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Asynchronous Message Blocks

The Agents Library provides several message-block types that enable you to propagate messages among application components in a thread-safe manner. These message-block types are often used with the various message-passing routines, such as concurrency::send, concurrency::asend, concurrency::receive, and concurrency::try_receive. For more information about the message passing routines that are defined by the Agents Library, see Message Passing Functions.

Sources and targets are two important participants in message passing. A source refers to an endpoint of communication that sends messages. A target refers to an endpoint of communication that receives messages. You can think of a source as an endpoint that you read from and a target as an endpoint that you write to. Applications connect sources and targets together to form messaging networks.

The Agents Library uses two abstract classes to represent sources and targets: concurrency::ISource and concurrency::ITarget. Message block types that act as sources derive from ISource; message block types that act as targets derive from ITarget. Message block types that act as sources and targets derive from both ISource and ITarget.

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Message propagation is the act of sending a message from one component to another. When a message block is offered a message, it can accept, decline, or postpone that message. Every message block type stores and transmits messages in different ways. For example, the unbounded_buffer class stores an unlimited number of messages, the overwrite_buffer class stores a single message at a time, and the transformer class stores an altered version of each message. These message block types are described in more detail later in this document.

When a message block accepts a message, it can optionally perform work and, if the message block is a source, pass the resulting message to another member of the network. A message block can use a filter function to decline messages that it does not want to receive. Filters are described in more detail later in this topic, in the section Message Filtering. A message block that postpones a message can reserve that message and consume it later. Message reservation is described in more detail later in this topic, in the section Message Reservation.

The Agents Library enables message blocks to asynchronously or synchronously pass messages. When you pass a message to a message block synchronously, for example, by using the send function, the runtime blocks the current context until the target block either accepts or rejects the message. When you pass a message to a message block asynchronously, for example, by using the asend function, the runtime offers the message to the target, and if the target accepts the message, the runtime schedules an asynchronous task that propagates the message to the receiver. The runtime uses lightweight tasks to propagate messages in a cooperative manner. For more information about lightweight tasks, see Task Scheduler (Concurrency Runtime).

Applications connect sources and targets together to form messaging networks. Typically, you link the network and call send or asend to pass data to the network. To connect a source message block to a target, call the concurrency::ISource::link_target method. To disconnect a source block from a target, call the concurrency::ISource::unlink_target method. To disconnect a source block from all of its targets, call the concurrency::ISource::unlink_targets method. When one of the predefined message block types leaves scope or is destroyed, it automatically disconnects itself from any target blocks. Some message block types restrict the maximum number of targets that they can write to. The following section describes the restrictions that apply to the predefined message block types.

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The following table briefly describes the role of the important message-block types.

unbounded_buffer

Stores a queue of messages.

overwrite_buffer

Stores one message that can be written to and read from multiple times.

single_assignment

Stores one message that can be written to one time and read from multiple times.

call

Performs work when it receives a message.

transformer

Performs work when it receives data and sends the result of that work to another target block. The transformer class can act on different input and output types.

choice

Selects the first available message from a set of sources.

join and multitype join

Wait for all messages to be received from a set of sources and then combine the messages into one message for another message block.

timer

Sends a message to a target block on a regular interval.

These message-block types have different characteristics that make them useful for different situations. These are some of the characteristics:

  • Propagation type: Whether the message block acts as a source of data, a receiver of data, or both.

  • Message ordering: Whether the message block maintains the original order in which messages are sent or received. Each predefined message block type maintains the original order in which it sends or receives messages.

  • Source count: The maximum number of sources that the message block can read from.

  • Target count: The maximum number of targets that the message block can write to.

The following table shows how these characteristics relate to the various message-block types.

Message block type

Propagation type (Source, Target, or Both)

Message ordering (Ordered or Unordered)

Source count

Target count

unbounded_buffer

Both

Ordered

Unbounded

Unbounded

overwrite_buffer

Both

Ordered

Unbounded

Unbounded

single_assignment

Both

Ordered

Unbounded

Unbounded

call

Target

Ordered

Unbounded

Not Applicable

transformer

Both

Ordered

Unbounded

1

choice

Both

Ordered

10

1

join

Both

Ordered

Unbounded

1

multitype_join

Both

Ordered

10

1

timer

Source

Not Applicable

Not Applicable

1

The following sections describe the message-block types in more detail.

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The concurrency::unbounded_buffer class represents a general-purpose asynchronous messaging structure. This class stores a first in, first out (FIFO) queue of messages that can be written to by multiple sources or read from by multiple targets. When a target receives a message from an unbounded_buffer object, that message is removed from the message queue. Therefore, although an unbounded_buffer object can have multiple targets, only one target will receive each message. The unbounded_buffer class is useful when you want to pass multiple messages to another component, and that component must receive each message.

The following example shows the basic structure of how to work with the unbounded_buffer class. This example sends three values to an unbounded_buffer object and then reads those values back from the same object.

// unbounded_buffer-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // Create an unbounded_buffer object that works with 
   // int data.
   unbounded_buffer<int> items;

   // Send a few items to the unbounded_buffer object.
   send(items, 33);
   send(items, 44);
   send(items, 55);

   // Read the items from the unbounded_buffer object and print 
   // them to the console.
   wcout << receive(items) << endl;
   wcout << receive(items) << endl;
   wcout << receive(items) << endl;
}

This example produces the following output:

33
44
55

For a complete example that shows how to use the unbounded_buffer class, see How to: Implement Various Producer-Consumer Patterns.

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The concurrency::overwrite_buffer class resembles the unbounded_buffer class, except that an overwrite_buffer object stores just one message. In addition, when a target receives a message from an overwrite_buffer object, that message is not removed from the buffer. Therefore, multiple targets receive a copy of the message.

The overwrite_buffer class is useful when you want to pass multiple messages to another component, but that component needs only the most recent value. This class is also useful when you want to broadcast a message to multiple components.

The following example shows the basic structure of how to work with the overwrite_buffer class. This example sends three values to an overwrite _buffer object and then reads the current value from the same object three times. This example is similar to the example for the unbounded_buffer class. However, the overwrite_buffer class stores just one message. In addition, the runtime does not remove the message from an overwrite_buffer object after it is read.

// overwrite_buffer-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // Create an overwrite_buffer object that works with 
   // int data.
   overwrite_buffer<int> item;

   // Send a few items to the overwrite_buffer object.
   send(item, 33);
   send(item, 44);
   send(item, 55);

   // Read the current item from the overwrite_buffer object and print 
   // it to the console three times.
   wcout << receive(item) << endl;
   wcout << receive(item) << endl;
   wcout << receive(item) << endl;
}

This example produces the following output:

55
55
55

For a complete example that shows how to use the overwrite_buffer class, see How to: Implement Various Producer-Consumer Patterns.

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The concurrency::single_assignment class resembles the overwrite_buffer class, except that a single_assignment object can be written to one time only. Like the overwrite_buffer class, when a target receives a message from a single_assignment object, that message is not removed from that object. Therefore, multiple targets receive a copy of the message. The single_assignment class is useful when you want to broadcast one message to multiple components.

The following example shows the basic structure of how to work with the single_assignment class. This example sends three values to a single_assignment object and then reads the current value from the same object three times. This example is similar to the example for the overwrite_buffer class. Although both the overwrite_buffer and single_assignment classes store a single message, the single_assignment class can be written to one time only.

// single_assignment-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // Create an single_assignment object that works with 
   // int data.
   single_assignment<int> item;

   // Send a few items to the single_assignment object.
   send(item, 33);
   send(item, 44);
   send(item, 55);

   // Read the current item from the single_assignment object and print 
   // it to the console three times.
   wcout << receive(item) << endl;
   wcout << receive(item) << endl;
   wcout << receive(item) << endl;
}

This example produces the following output:

33
33
33

For a complete example that shows how to use the single_assignment class, see Walkthrough: Implementing Futures.

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The concurrency::call class acts as a message receiver that performs a work function when it receives data. This work function can be a lambda expression, a function object, or a function pointer. A call object behaves differently than an ordinary function call because it acts in parallel to other components that send messages to it. If a call object is performing work when it receives a message, it adds that message to a queue. Every call object processes queued messages in the order in which they are received.

The following example shows the basic structure of how to work with the call class. This example creates a call object that prints each value that it receives to the console. The example then sends three values to the call object. Because the call object processes messages on a separate thread, this example also uses a counter variable and an event object to ensure that the call object processes all messages before the wmain function returns.

// call-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // An event that is set when the call object receives all values. 
   event received_all;

   // Counts the  
   long receive_count = 0L;
   long max_receive_count = 3L;

   // Create an call object that works with int data.
   call<int> target([&received_all,&receive_count,max_receive_count](int n) {
      // Print the value that the call object receives to the console.
      wcout << n << endl;

      // Set the event when all messages have been processed. 
      if (++receive_count == max_receive_count)
         received_all.set();
   });

   // Send a few items to the call object.
   send(target, 33);
   send(target, 44);
   send(target, 55);

   // Wait for the call object to process all items.
   received_all.wait();
}

This example produces the following output:

33
44
55

For a complete example that shows how to use the call class, see How to: Provide Work Functions to the call and transformer Classes.

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The concurrency::transformer class acts as both a message receiver and as a message sender. The transformer class resembles the call class because it performs a user-defined work function when it receives data. However, the transformer class also sends the result of the work function to receiver objects. Like a call object, a transformer object acts in parallel to other components that send messages to it. If a transformer object is performing work when it receives a message, it adds that message to a queue. Every transformer object processes its queued messages in the order in which they are received.

The transformer class sends its message to one target. If you set the _PTarget parameter in the constructor to NULL, you can later specify the target by calling the concurrency::link_target method.

Unlike all other asynchronous message block types that are provided by the Agents Library, the transformer class can act on different input and output types. This ability to transform data from one type to another makes the transformer class a key component in many concurrent networks. In addition, you can add more fine-grained parallel functionality in the work function of a transformer object.

The following example shows the basic structure of how to work with the transformer class. This example creates a transformer object that multiples each input int value by 0.33 in order to produce a double value as output. The example then receives the transformed values from the same transformer object and prints them to the console.

// transformer-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // Create an transformer object that receives int data and  
   // sends double data.
   transformer<int, double> third([](int n) {
      // Return one-third of the input value. 
      return n * 0.33;
   });

   // Send a few items to the transformer object.
   send(third, 33);
   send(third, 44);
   send(third, 55);

   // Read the processed items from the transformer object and print 
   // them to the console.
   wcout << receive(third) << endl;
   wcout << receive(third) << endl;
   wcout << receive(third) << endl;
}

This example produces the following output:

10.89
14.52
18.15

For a complete example that shows how to use the transformer class, see How to: Use transformer in a Data Pipeline.

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The concurrency::choice class selects the first available message from a set of sources. The choice class represents a control-flow mechanism instead of a dataflow mechanism (the topic Asynchronous Agents Library describes the differences between dataflow and control-flow).

Reading from a choice object resembles calling the Windows API function WaitForMultipleObjects when it has the bWaitAll parameter set to FALSE. However, the choice class binds data to the event itself instead of to an external synchronization object.

Typically, you use the choice class together with the concurrency::receive function to drive control-flow in your application. Use the choice class when you have to select among message buffers that have different types. Use the single_assignment class when you have to select among message buffers that have the same type.

The order in which you link sources to a choice object is important because it can determine which message is selected. For example, consider the case where you link multiple message buffers that already contain a message to a choice object. The choice object selects the message from the first source that it is linked to. After you link all sources, the choice object preserves the order in which each source receives a message.

The following example shows the basic structure of how to work with the choice class. This example uses the concurrency::make_choice function to create a choice object that selects among three message blocks. The example then computes various Fibonacci numbers and stores each result in a different message block. The example then prints to the console a message that is based on the operation that finished first.

// choice-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <ppl.h>
#include <iostream>

using namespace concurrency;
using namespace std;

// Computes the nth Fibonacci number. 
// This function illustrates a lengthy operation and is therefore 
// not optimized for performance. 
int fibonacci(int n)
{
   if (n < 2)
      return n;
   return fibonacci(n-1) + fibonacci(n-2);
}

int wmain()
{
   // Although the following thee message blocks are written to one time only,  
   // this example illustrates the fact that the choice class works with  
   // different message block types. 

   // Holds the 35th Fibonacci number.
   single_assignment<int> fib35;
   // Holds the 37th Fibonacci number.
   overwrite_buffer<int> fib37;
   // Holds half of the 42nd Fibonacci number.
   unbounded_buffer<double> half_of_fib42;   

   // Create a choice object that selects the first single_assignment  
   // object that receives a value.
   auto select_one = make_choice(&fib35, &fib37, &half_of_fib42);

   // Execute a few lengthy operations in parallel. Each operation sends its  
   // result to one of the single_assignment objects.
   parallel_invoke(
      [&fib35] { send(fib35, fibonacci(35)); },
      [&fib37] { send(fib37, fibonacci(37)); },
      [&half_of_fib42] { send(half_of_fib42, fibonacci(42) * 0.5); }
   );

   // Print a message that is based on the operation that finished first. 
   switch (receive(select_one))
   {
   case 0:
      wcout << L"fib35 received its value first. Result = " 
            << receive(fib35) << endl;
      break;
   case 1:
      wcout << L"fib37 received its value first. Result = " 
            << receive(fib37) << endl;
      break;
   case 2:
      wcout << L"half_of_fib42 received its value first. Result = " 
            << receive(half_of_fib42) << endl;
      break;
   default:
      wcout << L"Unexpected." << endl;
      break;
   }
}

This example produces the following sample output:

fib35 received its value first. Result = 9227465

Because the task that computes the 35th Fibonacci number is not guaranteed to finish first, the output of this example can vary.

This example uses the concurrency::parallel_invoke algorithm to compute the Fibonacci numbers in parallel. For more information about parallel_invoke, see Parallel Algorithms.

For a complete example that shows how to use the choice class, see How to: Select Among Completed Tasks.

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The concurrency::join and concurrency::multitype_join classes let you wait for each member of a set of sources to receive a message. The join class acts on source objects that have a common message type. The multitype_join class acts on source objects that can have different message types.

Reading from a join or multitype_join object resembles calling the Windows API function WaitForMultipleObjects when it has the bWaitAll parameter set to TRUE. However, just like a choice object, join and multitype_join objects use an event mechanism that binds data to the event itself instead of to an external synchronization object.

Reading from a join object produces a std::vector object. Reading from a multitype_join object produces a std::tuple object. Elements appear in these objects in the same order as their corresponding source buffers are linked to the join or multitype_join object. Because the order in which you link source buffers to a join or multitype_join object is associated with the order of elements in the resulting vector or tuple object, we recommend that you do not unlink an existing source buffer from a join. Doing so can result in unspecified behavior.

The join and multitype_join classes support the concept of greedy and non-greedy joins. A greedy join accepts a message from each of its sources as messages become available until all message are available. A non-greedy join receives messages in two phases. First, a non-greedy join waits until it is offered a message from each of its sources. Second, after all source messages are available, a non-greedy join attempts to reserve each of those messages. If it can reserve each message, it consumes all messages and propagates them to its target. Otherwise, it releases, or cancels, the message reservations and again waits for each source to receive a message.

Greedy joins perform better than non-greedy joins because they accept messages immediately. However, in rare cases, greedy joins can lead to deadlocks. Use a non-greedy join when you have multiple joins that contain one or more shared source objects.

The following example shows the basic structure of how to work with the join class. This example uses the concurrency::make_join function to create a join object that receives from three single_assignment objects. This example computes various Fibonacci numbers, stores each result in a different single_assignment object, and then prints to the console each result that the join object holds. This example is similar to the example for the choice class, except that the join class waits for all source message blocks to receive a message.

// join-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <ppl.h>
#include <iostream>

using namespace concurrency;
using namespace std;

// Computes the nth Fibonacci number. 
// This function illustrates a lengthy operation and is therefore 
// not optimized for performance. 
int fibonacci(int n)
{
   if (n < 2)
      return n;
   return fibonacci(n-1) + fibonacci(n-2);
}

int wmain()
{
   // Holds the 35th Fibonacci number.
   single_assignment<int> fib35;
   // Holds the 37th Fibonacci number.
   single_assignment<int> fib37;
   // Holds half of the 42nd Fibonacci number.
   single_assignment<double> half_of_fib42;   

   // Create a join object that selects the values from each of the 
   // single_assignment objects.
   auto join_all = make_join(&fib35, &fib37, &half_of_fib42);

   // Execute a few lengthy operations in parallel. Each operation sends its  
   // result to one of the single_assignment objects.
   parallel_invoke(
      [&fib35] { send(fib35, fibonacci(35)); },
      [&fib37] { send(fib37, fibonacci(37)); },
      [&half_of_fib42] { send(half_of_fib42, fibonacci(42) * 0.5); }
   );

   auto result = receive(join_all);
   wcout << L"fib35 = " << get<0>(result) << endl;
   wcout << L"fib37 = " << get<1>(result) << endl;
   wcout << L"half_of_fib42 = " << get<2>(result) << endl;
}

This example produces the following output:

fib35 = 9227465
fib37 = 24157817
half_of_fib42 = 1.33957e+008

This example uses the concurrency::parallel_invoke algorithm to compute the Fibonacci numbers in parallel. For more information about parallel_invoke, see Parallel Algorithms.

For complete examples that show how to use the join class, see How to: Select Among Completed Tasks and Walkthrough: Using join to Prevent Deadlock.

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The concurrency::timer class acts as a message source. A timer object sends a message to a target after a specified period of time has elapsed. The timer class is useful when you must delay sending a message or you want to send a message at a regular interval.

The timer class sends its message to just one target. If you set the _PTarget parameter in the constructor to NULL, you can later specify the target by calling the concurrency::ISource::link_target method.

A timer object can be repeating or non-repeating. To create a repeating timer, pass true for the _Repeating parameter when you call the constructor. Otherwise, pass false for the _Repeating parameter to create a non-repeating timer. If the timer is repeating, it sends the same message to its target after each interval.

The Agents Library creates timer objects in the non-started state. To start a timer object, call the concurrency::timer::start method. To stop a timer object, destroy the object or call the concurrency::timer::stop method. To pause a repeating timer, call the concurrency::timer::pause method.

The following example shows the basic structure of how to work with the timer class. The example uses timer and call objects to report the progress of a lengthy operation.

// timer-structure.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

// Computes the nth Fibonacci number. 
// This function illustrates a lengthy operation and is therefore 
// not optimized for performance. 
int fibonacci(int n)
{
   if (n < 2)
      return n;
   return fibonacci(n-1) + fibonacci(n-2);
}

int wmain()
{
   // Create a call object that prints characters that it receives  
   // to the console.
   call<wchar_t> print_character([](wchar_t c) {
      wcout << c;
   });

   // Create a timer object that sends the period (.) character to  
   // the call object every 100 milliseconds.
   timer<wchar_t> progress_timer(100u, L'.', &print_character, true);

   // Start the timer.
   wcout << L"Computing fib(42)";
   progress_timer.start();

   // Compute the 42nd Fibonacci number. 
   int fib42 = fibonacci(42);

   // Stop the timer and print the result.
   progress_timer.stop();
   wcout << endl << L"result is " << fib42 << endl;
}

This example produces the following sample output:

Computing fib(42)..................................................
result is 267914296

For a complete example that shows how to use the timer class, see How to: Send a Message at a Regular Interval.

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When you create a message block object, you can supply a filter function that determines whether the message block accepts or rejects a message. A filter function is a useful way to guarantee that a message block receives only certain values.

The following example shows how to create an unbounded_buffer object that uses a filter function to accept only even numbers. The unbounded_buffer object rejects odd numbers, and therefore does not propagate odd numbers to its target blocks.

// filter-function.cpp 
// compile with: /EHsc
#include <agents.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
   // Create an unbounded_buffer object that uses a filter 
   // function to accept only even numbers.
   unbounded_buffer<int> accept_evens(
      [](int n) {
         return (n%2) == 0;
      });

   // Send a few values to the unbounded_buffer object. 
   unsigned int accept_count = 0;
   for (int i = 0; i < 10; ++i)
   {
      // The asend function returns true only if the target 
      // accepts the message. This enables us to determine 
      // how many elements are stored in the unbounded_buffer 
      // object. 
      if (asend(accept_evens, i))
      {
         ++accept_count;
      }
   }

   // Print to the console each value that is stored in the  
   // unbounded_buffer object. The unbounded_buffer object should 
   // contain only even numbers. 
   while (accept_count > 0)
   {
      wcout << receive(accept_evens) << L' ';
      --accept_count;
   }
}

This example produces the following output:

0 2 4 6 8

A filter function can be a lambda function, a function pointer, or a function object. Every filter function takes one of the following forms.

bool (_Type)
bool (_Type const &)

To eliminate the unnecessary copying of data, use the second form when you have an aggregate type that is propagated by value.

Message filtering supports the dataflow programming model, in which components perform computations when they receive data. For examples that use filter functions to control the flow of data in a message passing network, see How to: Use a Message Block Filter, Walkthrough: Creating a Dataflow Agent, and Walkthrough: Creating an Image-Processing Network.

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Message reservation enables a message block to reserve a message for later use. Typically, message reservation is not used directly. However, understanding message reservation can help you better understand the behavior of some of the predefined message block types.

Consider non-greedy and greedy joins. Both of these use message reservation to reserve messages for later use. A described earlier, a non-greedy join receives messages in two phases. During the first phase, a non-greedy join object waits for each of its sources to receive a message. A non-greedy join then attempts to reserve each of those messages. If it can reserve each message, it consumes all messages and propagates them to its target. Otherwise, it releases, or cancels, the message reservations and again waits for each source to receive a message.

A greedy join, which also reads input messages from a number of sources, uses message reservation to read additional messages while it waits to receive a message from each source. For example, consider a greedy join that receives messages from message blocks A and B. If the greedy join receives two messages from B but has not yet received a message from A, the greedy join saves the unique message identifier for the second message from B. After the greedy join receives a message from A and propagates out these messages, it uses the saved message identifier to see if the second message from B is still available.

You can use message reservation when you implement your own custom message block types. For an example about how to create a custom message block type, see Walkthrough: Creating a Custom Message Block.

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