asio/src/doc/overview/async.qbk - third_party/asio - Git at Google

 [/
  / Copyright (c) 2003-2016 Christopher M. Kohlhoff (chris at kohlhoff dot com)
  /
  / Distributed under the Boost Software License, Version 1.0. (See accompanying
  / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  /]

 [section:async The Proactor Design Pattern: Concurrency Without Threads]

 The Asio library offers side-by-side support for synchronous and asynchronous
 operations. The asynchronous support is based on the Proactor design pattern
 [link asio.overview.core.async.references \[POSA2\]]. The advantages and
 disadvantages of this approach, when compared to a synchronous-only or Reactor
 approach, are outlined below.

 [heading Proactor and Asio]

 Let us examine how the Proactor design pattern is implemented in Asio,
 without reference to platform-specific details.

 [$proactor.png]

 [*Proactor design pattern (adapted from \[POSA2\])]

 [mdash] Asynchronous Operation

 [:Defines an operation that is executed asynchronously, such as an asynchronous
 read or write on a socket.]

 [mdash] Asynchronous Operation Processor

 [:Executes asynchronous operations and queues events on a completion event
 queue when operations complete. From a high-level point of view, internal
 services like `reactive_socket_service` are asynchronous operation processors.]

 [mdash] Completion Event Queue

 [:Buffers completion events until they are dequeued by an asynchronous event
 demultiplexer.]

 [mdash] Completion Handler

 [:Processes the result of an asynchronous operation. These are function
 objects, often created using `boost::bind`.]

 [mdash] Asynchronous Event Demultiplexer

 [:Blocks waiting for events to occur on the completion event queue, and returns
 a completed event to its caller.]

 [mdash] Proactor

 [:Calls the asynchronous event demultiplexer to dequeue events, and dispatches
 the completion handler (i.e. invokes the function object) associated with the
 event. This abstraction is represented by the `io_context` class.]

 [mdash] Initiator

 [:Application-specific code that starts asynchronous operations. The initiator
 interacts with an asynchronous operation processor via a high-level interface
 such as `basic_stream_socket`, which in turn delegates to a service like
 `reactive_socket_service`.]

 [heading Implementation Using Reactor]

 On many platforms, Asio implements the Proactor design pattern in terms
 of a Reactor, such as `select`, `epoll` or `kqueue`. This implementation
 approach corresponds to the Proactor design pattern as follows:

 [mdash] Asynchronous Operation Processor

 [:A reactor implemented using `select`, `epoll` or `kqueue`. When the reactor
 indicates that the resource is ready to perform the operation, the processor
 executes the asynchronous operation and enqueues the associated completion
 handler on the completion event queue.]

 [mdash] Completion Event Queue

 [:A linked list of completion handlers (i.e. function objects).]

 [mdash] Asynchronous Event Demultiplexer

 [:This is implemented by waiting on an event or condition variable until a
 completion handler is available in the completion event queue.]

 [heading Implementation Using Windows Overlapped I/O]

 On Windows NT, 2000 and XP, Asio takes advantage of overlapped I/O to
 provide an efficient implementation of the Proactor design pattern. This
 implementation approach corresponds to the Proactor design pattern as follows:

 [mdash] Asynchronous Operation Processor

 [:This is implemented by the operating system. Operations are initiated by
 calling an overlapped function such as `AcceptEx`.]

 [mdash] Completion Event Queue

 [:This is implemented by the operating system, and is associated with an I/O
 completion port. There is one I/O completion port for each `io_context`
 instance.]

 [mdash] Asynchronous Event Demultiplexer

 [:Called by Asio to dequeue events and their associated completion
 handlers.]

 [heading Advantages]

 [mdash] Portability.

 [:Many operating systems offer a native asynchronous I/O API (such as
 overlapped I/O on __Windows__) as the preferred option for developing high
 performance network applications. The library may be implemented in terms of
 native asynchronous I/O. However, if native support is not available, the
 library may also be implemented using synchronous event demultiplexors that
 typify the Reactor pattern, such as __POSIX__ `select()`.]

 [mdash] Decoupling threading from concurrency.

 [:Long-duration operations are performed asynchronously by the implementation
 on behalf of the application. Consequently applications do not need to spawn
 many threads in order to increase concurrency.]

 [mdash] Performance and scalability.

 [:Implementation strategies such as thread-per-connection (which a
 synchronous-only approach would require) can degrade system performance, due to
 increased context switching, synchronisation and data movement among CPUs. With
 asynchronous operations it is possible to avoid the cost of context switching
 by minimising the number of operating system threads [mdash] typically a
 limited resource [mdash] and only activating the logical threads of control
 that have events to process.]

 [mdash] Simplified application synchronisation.

 [:Asynchronous operation completion handlers can be written as though they
 exist in a single-threaded environment, and so application logic can be
 developed with little or no concern for synchronisation issues.]

 [mdash] Function composition.

 [:Function composition refers to the implementation of functions to provide a
 higher-level operation, such as sending a message in a particular format. Each
 function is implemented in terms of multiple calls to lower-level read or write
 operations.]

 [:For example, consider a protocol where each message consists of a
 fixed-length header followed by a variable length body, where the length of the
 body is specified in the header. A hypothetical read_message operation could be
 implemented using two lower-level reads, the first to receive the header and,
 once the length is known, the second to receive the body.]

 [:To compose functions in an asynchronous model, asynchronous operations can be
 chained together. That is, a completion handler for one operation can initiate
 the next. Starting the first call in the chain can be encapsulated so that the
 caller need not be aware that the higher-level operation is implemented as a
 chain of asynchronous operations.]

 [:The ability to compose new operations in this way simplifies the development
 of higher levels of abstraction above a networking library, such as functions
 to support a specific protocol.]

 [heading Disadvantages]

 [mdash] Program complexity.

 [:It is more difficult to develop applications using asynchronous mechanisms
 due to the separation in time and space between operation initiation and
 completion. Applications may also be harder to debug due to the inverted flow
 of control.]

 [mdash] Memory usage.

 [:Buffer space must be committed for the duration of a read or write operation,
 which may continue indefinitely, and a separate buffer is required for each
 concurrent operation. The Reactor pattern, on the other hand, does not require
 buffer space until a socket is ready for reading or writing.]

 [heading References]

 \[POSA2\] D. Schmidt et al, ['Pattern Oriented Software Architecture, Volume
 2]. Wiley, 2000.

 [endsect]
	[/
	/ Copyright (c) 2003-2016 Christopher M. Kohlhoff (chris at kohlhoff dot com)
	/
	/ Distributed under the Boost Software License, Version 1.0. (See accompanying
	/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
	/]

	[section:async The Proactor Design Pattern: Concurrency Without Threads]

	The Asio library offers side-by-side support for synchronous and asynchronous
	operations. The asynchronous support is based on the Proactor design pattern
	[link asio.overview.core.async.references \[POSA2\]]. The advantages and
	disadvantages of this approach, when compared to a synchronous-only or Reactor
	approach, are outlined below.

	[heading Proactor and Asio]

	Let us examine how the Proactor design pattern is implemented in Asio,
	without reference to platform-specific details.

	[$proactor.png]

	[*Proactor design pattern (adapted from \[POSA2\])]

	[mdash] Asynchronous Operation

	[:Defines an operation that is executed asynchronously, such as an asynchronous
	read or write on a socket.]

	[mdash] Asynchronous Operation Processor

	[:Executes asynchronous operations and queues events on a completion event
	queue when operations complete. From a high-level point of view, internal
	services like `reactive_socket_service` are asynchronous operation processors.]

	[mdash] Completion Event Queue

	[:Buffers completion events until they are dequeued by an asynchronous event
	demultiplexer.]

	[mdash] Completion Handler

	[:Processes the result of an asynchronous operation. These are function
	objects, often created using `boost::bind`.]

	[mdash] Asynchronous Event Demultiplexer

	[:Blocks waiting for events to occur on the completion event queue, and returns
	a completed event to its caller.]

	[mdash] Proactor

	[:Calls the asynchronous event demultiplexer to dequeue events, and dispatches
	the completion handler (i.e. invokes the function object) associated with the
	event. This abstraction is represented by the `io_context` class.]

	[mdash] Initiator

	[:Application-specific code that starts asynchronous operations. The initiator
	interacts with an asynchronous operation processor via a high-level interface
	such as `basic_stream_socket`, which in turn delegates to a service like
	`reactive_socket_service`.]

	[heading Implementation Using Reactor]

	On many platforms, Asio implements the Proactor design pattern in terms
	of a Reactor, such as `select`, `epoll` or `kqueue`. This implementation
	approach corresponds to the Proactor design pattern as follows:

	[mdash] Asynchronous Operation Processor

	[:A reactor implemented using `select`, `epoll` or `kqueue`. When the reactor
	indicates that the resource is ready to perform the operation, the processor
	executes the asynchronous operation and enqueues the associated completion
	handler on the completion event queue.]

	[mdash] Completion Event Queue

	[:A linked list of completion handlers (i.e. function objects).]

	[mdash] Asynchronous Event Demultiplexer

	[:This is implemented by waiting on an event or condition variable until a
	completion handler is available in the completion event queue.]

	[heading Implementation Using Windows Overlapped I/O]

	On Windows NT, 2000 and XP, Asio takes advantage of overlapped I/O to
	provide an efficient implementation of the Proactor design pattern. This
	implementation approach corresponds to the Proactor design pattern as follows:

	[mdash] Asynchronous Operation Processor

	[:This is implemented by the operating system. Operations are initiated by
	calling an overlapped function such as `AcceptEx`.]

	[mdash] Completion Event Queue

	[:This is implemented by the operating system, and is associated with an I/O
	completion port. There is one I/O completion port for each `io_context`
	instance.]

	[mdash] Asynchronous Event Demultiplexer

	[:Called by Asio to dequeue events and their associated completion
	handlers.]

	[heading Advantages]

	[mdash] Portability.

	[:Many operating systems offer a native asynchronous I/O API (such as
	overlapped I/O on __Windows__) as the preferred option for developing high
	performance network applications. The library may be implemented in terms of
	native asynchronous I/O. However, if native support is not available, the
	library may also be implemented using synchronous event demultiplexors that
	typify the Reactor pattern, such as __POSIX__ `select()`.]

	[mdash] Decoupling threading from concurrency.

	[:Long-duration operations are performed asynchronously by the implementation
	on behalf of the application. Consequently applications do not need to spawn
	many threads in order to increase concurrency.]

	[mdash] Performance and scalability.

	[:Implementation strategies such as thread-per-connection (which a
	synchronous-only approach would require) can degrade system performance, due to
	increased context switching, synchronisation and data movement among CPUs. With
	asynchronous operations it is possible to avoid the cost of context switching
	by minimising the number of operating system threads [mdash] typically a
	limited resource [mdash] and only activating the logical threads of control
	that have events to process.]

	[mdash] Simplified application synchronisation.

	[:Asynchronous operation completion handlers can be written as though they
	exist in a single-threaded environment, and so application logic can be
	developed with little or no concern for synchronisation issues.]

	[mdash] Function composition.

	[:Function composition refers to the implementation of functions to provide a
	higher-level operation, such as sending a message in a particular format. Each
	function is implemented in terms of multiple calls to lower-level read or write
	operations.]

	[:For example, consider a protocol where each message consists of a
	fixed-length header followed by a variable length body, where the length of the
	body is specified in the header. A hypothetical read_message operation could be
	implemented using two lower-level reads, the first to receive the header and,
	once the length is known, the second to receive the body.]

	[:To compose functions in an asynchronous model, asynchronous operations can be
	chained together. That is, a completion handler for one operation can initiate
	the next. Starting the first call in the chain can be encapsulated so that the
	caller need not be aware that the higher-level operation is implemented as a
	chain of asynchronous operations.]

	[:The ability to compose new operations in this way simplifies the development
	of higher levels of abstraction above a networking library, such as functions
	to support a specific protocol.]

	[heading Disadvantages]

	[mdash] Program complexity.

	[:It is more difficult to develop applications using asynchronous mechanisms
	due to the separation in time and space between operation initiation and
	completion. Applications may also be harder to debug due to the inverted flow
	of control.]

	[mdash] Memory usage.

	[:Buffer space must be committed for the duration of a read or write operation,
	which may continue indefinitely, and a separate buffer is required for each
	concurrent operation. The Reactor pattern, on the other hand, does not require
	buffer space until a socket is ready for reading or writing.]

	[heading References]

	\[POSA2\] D. Schmidt et al, ['Pattern Oriented Software Architecture, Volume
	2]. Wiley, 2000.

	[endsect]