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<html>
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<title>The Proactor Design Pattern: Concurrency Without Threads</title>
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<div class="section" title="The Proactor Design Pattern: Concurrency Without Threads">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.async"></a><a class="link" href="async.html" title="The Proactor Design Pattern: Concurrency Without Threads"> The Proactor Design
Pattern: Concurrency Without Threads</a>
</h4></div></div></div>
<p>
The Boost.Asio library offers side-by-side support for synchronous and
asynchronous operations. The asynchronous support is based on the Proactor
design pattern <a class="link" href="async.html#boost_asio.overview.core.async.references">[POSA2]</a>.
The advantages and disadvantages of this approach, when compared to a synchronous-only
or Reactor approach, are outlined below.
</p>
<a name="boost_asio.overview.core.async.proactor_and_boost_asio"></a><h6>
<a name="id555401"></a>
<a class="link" href="async.html#boost_asio.overview.core.async.proactor_and_boost_asio">Proactor
and Boost.Asio</a>
</h6>
<p>
Let us examine how the Proactor design pattern is implemented in Boost.Asio,
without reference to platform-specific details.
</p>
<p>
<span class="inlinemediaobject"><img src="../../proactor.png" alt="proactor"></span>
</p>
<p>
<span class="bold"><strong>Proactor design pattern (adapted from [POSA2])</strong></span>
</p>
<p>
— Asynchronous Operation
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Defines an operation that is executed asynchronously, such as an asynchronous
read or write on a socket.
</p>
<p>
</p>
</blockquote></div>
<p>
— Asynchronous Operation Processor
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Executes asynchronous operations and queues events on a completion
event queue when operations complete. From a high-level point of view,
services like <code class="computeroutput"><span class="identifier">stream_socket_service</span></code>
are asynchronous operation processors.
</p>
<p>
</p>
</blockquote></div>
<p>
— Completion Event Queue
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Buffers completion events until they are dequeued by an asynchronous
event demultiplexer.
</p>
<p>
</p>
</blockquote></div>
<p>
— Completion Handler
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Processes the result of an asynchronous operation. These are function
objects, often created using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span></code>.
</p>
<p>
</p>
</blockquote></div>
<p>
— Asynchronous Event Demultiplexer
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Blocks waiting for events to occur on the completion event queue, and
returns a completed event to its caller.
</p>
<p>
</p>
</blockquote></div>
<p>
— Proactor
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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 <code class="computeroutput"><span class="identifier">io_service</span></code> class.
</p>
<p>
</p>
</blockquote></div>
<p>
— Initiator
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Application-specific code that starts asynchronous operations. The
initiator interacts with an asynchronous operation processor via a
high-level interface such as <code class="computeroutput"><span class="identifier">basic_stream_socket</span></code>,
which in turn delegates to a service like <code class="computeroutput"><span class="identifier">stream_socket_service</span></code>.
</p>
<p>
</p>
</blockquote></div>
<a name="boost_asio.overview.core.async.implementation_using_reactor"></a><h6>
<a name="id555625"></a>
<a class="link" href="async.html#boost_asio.overview.core.async.implementation_using_reactor">Implementation
Using Reactor</a>
</h6>
<p>
On many platforms, Boost.Asio implements the Proactor design pattern in
terms of a Reactor, such as <code class="computeroutput"><span class="identifier">select</span></code>,
<code class="computeroutput"><span class="identifier">epoll</span></code> or <code class="computeroutput"><span class="identifier">kqueue</span></code>. This implementation approach
corresponds to the Proactor design pattern as follows:
</p>
<p>
— Asynchronous Operation Processor
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
A reactor implemented using <code class="computeroutput"><span class="identifier">select</span></code>,
<code class="computeroutput"><span class="identifier">epoll</span></code> or <code class="computeroutput"><span class="identifier">kqueue</span></code>. 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.
</p>
<p>
</p>
</blockquote></div>
<p>
— Completion Event Queue
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
A linked list of completion handlers (i.e. function objects).
</p>
<p>
</p>
</blockquote></div>
<p>
— Asynchronous Event Demultiplexer
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
This is implemented by waiting on an event or condition variable until
a completion handler is available in the completion event queue.
</p>
<p>
</p>
</blockquote></div>
<a name="boost_asio.overview.core.async.implementation_using_windows_overlapped_i_o"></a><h6>
<a name="id555756"></a>
<a class="link" href="async.html#boost_asio.overview.core.async.implementation_using_windows_overlapped_i_o">Implementation
Using Windows Overlapped I/O</a>
</h6>
<p>
On Windows NT, 2000 and XP, Boost.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:
</p>
<p>
— Asynchronous Operation Processor
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
This is implemented by the operating system. Operations are initiated
by calling an overlapped function such as <code class="computeroutput"><span class="identifier">AcceptEx</span></code>.
</p>
<p>
</p>
</blockquote></div>
<p>
— Completion Event Queue
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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
<code class="computeroutput"><span class="identifier">io_service</span></code> instance.
</p>
<p>
</p>
</blockquote></div>
<p>
— Asynchronous Event Demultiplexer
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Called by Boost.Asio to dequeue events and their associated completion
handlers.
</p>
<p>
</p>
</blockquote></div>
<a name="boost_asio.overview.core.async.advantages"></a><h6>
<a name="id555851"></a>
<a class="link" href="async.html#boost_asio.overview.core.async.advantages">Advantages</a>
</h6>
<p>
— Portability.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Many operating systems offer a native asynchronous I/O API (such as
overlapped I/O on <span class="emphasis"><em>Windows</em></span>) 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 <span class="emphasis"><em>POSIX</em></span> <code class="computeroutput"><span class="identifier">select</span><span class="special">()</span></code>.
</p>
<p>
</p>
</blockquote></div>
<p>
— Decoupling threading from concurrency.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<p>
— Performance and scalability.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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 — typically
a limited resource — and only activating the logical threads of control
that have events to process.
</p>
<p>
</p>
</blockquote></div>
<p>
— Simplified application synchronisation.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<p>
— Function composition.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<a name="boost_asio.overview.core.async.disadvantages"></a><h6>
<a name="id556030"></a>
<a class="link" href="async.html#boost_asio.overview.core.async.disadvantages">Disadvantages</a>
</h6>
<p>
— Program complexity.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<p>
— Memory usage.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
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.
</p>
<p>
</p>
</blockquote></div>
<a name="boost_asio.overview.core.async.references"></a><h6>
<a name="id556088"></a>
<a class="link" href="async.html#boost_asio.overview.core.async.references">References</a>
</h6>
<p>
[POSA2] D. Schmidt et al, <span class="emphasis"><em>Pattern Oriented Software Architecture,
Volume 2</em></span>. Wiley, 2000.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003 - 2010 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
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