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Module: Concurrent::Async
— Concurrent Ruby
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<div id="content"><h1>Module: Concurrent::Async
</h1>
<div class="box_info">
<dl>
<dt>Defined in:</dt>
<dd>lib/concurrent-ruby/concurrent/async.rb</dd>
</dl>
</div>
<h2>Overview</h2><div class="docstring">
<div class="discussion">
<p>A mixin module that provides simple asynchronous behavior to a class,
turning it into a simple actor. Loosely based on Erlang's
<a href="http://www.erlang.org/doc/man/gen_server.html">gen_server</a>, but without
supervision or linking.</p>
<p>A more feature-rich <span class='object_link'><a href="Actor.html" title="Concurrent::Actor (module)">Actor</a></span> is also available when the
capabilities of <code>Async</code> are too limited.</p>
<pre class="code cucumber"><code class="cucumber">Feature:
As a stateful, plain old Ruby class
I want safe, asynchronous behavior
So my long-running methods don't block the main thread
</code></pre>
<p>The <code>Async</code> module is a way to mix simple yet powerful asynchronous
capabilities into any plain old Ruby object or class, turning each object
into a simple Actor. Method calls are processed on a background thread. The
caller is free to perform other actions while processing occurs in the
background.</p>
<p>Method calls to the asynchronous object are made via two proxy methods:
<code>async</code> (alias <code>cast</code>) and <code>await</code> (alias <code>call</code>). These proxy methods post
the method call to the object's background thread and return a "future"
which will eventually contain the result of the method call.</p>
<p>This behavior is loosely patterned after Erlang's <code>gen_server</code> behavior.
When an Erlang module implements the <code>gen_server</code> behavior it becomes
inherently asynchronous. The <code>start</code> or <code>start_link</code> function spawns a
process (similar to a thread but much more lightweight and efficient) and
returns the ID of the process. Using the process ID, other processes can
send messages to the <code>gen_server</code> via the <code>cast</code> and <code>call</code> methods. Unlike
Erlang's <code>gen_server</code>, however, <code>Async</code> classes do not support linking or
supervision trees.</p>
<h2>Basic Usage</h2>
<p>When this module is mixed into a class, objects of the class become inherently
asynchronous. Each object gets its own background thread on which to post
asynchronous method calls. Asynchronous method calls are executed in the
background one at a time in the order they are received.</p>
<p>To create an asynchronous class, simply mix in the <code>Concurrent::Async</code> module:</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>class</span> <span class='const'>Hello</span>
<span class='id identifier rubyid_include'>include</span> <span class='const'><span class='object_link'><a href="../Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'>Async</span>
<span class='kw'>def</span> <span class='id identifier rubyid_hello'>hello</span><span class='lparen'>(</span><span class='id identifier rubyid_name'>name</span><span class='rparen'>)</span>
<span class='tstring'><span class='tstring_beg'>"</span><span class='tstring_content'>Hello, </span><span class='embexpr_beg'>#{</span><span class='id identifier rubyid_name'>name</span><span class='embexpr_end'>}</span><span class='tstring_content'>!</span><span class='tstring_end'>"</span></span>
<span class='kw'>end</span>
<span class='kw'>end</span>
</code></pre>
<p>When defining a constructor it is critical that the first line be a call to
<code>super</code> with no arguments. The <code>super</code> method initializes the background
thread and other asynchronous components.</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>class</span> <span class='const'>BackgroundLogger</span>
<span class='id identifier rubyid_include'>include</span> <span class='const'><span class='object_link'><a href="../Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'>Async</span>
<span class='kw'>def</span> <span class='id identifier rubyid_initialize'>initialize</span><span class='lparen'>(</span><span class='id identifier rubyid_level'>level</span><span class='rparen'>)</span>
<span class='kw'>super</span><span class='lparen'>(</span><span class='rparen'>)</span>
<span class='ivar'>@logger</span> <span class='op'>=</span> <span class='const'>Logger</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='const'>STDOUT</span><span class='rparen'>)</span>
<span class='ivar'>@logger</span><span class='period'>.</span><span class='id identifier rubyid_level'>level</span> <span class='op'>=</span> <span class='id identifier rubyid_level'>level</span>
<span class='kw'>end</span>
<span class='kw'>def</span> <span class='id identifier rubyid_info'>info</span><span class='lparen'>(</span><span class='id identifier rubyid_msg'>msg</span><span class='rparen'>)</span>
<span class='ivar'>@logger</span><span class='period'>.</span><span class='id identifier rubyid_info'>info</span><span class='lparen'>(</span><span class='id identifier rubyid_msg'>msg</span><span class='rparen'>)</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
</code></pre>
<p>Mixing this module into a class provides each object two proxy methods:
<code>async</code> and <code>await</code>. These methods are thread safe with respect to the
enclosing object. The former proxy allows methods to be called
asynchronously by posting to the object's internal thread. The latter proxy
allows a method to be called synchronously but does so safely with respect
to any pending asynchronous method calls and ensures proper ordering. Both
methods return a <span class='object_link'><a href="IVar.html" title="Concurrent::IVar (class)">IVar</a></span> which can be inspected for the result
of the proxied method call. Calling a method with <code>async</code> will return a
<code>:pending</code> <code>IVar</code> whereas <code>await</code> will return a <code>:complete</code> <code>IVar</code>.</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>class</span> <span class='const'>Echo</span>
<span class='id identifier rubyid_include'>include</span> <span class='const'><span class='object_link'><a href="../Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'>Async</span>
<span class='kw'>def</span> <span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='id identifier rubyid_msg'>msg</span><span class='rparen'>)</span>
<span class='id identifier rubyid_print'>print</span> <span class='tstring'><span class='tstring_beg'>"</span><span class='embexpr_beg'>#{</span><span class='id identifier rubyid_msg'>msg</span><span class='embexpr_end'>}</span><span class='tstring_content'>\n</span><span class='tstring_end'>"</span></span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_horn'>horn</span> <span class='op'>=</span> <span class='const'>Echo</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span>
<span class='id identifier rubyid_horn'>horn</span><span class='period'>.</span><span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>zero</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='comment'># synchronous, not thread-safe
</span> <span class='comment'># returns the actual return value of the method
</span>
<span class='id identifier rubyid_horn'>horn</span><span class='period'>.</span><span class='id identifier rubyid_async'>async</span><span class='period'>.</span><span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>one</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='comment'># asynchronous, non-blocking, thread-safe
</span> <span class='comment'># returns an IVar in the :pending state
</span>
<span class='id identifier rubyid_horn'>horn</span><span class='period'>.</span><span class='id identifier rubyid_await'>await</span><span class='period'>.</span><span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>two</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='comment'># synchronous, blocking, thread-safe
</span> <span class='comment'># returns an IVar in the :complete state
</span></code></pre>
<h2>Let It Fail</h2>
<p>The <code>async</code> and <code>await</code> proxy methods have built-in error protection based
on Erlang's famous "let it fail" philosophy. Instance methods should not be
programmed defensively. When an exception is raised by a delegated method
the proxy will rescue the exception, expose it to the caller as the <code>reason</code>
attribute of the returned future, then process the next method call.</p>
<h2>Calling Methods Internally</h2>
<p>External method calls should <em>always</em> use the <code>async</code> and <code>await</code> proxy
methods. When one method calls another method, the <code>async</code> proxy should
rarely be used and the <code>await</code> proxy should <em>never</em> be used.</p>
<p>When an object calls one of its own methods using the <code>await</code> proxy the
second call will be enqueued <em>behind</em> the currently running method call.
Any attempt to wait on the result will fail as the second call will never
run until after the current call completes.</p>
<p>Calling a method using the <code>await</code> proxy from within a method that was
itself called using <code>async</code> or <code>await</code> will irreversibly deadlock the
object. Do <em>not</em> do this, ever.</p>
<h2>Instance Variables and Attribute Accessors</h2>
<p>Instance variables do not need to be thread-safe so long as they are private.
Asynchronous method calls are processed in the order they are received and
are processed one at a time. Therefore private instance variables can only
be accessed by one thread at a time. This is inherently thread-safe.</p>
<p>When using private instance variables within asynchronous methods, the best
practice is to read the instance variable into a local variable at the start
of the method then update the instance variable at the <em>end</em> of the method.
This way, should an exception be raised during method execution the internal
state of the object will not have been changed.</p>
<h3>Reader Attributes</h3>
<p>The use of <code>attr_reader</code> is discouraged. Internal state exposed externally,
when necessary, should be done through accessor methods. The instance
variables exposed by these methods <em>must</em> be thread-safe, or they must be
called using the <code>async</code> and <code>await</code> proxy methods. These two approaches are
subtly different.</p>
<p>When internal state is accessed via the <code>async</code> and <code>await</code> proxy methods,
the returned value represents the object's state <em>at the time the call is
processed</em>, which may <em>not</em> be the state of the object at the time the call
is made.</p>
<p>To get the state <em>at the current</em> time, irrespective of an enqueued method
calls, a reader method must be called directly. This is inherently unsafe
unless the instance variable is itself thread-safe, preferably using one
of the thread-safe classes within this library. Because the thread-safe
classes within this library are internally-locking or non-locking, they can
be safely used from within asynchronous methods without causing deadlocks.</p>
<p>Generally speaking, the best practice is to <em>not</em> expose internal state via
reader methods. The best practice is to simply use the method's return value.</p>
<h3>Writer Attributes</h3>
<p>Writer attributes should never be used with asynchronous classes. Changing
the state externally, even when done in the thread-safe way, is not logically
consistent. Changes to state need to be timed with respect to all asynchronous
method calls which my be in-process or enqueued. The only safe practice is to
pass all necessary data to each method as arguments and let the method update
the internal state as necessary.</p>
<h2>Class Constants, Variables, and Methods</h2>
<h3>Class Constants</h3>
<p>Class constants do not need to be thread-safe. Since they are read-only and
immutable they may be safely read both externally and from within
asynchronous methods.</p>
<h3>Class Variables</h3>
<p>Class variables should be avoided. Class variables represent shared state.
Shared state is anathema to concurrency. Should there be a need to share
state using class variables they <em>must</em> be thread-safe, preferably
using the thread-safe classes within this library. When updating class
variables, never assign a new value/object to the variable itself. Assignment
is not thread-safe in Ruby. Instead, use the thread-safe update functions
of the variable itself to change the value.</p>
<p>The best practice is to <em>never</em> use class variables with <code>Async</code> classes.</p>
<h3>Class Methods</h3>
<p>Class methods which are pure functions are safe. Class methods which modify
class variables should be avoided, for all the reasons listed above.</p>
<h2>An Important Note About Thread Safe Guarantees</h2>
<blockquote>
<p>Thread safe guarantees can only be made when asynchronous method calls
are not mixed with direct method calls. Use only direct method calls
when the object is used exclusively on a single thread. Use only
<code>async</code> and <code>await</code> when the object is shared between threads. Once you
call a method using <code>async</code> or <code>await</code>, you should no longer call methods
directly on the object. Use <code>async</code> and <code>await</code> exclusively from then on.</p>
</blockquote>
</div>
</div>
<div class="tags">
<div class="examples">
<p class="tag_title">Examples:</p>
<pre class="example code"><code>
<span class='kw'>class</span> <span class='const'>Echo</span>
<span class='id identifier rubyid_include'>include</span> <span class='const'><span class='object_link'><a href="../Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'>Async</span>
<span class='kw'>def</span> <span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='id identifier rubyid_msg'>msg</span><span class='rparen'>)</span>
<span class='id identifier rubyid_print'>print</span> <span class='tstring'><span class='tstring_beg'>"</span><span class='embexpr_beg'>#{</span><span class='id identifier rubyid_msg'>msg</span><span class='embexpr_end'>}</span><span class='tstring_content'>\n</span><span class='tstring_end'>"</span></span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_horn'>horn</span> <span class='op'>=</span> <span class='const'>Echo</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span>
<span class='id identifier rubyid_horn'>horn</span><span class='period'>.</span><span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>zero</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='comment'># synchronous, not thread-safe
</span> <span class='comment'># returns the actual return value of the method
</span>
<span class='id identifier rubyid_horn'>horn</span><span class='period'>.</span><span class='id identifier rubyid_async'>async</span><span class='period'>.</span><span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>one</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='comment'># asynchronous, non-blocking, thread-safe
</span> <span class='comment'># returns an IVar in the :pending state
</span>
<span class='id identifier rubyid_horn'>horn</span><span class='period'>.</span><span class='id identifier rubyid_await'>await</span><span class='period'>.</span><span class='id identifier rubyid_echo'>echo</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>two</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='comment'># synchronous, blocking, thread-safe
</span> <span class='comment'># returns an IVar in the :complete state</span></code></pre>
</div>
<p class="tag_title">See Also:</p>
<ul class="see">
<li><span class='object_link'><a href="Actor.html" title="Concurrent::Actor (module)">Actor</a></span></li>
<li><a href="https://en.wikipedia.org/wiki/Actor_model" target="_parent" title=""Actor Model" at Wikipedia">"Actor Model" at Wikipedia</a></li>
<li><a href="http://www.erlang.org/doc/man/gen_server.html" target="_parent" title="Erlang gen_server">Erlang gen_server</a></li>
<li><a href="http://c2.com/cgi/wiki?LetItCrash" target="_parent" title=""Let It Crash" at http://c2.com/">"Let It Crash" at http://c2.com/</a></li>
</ul>
</div>
<h2>
Class Method Summary
<small><a href="#" class="summary_toggle">collapse</a></small>
</h2>
<ul class="summary">
<li class="public ">
<span class="summary_signature">
<a href="#new-class_method" title="new (class method)">.<strong>new</strong>(*args, &block) ⇒ Object </a>
</span>
<span class="summary_desc"><div class='inline'><p>Instanciate a new object and ensure proper initialization of the synchronization mechanisms.</p>
</div></span>
</li>
</ul>
<h2>
Instance Method Summary
<small><a href="#" class="summary_toggle">collapse</a></small>
</h2>
<ul class="summary">
<li class="public ">
<span class="summary_signature">
<a href="#async-instance_method" title="#async (instance method)">#<strong>async</strong> ⇒ Concurrent::IVar </a>
(also: #cast)
</span>
<span class="summary_desc"><div class='inline'><p>Causes the chained method call to be performed asynchronously on the object's thread.</p>
</div></span>
</li>
<li class="public ">
<span class="summary_signature">
<a href="#await-instance_method" title="#await (instance method)">#<strong>await</strong> ⇒ Concurrent::IVar </a>
(also: #call)
</span>
<span class="summary_desc"><div class='inline'><p>Causes the chained method call to be performed synchronously on the current thread.</p>
</div></span>
</li>
</ul>
<div id="class_method_details" class="method_details_list">
<h2>Class Method Details</h2>
<div class="method_details first">
<h3 class="signature first" id="new-class_method">
.<strong>new</strong>(*args, &block) ⇒ <tt>Object</tt>
</h3><div class="docstring">
<div class="discussion">
<p>Instanciate a new object and ensure proper initialization of the
synchronization mechanisms.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Parameters:</p>
<ul class="param">
<li>
<span class='name'>args</span>
<span class='type'>(<tt><span class='object_link'><a href="Array.html" title="Concurrent::Array (class)">Array</a></span><Object></tt>)</span>
—
<div class='inline'><p>Zero or more arguments to be passed to the
object's initializer.</p>
</div>
</li>
<li>
<span class='name'>block</span>
<span class='type'>(<tt>Proc</tt>)</span>
—
<div class='inline'><p>Optional block to pass to the object's initializer.</p>
</div>
</li>
</ul>
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt>Object</tt>)</span>
—
<div class='inline'><p>A properly initialized object of the asynchronous class.</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent-ruby/concurrent/async.rb', line 239</span>
</pre>
</td>
</tr>
</table>
</div>
</div>
<div id="instance_method_details" class="method_details_list">
<h2>Instance Method Details</h2>
<div class="method_details first">
<h3 class="signature first" id="async-instance_method">
#<strong>async</strong> ⇒ <tt><span class='object_link'><a href="IVar.html" title="Concurrent::IVar (class)">Concurrent::IVar</a></span></tt>
<span class="aliases">Also known as:
<span class="names"><span id='cast-instance_method'>cast</span></span>
</span>
</h3><div class="docstring">
<div class="discussion">
<div class="note notetag">
<strong>Note:</strong>
<div class='inline'><p>The method call is guaranteed to be thread safe with respect to
all other method calls against the same object that are called with
either <code>async</code> or <code>await</code>. The mutable nature of Ruby references
(and object orientation in general) prevent any other thread safety
guarantees. Do NOT mix direct method calls with delegated method calls.
Use <em>only</em> delegated method calls when sharing the object between threads.</p>
</div>
</div>
<p>Causes the chained method call to be performed asynchronously on the
object's thread. The delegated method will return a future in the
<code>:pending</code> state and the method call will have been scheduled on the
object's thread. The final disposition of the method call can be obtained
by inspecting the returned future.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt><span class='object_link'><a href="IVar.html" title="Concurrent::IVar (class)">Concurrent::IVar</a></span></tt>)</span>
—
<div class='inline'><p>the pending result of the asynchronous operation</p>
</div>
</li>
</ul>
<p class="tag_title">Raises:</p>
<ul class="raise">
<li>
<span class='type'>(<tt>NameError</tt>)</span>
—
<div class='inline'><p>the object does not respond to the requested method</p>
</div>
</li>
<li>
<span class='type'>(<tt>ArgumentError</tt>)</span>
—
<div class='inline'><p>the given <code>args</code> do not match the arity of
the requested method</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
422
423
424</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent-ruby/concurrent/async.rb', line 422</span>
<span class='kw'>def</span> <span class='id identifier rubyid_async'>async</span>
<span class='ivar'>@__async_delegator__</span>
<span class='kw'>end</span></pre>
</td>
</tr>
</table>
</div>
<div class="method_details ">
<h3 class="signature " id="await-instance_method">
#<strong>await</strong> ⇒ <tt><span class='object_link'><a href="IVar.html" title="Concurrent::IVar (class)">Concurrent::IVar</a></span></tt>
<span class="aliases">Also known as:
<span class="names"><span id='call-instance_method'>call</span></span>
</span>
</h3><div class="docstring">
<div class="discussion">
<div class="note notetag">
<strong>Note:</strong>
<div class='inline'><p>The method call is guaranteed to be thread safe with respect to
all other method calls against the same object that are called with
either <code>async</code> or <code>await</code>. The mutable nature of Ruby references
(and object orientation in general) prevent any other thread safety
guarantees. Do NOT mix direct method calls with delegated method calls.
Use <em>only</em> delegated method calls when sharing the object between threads.</p>
</div>
</div>
<p>Causes the chained method call to be performed synchronously on the
current thread. The delegated will return a future in either the
<code>:fulfilled</code> or <code>:rejected</code> state and the delegated method will have
completed. The final disposition of the delegated method can be obtained
by inspecting the returned future.</p>
</div>
</div>
<div class="tags">
<p class="tag_title">Returns:</p>
<ul class="return">
<li>
<span class='type'>(<tt><span class='object_link'><a href="IVar.html" title="Concurrent::IVar (class)">Concurrent::IVar</a></span></tt>)</span>
—
<div class='inline'><p>the completed result of the synchronous operation</p>
</div>
</li>
</ul>
<p class="tag_title">Raises:</p>
<ul class="raise">
<li>
<span class='type'>(<tt>NameError</tt>)</span>
—
<div class='inline'><p>the object does not respond to the requested method</p>
</div>
</li>
<li>
<span class='type'>(<tt>ArgumentError</tt>)</span>
—
<div class='inline'><p>the given <code>args</code> do not match the arity of the
requested method</p>
</div>
</li>
</ul>
</div><table class="source_code">
<tr>
<td>
<pre class="lines">
440
441
442</pre>
</td>
<td>
<pre class="code"><span class="info file"># File 'lib/concurrent-ruby/concurrent/async.rb', line 440</span>
<span class='kw'>def</span> <span class='id identifier rubyid_await'>await</span>
<span class='ivar'>@__await_delegator__</span>
<span class='kw'>end</span></pre>
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