Asynchronous Programming in C# 5.0 part two: Whence await?

Asynchronous Programming in C# 5.0 part two: Whence await?

Rate This

I want to start by being absolutely positively clear about two things, because our usability research has shown this to be confusing. Remember our little program from last time?

async void ArchiveDocuments(List<Url> urls)
{
  Task archive = null;
  for(int i = 0; i < urls.Count; ++i)
  {
    var document = await FetchAsync(urls[i]);
    if (archive != null)
      await archive;
    archive = ArchiveAsync(document);
  }
}

The two things are:

1) The “async” modifier on the method does not mean “this method is automatically scheduled to run on a worker thread asynchronously”. It means the opposite of that; it means “this method contains control flow that involves awaiting asynchronous operations and will therefore be rewritten by the compiler into continuation passing style to ensure that the asynchronous operations can resume this method at the right spot.” The whole point of async methods it that you stay on the current thread as much as possible. They’re like coroutines: async methods bring single-threaded cooperative multitasking to C#. (At a later date I’ll discuss the reasons behind requiring the async modifier rather than inferring it.)

2) The “await” operator used twice in that method does not mean “this method now blocks the current thread until the asynchronous operation returns”. That would be making the asynchronous operation back into a synchronous operation, which is precisely what we are attempting to avoid. Rather, it means the opposite of that; it means “if the task we are awaiting has not yet completed then sign up the rest of this method as the continuation of that task, and then return to your caller immediately; the task will invoke the continuation when it completes.

It is unfortunate that people’s intuition upon first exposure regarding what the “async” and “await” contextual keywords mean is frequently the opposite of their actual meanings. Many attempts to come up with better keywords failed to find anything better. If you have ideas for a keyword or combination of keywords that is short, snappy, and gets across the correct ideas, I am happy to hear them. Some ideas that we already had and rejected for various reasons were:

wait for FetchAsync(…)
yield with FetchAsync(…)
yield FetchAsync(…)
while away the time FetchAsync(…)
hearken unto FetchAsync(…)
for sooth Romeo wherefore art thou FetchAsync(…)

Moving on. We’ve got a lot of ground to cover. The next thing I want to talk about is “what exactly are those ‘thingies’ that I handwaved about last time?”

Last time I implied that the C# 5.0 expression

document = await FetchAsync(urls[i])

gets realized as:

state = State.AfterFetch;
fetchThingy = FetchAsync(urls[i]);
if (fetchThingy.SetContinuation(archiveDocuments))
  return;
AfterFetch: ;
document = fetchThingy.GetValue();

what’s the thingy?

In our model for asynchrony an asynchronous method typically returns a Task<T>; let’s assume for now that FetchAsync returns a Task<Document>. (Again, I’ll discuss the reasons behind this "Task-based Asynchrony Pattern" at a later date.) The actual code will be realized as:

fetchAwaiter = FetchAsync(urls[i]).GetAwaiter();
state = State.AfterFetch;
if (fetchAwaiter.BeginAwait(archiveDocuments))
  return;
AfterFetch: ;
document = fetchAwaiter.EndAwait();

The call to FetchAsync creates and returns a Task<Document> - that is, an object which represents a “hot” running task. Calling this method immediately returns a Task<Document> which is then somehow asynchronously fetches the desired document. Perhaps it runs on another thread, or perhaps it posts itself to some Windows message queue on this thread that some message loop is polling for information about work that needs to be done in idle time, or whatever. That’s its business. What we know is that we need something to happen when it completes. (Again, I’ll discuss single-threaded asynchrony at a later date.)

To make something happen when it completes, we ask the task for an Awaiter, which exposes two methods. BeginAwait signs up a continuation for this task; when the task completes, a miracle happens: somehow the continuation gets called. (Again, how exactly this is orchestrated is a subject for another day.) If BeginAwait returns true then the continuation will be called; if not, then that’s because the task has already completed and there is no need to use the continuation mechanism.

EndAwait extracts the result that was the result of the completed task.

We will provide implementations of BeginAwait and EndAwait on Task (for tasks that are logically void returning) and Task<T> (for tasks that return a value). But what about asynchronous methods that do not return a Task or Task<T> object? Here we’re going to use the same strategy we used for LINQ. In LINQ if you say

from c in customers where c.City == "London" blah blah blah

then that gets translated into

customers.Where(c=>c.City=="London") …

and overload resolution tries to find the best possible Where method by checking to see if customers implements such a method, or, if not, by going to extension methods. The GetAwaiter / BeginAwait / EndAwait pattern will be the same; we’ll just do overload resolution on the transformed expression and see what it comes up with. If we need to go to extension methods, we will.

Finally: why "Task"?

The insight here is that asynchrony does not require parallelism, but parallelism does require asynchrony, and many of the tools useful for parallelism can be used just as easily for non-parallel asynchrony. There is no inherent parallelism in Task; that the Task Parallel Library uses a task-based pattern to represent units of pending work that can be parallelized does not require multithreading.

As I've pointed out a few times, from the point of view of the code that is waiting for a result it really doesn't matter whether that result is being computed in idle time on this thread, in a worker thread in this process, in another process on this machine, on a storage device, or on a machine halfway around the world. What matters is that it's going to take time to compute the result, and this CPU could be doing something else while it is waiting, if only we let it.

The Task class from the TPL already has a lot of investment in it; it's got a cancellation mechanism and other useful features. Rather than invent some new thing, like some new "IFuture" type, we can just extend the existing task-based code to meet our asynchrony needs.

Next time: How to further compose asynchronous tasks.

  • I'm fine with "await".  As alpha coders I believe we tend to think too much about the "how" (and in excruciating detail).  If you look at these async methods, they're kind of like work flows that operate over discontiguous chunks of time and possibly threads.  So if my workflow says that a result is awaiting the completion of a certain async operation that make sense to me.  

    If you want to get pedantic about keywords reflecting reality, then seems like there should be an uproar over the use of "return" in one of the new async methods.  After all, the method will typically return well before it hits the "return" statement.  But again, from a workflow point of value, that makes sense to me because it is returning the final result from the workflow.

    Now I'm not so sure about the need for "async" but if a modifier is absolutely needed, then I prefer "async".

  • BTW "await" doesn't just yield the thread timeslice, it yields the thread itself!  So "yield *" in its place would definitely make sense.

    My vote would be for "yield while" or "yield until" except when you consider that this new thread-agnostic method is destined to become the way all .NET asynchronous code is written... you would tire of writing that when a short "await" could be used in place.

  • 'eventually' - following the statement to yield with.

    var document = FetchAsync(urls[i]) eventually;

    if (archive != null)

       archive eventually;

    Alternatively, 'eventually then'

    var document = FetchAsync(urls[i]) eventually then;

    if (archive != null)

       archive eventually then;

    archive = ArchiveAsync(document);

  • Eric,

    Here are my 2 cents at the first view of these new concepts

    replace async with 'blocked' and await with 'deferred'

    your code becomes:

    blocked  void ArchiveDocuments(List<Url> urls)

    {

     Task archive = null;

     for(int i = 0; i < urls.Count; ++i)

     {

       var document = deferred  FetchAsync(urls[i]);

       if (archive != null)

         deferred archive;

       archive = ArchiveAsync(document);

     }

    }

    the intent is decribed better in my opinion

    another good choice would be  async  -> 'finalized' and await -> 'nonfinalized'

  • This is a commendable feature but I know developers will abuse it. They will call "async" functions treating them like synchronous function and they will then start seeing horrible race conditions and deadlocks. The confusion that you fight against at the beginning of this post won't just go away.  Think very carefully before you release this feature.

  • Instead of 'wait' it should say 'resumable'...

  • How about the using the keyword "start"?

  • I would like to replace 'await' with 'async'. Then replace 'async' with 'task' or something else.

  • I would have liked to see the keyword "defer" used in place of "await".

  • My proposal: "afterwards" and  simplify the syntax as follows:

    async void ArchiveDocuments(List<Url> urls)

    {

    Task archive = null;

    for(int i = 0; i < urls.Count; ++i)

    {

      var document = FetchAsync(urls[i]):

      afterftwerards archive:

      aftgerwards archive = ArchiveAsync(document);

    }

    }

    Do you like it?

  • Nevermind, it just seems Part Two has a different title that the other parts...

Page 11 of 11 (161 items) «7891011