vb net datamatrix 2d barcode REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING in VB.NET

Printing Data Matrix ECC200 in VB.NET REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING

CHAPTER 13 REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING
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Looking at Listing 13-10, note that calling the Start method causes the processing agent to enter loop with n = 0. The agent then performs an asynchronous Receive request on the inbox for the MailboxProcessor; that is, the agent waits asynchronously until a message has been received. When the message msg is received, the program calls loop (n+msg). As additional messages are received, the internal counter (actually an argument) is incremented further. You post messages to the agent using counter.Post. The type of inbox.Receive is as follows: member Receive: unit -> Async<'Message> Using an asynchronous receive ensures that no real threads are blocked for the duration of the wait. This means the previous techniques scale to many thousands of concurrent agents.
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Message Processing and State Machines
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Listing 13-10 shares a common structure with many of the other message-processing components you see in this chapter, all of which are state machines. This general structure is as follows:
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let agent = MailboxProcessor.Start(fun inbox -> // The states of the let rec state1(args) and state2(args) ... and stateN(args) state machine = async { ... } = async { ... } = async { ... }
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// Enter the initial state state1(initialArgs))
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That is, message-processing components typically use sets of recursive functions, each defining an asynchronous computation. Each of these functions can be thought of as a state, and one of these states is identified as the initial state. You can pass arguments between these states just as you pass them between any other set of recursive functions.
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Creating Objects That React to Messages
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Often, it s wise to hide the internals of an asynchronous computation behind an object, because the use of message passing can be seen as an implementation detail. Listing 13-10 doesn t show you how to retrieve information from the counter, except by printing it to the standard output. Furthermore, it doesn t show how to ask the processing agent to exit. Listing 13-11 shows how to implement an object wrapping an agent that supports Increment, Stop, and Fetch messages.
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CHAPTER 13 REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING
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Listing 13-11. Hiding a Mailbox and Supporting a Fetch Method /// The internal type of messages for the agent type internal msg = Increment of int | Fetch of AsyncReplyChannel<int> | Stop type CountingAgent() = let counter = MailboxProcessor.Start(fun inbox -> // The states of the message-processing state machine... let rec loop n = async { let! msg = inbox.Receive() match msg with | Increment m -> // increment and continue... return! loop(n+m) | Stop -> // exit return () | Fetch replyChannel -> // post response to reply channel and continue do replyChannel.Reply n return! loop n } // The initial state of the message-processing state machine... loop(0)) member a.Increment(n) = counter.Post(Increment n) member a.Stop() = counter.Post Stop member a.Fetch() = counter.PostAndReply(fun replyChannel -> Fetch replyChannel) The inferred public types indicate how the presence of a concurrent agent is successfully hidden by the use of an object: type CountingAgent = new : unit -> CountingAgent member Fetch : unit -> int member Increment : n:int -> unit member Stop : unit -> unit Here, you can see an instance of this object in action: > let counter = new CountingAgent();; val counter : CountingAgent > counter.Increment(1);; val it : unit = () > counter.Fetch();; val it : int = 1 > counter.Increment(2);; val it : unit = ()
CHAPTER 13 REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING
> counter.Fetch();; val it : int = 3 > counter.Stop();; val it : unit = () Listing 13-11 shows several important aspects of message passing and processing using the mailbox-processing model: Internal message protocols are often represented using discriminated unions. Here the type msg has cases Increment, Fetch, and Stop, corresponding to the three methods accepted by the object that wraps the overall agent implementation. Pattern matching over discriminated unions gives a succinct way to process messages. A common pattern is a call to inbox.Receive() or inbox.TryReceive() followed by a match on the message contents.
The PostAndReply on the MailboxProcessor type gives a way to post a message and wait for a reply. A temporary reply channel is created and should form part of the message. A reply channel is an object of type Microsoft.FSharp.Control.AsyncReplyChannel<'reply>, which in turn supports a Post method. The MailboxProcessor can use this to post a reply to the waiting caller. In Listing 13-11, the channel is sent to the underlying messageprocessing agent counter as part of the Fetch message. Table 13-6 summarizes the most important members available on the MailboxProcessor type. Table 13-6. Some Members of the MailboxProcessor<'Message> Type
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