[/
 / Copyright (c) 2003-2020 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]
