systems ● Known as overlapped I/O in Windows ● Posix AIO in Linux ● Socket AIO has arguably the most impact in web apps (database calls, HTTP, etc.)
• 9.
  Example: Dapper queries SqlConnection.Query()-> synchronous socket. Synchronous blocking SqlConnection.QueryAsync() -> asynchronous socket. Asynchrounous non blocking Task.Run(() => SqlConnection.Query() -> still a synchronous socket) Asynchrounous blocking. SqlConnection.QueryAsync().Result -> Asynchronous call and blocks the current thread
• 10.
  Why all thefuss about asynchronous anyway (Supposing asynchronous IO) ● Scalability, handling many requests in parallel ● Better handling of spikes. .NET throttle thread creation at 1 per half second. Async regardless of IO ● Freeing the UI thread in a desktop application ● Asynchronous flows
• 11.
  Async all thethings
• 12.
  Not so fast -Need to propagate the async all the way up to the caller - Overhead on pure CPU code - Exception handling is tricky. - Not meant for fire and forget. Hard to debug, monitor, error handling.
• 13.
  You can blockon async code. Don’t. Task.Result -> Congratulations, it’s a deadlock (probably) Blocking -> the async IO is now synchronous. Frameworks such as ASP.NET force a request to continue on the same thread. Doesn’t happen anymore in ASP.NET Core, no synchronization context.
• 14.
  State machine ● Theasync/await is replaced with a state machine by the compiler ● Async/await is a language construct -> doesn’t exist in IL. ● Could be done manually, lot of boilerplate
• 15.
  Quiz var tasks =Enumerable.Range(0, 100).Select(async _ => await Task.Delay(1000)).ToArray(); var tasks = Enumerable.Range(0, 100).Select(async _ => await Task.Run(() => Thread.Sleep(1000))).ToArray(); Task.WaitAll(tasks);
• 16.
  async _ =>await Task.Delay(1000)
• 17.
  async _ =>await Task.Run(() => Thread.Sleep(1000))
• 18.
  Beware of backgroundprocessing Easy to fire and forget -> doesn’t mean you should - Error handling is thrown out the window - No priority on what is executed - No way of cancelling anything - Hell to monitor/debug - Accessing thread scoped dependencies and all hell breaks loose. - Hangfire is a worthy alternative.
• 19.
  Cancellation ● Cancellation tokento cancel a task ● Essentially throws an exception, bubbling up the call chain ● Useful for cleanup of resources or stop intensive processing var tokenSource = new CancellationTokenSource(); var task = _httpClient..GetAsync("http://www.google.com", cts.Token); tokenSource.Cancel();
• 20.
  Takeaways ● Non-blocking I/Ois usually implied with async in .NET ● Asynchronous programming has many benefits but shouldn’t be used for all methods. ● Blocking on async code essentially nullifies all gain, if you’re lucky. Use await! ● If you’re not lucky, deadlock
• 21.
  Questions? Twitter: @plmaheu