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This is meant to illustrate the flow of control Notice that Beg i n F oo could return before, while, or after Foo finishes executing, depending on the way work is scheduled on the thread pool The meat of the implementation is omitted: the FooAsy n c R e s u l t class We'll explore a sample implementation of
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IAsyn c Re s u lt later Also, we don't necessarily need to run Foo on the thread
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pool In some specific circumstances, we could use Windows 1 / completion ports for asynchronous I/O, for instance, so that no thread ever has to block
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Rendezvousing: Four Ways After a thread kicks off asynchronous work, there is a decision to make: How will we rendezvous with the completion of that work so that the E n d F oo method can be called, possible exceptions handled, and the return value processed in an appropriate way This rendezvous may or may not involve the original thread In fact, four basic rendezvous patterns are supported:
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1 A thread can make a call to E n d F o o directly The APM provider is responsible for doing the right thing in this method : if already
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completed, it will return or throw right away; otherwise, it will block waiting for completion When the call returns or throws, the asynchronous operation is complete 2 Any thread with access to the IAsyn c Re s u lt can use the Asyn cWa it H a n d l e to block until the concurrent work has finished 3 Any thread with access to the IAsyn c R e s u l t (usually the thread that started the work) can "poll" for completion by checking the I s Com p l eted flag When the asynchronous work has finished, the I s Com p l et e d flag will be set to t r ue, and it is then safe to call E n d F oo 4 Finally, a callback may be supplied to Beg i n F oo, which is called when Foo finishes This typically executes on a thread pool thread, and inside the callback code you can make a call to E n d Foo to retrieve the results You can also mix a combination of these things, though you have to be somewhat careful You must ensure no two threads ever call E n d Foo on the same IAsyn c Re s u l t While some APM providers may handle this situation, it is not a standard part of the pattern Should you depend on one particu lar implementation handling this, you're apt to encounter race conditions and compatibility problems down the road Now we'll look at an example program that uses a synchronous method Foo and, specifically, how we can morph the program into using Begin Foo and each of these completion mechanisms instead This is more of a case study walkthrough of the completion mechanisms and will be useful to illustrate practical concerns that will arise when you try to consume the APM from your own code Here is the original synchronous program
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The markers S O 55 are meant t o indicate some set o f program state ments that are immaterial to the example itself What is important about them is the control flow and when they will execute For simplification pur poses, imagine that no references to t are found in any of the statements except for 53 That is, the call to Foo produces a value stored in t, which is returned from g to f, and then f returns it without inspecting the value Where are the opportunities for asynchronous execution here The pos sibility of race conditions and shared resources aside, Foo can run concur rently with respect to at least 55 and 51 due to the lack of control dependence It can run concurrently with 50 too, but because the call to Foo is dependent on the output of 52, we would need to restructure the code somehow, probably issuing 52 before SO We'll now work our way through the rendezvous techniques: from mechanism #1 to mechanism #4 You will find that #1 is generally the least different from the sequential code while #4 is generally the most different
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