CODEWITHSUNDEEP
concurrencyf#-async
ZHUOWEI YANG
ZHUOWEI YANG

Reputation: 29

what is the relationship among different approaches of F# concurrency

I'm recently learn F# asynchronous workflows, which is an important feature of F# concurrency. What confused me is that how many approaches to write concurrent code in F#? I read Except F#, and some blog about F# concurrency, I know things like background workers; IAsyncResult; If programming on local machine, there is shard-memory concurrency in F#; If programming on distributed system, there is message-passing concurrency. But I really not sure what is the relationship between these techniques, and how to classify them. I understand it is quite a "big" question cannot be answer with one or two sentences, so I would definitely appreciate if anyone can give me specific answer or recommend me some useful references.

Upvotes: 2

Views: 515

Answers (1)

Alfonso Garcia-Caro
Alfonso Garcia-Caro

Reputation: 428

I'm also rather new to F#, so I hope more answers come to complement this one :)

The first thing is you need to distinguish between .NET classes (which can be used from any .NET language) and F# unique ways to deal with asynchronous operations. In the first case as you mention and among others, you have:

  • System.ComponentModel.BackgroundWorker: This was used mainly in the first .NET versions with Windows Forms and it's not recommended anymore.

  • System.IAsyncResult: This is also an old .NET interface implemented by several classes (also Task) but I don't usually use it directly.

  • Windows.Foundation.IAsyncOperation: Another interface but used only in Windows Store apps. Most of the times you translate it directly to Task, so you don't have to worry too much about it.

  • System.Threading.Tasks.Task: This is the recommended way now to handle .NET asynchronous and parallel (with the Parallel Task Library) operations. It's the hidden force behind C# async/await keywords, which are just syntactic sugar to pass continuations to Tasks.


So now with F# unique ways: Asynchronous Workflows and MailboxProcessor. It can roughly be said the former corresponds to parallelism while the latter deals with concurrency.

  • Asynchronous Workflows: This is just a computation expression (aka monad) which happens to deal with asynchrony: operations that run in the background to prevent blocking the UI thread or parallel operations to get the maximum performance in multi-core systems.

    It's more or less the equivalent to C# async/await but we F# fans like to think it's a more elegant solution because it uses a more generic and flexible mechanism (computation expressions) which can be adapted for example to asynchronous sequences, events or even Javascript callbacks. It has also other advantages as Thomas Petricek explains here.

    Within an asynchronous workflow most of the time you'll be using the methods in Control.Async or the extensions to .NET classes (like WebRequest.AsyncGetResponse) from the F# Core Library. If necessary, you can also interact directly with .NET Tasks (Async.AwaitTask and Async.StartAsTask) or even easily create your own async operations with Async.StartWithContinuations.

    To learn more about asynchronous workflows you can consult the MSDN documentation, the magnificent Scott Wlaschin's site, Tomas Petricek's blog or the F# Wikibook.

  • Control.MailboxProcessor: Designed to deal with concurrency, that is, several processes running at the same time which usually need to share some information. The traditional .NET way to prevent memory corruption when several threads try to write a variable at the same time was the lock statement. Besides the fact that functional style prefers to use immutable values, memory locks are complicated to use properly and can also have a high performance penalty. So instead of this, MailboxProcessor uses an Erlang-like message-based (or actor-based) approach to concurrency.

    I have not used MailboxProcessor myself that much, but for more info you can check Scott Wlaschin's site or the F# Wikibook.


I hope this helps! If someone sees something not completely correct in this answer, please feel free to edit it.

Cheers!

Upvotes: 2

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