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authorgdamore <gdamore@users.noreply.github.com>2024-10-27 18:55:51 +0000
committergdamore <gdamore@users.noreply.github.com>2024-10-27 18:55:51 +0000
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+ <ol class="chapter"><li class="chapter-item expanded affix "><a href="../copyright.html">Copyright</a></li><li class="chapter-item expanded affix "><a href="../dedication.html">Dedication</a></li><li class="chapter-item expanded affix "><a href="../preface.html">Preface</a></li><li class="chapter-item expanded "><a href="../api/index.html"><strong aria-hidden="true">1.</strong> API Reference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../api/msg.html"><strong aria-hidden="true">1.1.</strong> Messages</a></li><li class="chapter-item expanded "><a href="../api/memory.html"><strong aria-hidden="true">1.2.</strong> Memory</a></li><li class="chapter-item expanded "><a href="../api/time.html"><strong aria-hidden="true">1.3.</strong> Time</a></li><li class="chapter-item expanded "><a href="../api/url.html"><strong aria-hidden="true">1.4.</strong> URLs</a></li><li class="chapter-item expanded "><a href="../api/aio.html" class="active"><strong aria-hidden="true">1.5.</strong> Asynchronous I/O</a></li><li class="chapter-item expanded "><a href="../api/synch.html"><strong aria-hidden="true">1.6.</strong> Synchronization</a></li><li class="chapter-item expanded "><a href="../api/thread.html"><strong aria-hidden="true">1.7.</strong> Threads</a></li><li class="chapter-item expanded "><a href="../api/logging.html"><strong aria-hidden="true">1.8.</strong> Logging</a></li><li class="chapter-item expanded "><a href="../api/stats.html"><strong aria-hidden="true">1.9.</strong> Statistics</a></li><li class="chapter-item expanded "><a href="../api/errors.html"><strong aria-hidden="true">1.10.</strong> Errors</a></li><li class="chapter-item expanded "><a href="../api/misc.html"><strong aria-hidden="true">1.11.</strong> Miscellaneous</a></li><li class="chapter-item expanded "><a href="../api/id_map.html"><strong aria-hidden="true">1.12.</strong> ID Map</a></li><li class="chapter-item expanded "><a href="../api/cmd_opts.html"><strong aria-hidden="true">1.13.</strong> Command Options</a></li></ol></li><li class="chapter-item expanded "><a href="../proto/index.html"><strong aria-hidden="true">2.</strong> Protocols</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../proto/bus.html"><strong aria-hidden="true">2.1.</strong> BUS Protocol</a></li><li class="chapter-item expanded "><a href="../proto/pair.html"><strong aria-hidden="true">2.2.</strong> PAIR Protocol</a></li><li class="chapter-item expanded "><a href="../proto/pub.html"><strong aria-hidden="true">2.3.</strong> PUB Protocol</a></li><li class="chapter-item expanded "><a href="../proto/pull.html"><strong aria-hidden="true">2.4.</strong> PULL Protocol</a></li><li class="chapter-item expanded "><a href="../proto/push.html"><strong aria-hidden="true">2.5.</strong> PUSH Protocol</a></li><li class="chapter-item expanded "><a href="../proto/rep.html"><strong aria-hidden="true">2.6.</strong> REP Protocol</a></li><li class="chapter-item expanded "><a href="../proto/req.html"><strong aria-hidden="true">2.7.</strong> REQ Protocol</a></li><li class="chapter-item expanded "><a href="../proto/respondent.html"><strong aria-hidden="true">2.8.</strong> RESPONDENT Protocol</a></li><li class="chapter-item expanded "><a href="../proto/sub.html"><strong aria-hidden="true">2.9.</strong> SUB Protocol</a></li><li class="chapter-item expanded "><a href="../proto/surveyor.html"><strong aria-hidden="true">2.10.</strong> SURVEYOR Protocol</a></li></ol></li><li class="chapter-item expanded "><a href="../tran/index.html"><strong aria-hidden="true">3.</strong> Transports</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tran/inproc.html"><strong aria-hidden="true">3.1.</strong> Intra-Process Transport</a></li><li class="chapter-item expanded "><a href="../tran/ipc.html"><strong aria-hidden="true">3.2.</strong> Inter-Process Transport</a></li><li class="chapter-item expanded "><a href="../tran/socket.html"><strong aria-hidden="true">3.3.</strong> BSD Socket (Experimental)</a></li><li class="chapter-item expanded "><a href="../tran/udp.html"><strong aria-hidden="true">3.4.</strong> UDP Transport (Experimental)</a></li></ol></li><li class="chapter-item expanded "><a href="../migrate/index.html"><strong aria-hidden="true">4.</strong> Migration Guides</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../migrate/nng1.html"><strong aria-hidden="true">4.1.</strong> Migrating from NNG 1.x</a></li><li class="chapter-item expanded "><a href="../migrate/nanomsg.html"><strong aria-hidden="true">4.2.</strong> Migrating from libnanomsg</a></li></ol></li><li class="chapter-item expanded "><a href="../indexing.html">Index</a></li></ol>
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+<h1 id="asynchronous-operations"><a class="header" href="#asynchronous-operations">Asynchronous Operations</a></h1>
+<p>In order to obtain significant scalability, with low-latency, and minimal
+overheads, <em>NNG</em> supports performing operations asynchronously.</p>
+<p>One way that applications can perform work asynchronously and concurrently
+is by using <a href="/api/thread.html">threads</a>, but threads carry significant resource overheads
+and frequently there are limits on the number that can easily be created.</p>
+<p>Additionally, with most network applications, the flow of execution will spend
+the bulk of its time waiting for traffic from a peer.</p>
+<p>For these kinds of applications, it is far more efficient to use asynchronous operations
+using the mechanisms described in this chapter.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>To get the highest performance with the least overhead, applications should use
+asynchronous operations described in this chapter whenever possible.</p>
+</div>
+<h2 id="asynchronous-io-handle"><a class="header" href="#asynchronous-io-handle">Asynchronous I/O Handle</a></h2>
+<pre><code class="language-c">typedef struct nng_aio nng_aio;
+</code></pre>
+<p>An <a name="a001"></a><code>nng_aio</code><a name="a002"></a> is an opaque structure used in conjunction with
+<a name="a003"></a>asynchronous I/O operations.
+Every asynchronous operation uses one of these structures, each of which
+can only be used with a single operation at a time.</p>
+<p>Asynchronous operations are performed without blocking calling application threads.
+Instead the application registers a callback function to be executed
+when the operation is complete (whether successfully or not).
+This callback will be executed exactly once.</p>
+<p>The asynchronous I/O framework also supports <a href="/api/aio.html#cancellation">cancellation</a> of
+operations that are already in progress as well setting a maximum
+<a href="/TODO.html">timeout</a> for them to complete.</p>
+<p>It is also possible to initiate an asynchronous operation, and <a href="/api/api.html#wait-for-completion">wait</a> for it to
+complete, creating a synchronous flow from an asynchronous one.</p>
+<h2 id="create-handle"><a class="header" href="#create-handle">Create Handle</a></h2>
+<pre><code class="language-c">int nng_aio_alloc(nng_aio **aiop, void (*callb)(void *), void *arg);
+</code></pre>
+<p>The <a name="a004"></a><code>nng_aio_alloc</code> function creates an <a href="/TODO.html"><code>nng_aio</code></a> object, with the
+<a name="a005"></a>callback <em>callb</em> taking the argument <em>arg</em>, and returns it in <em>aiop</em>.</p>
+<p>If this succeeds, the function returns zero. Otherwise it may return <a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a>.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>The <em>arg</em> should normally be a structure that contains a pointer to the <em>aiop</em>,
+or from which it can be located. This allows <em>callb</em> to check the handle for
+success using <a href="/api/api.html#result-of-operation"><code>nng_aio_result</code></a>, as well access other properties of <em>aiop</em>.</p>
+</div>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>The <em>callb</em> may be executed on another <a href="/api/thread.html">thread</a>, so it may be necessary to use
+<a href="/api/synch.html">synchronization</a> methods in <em>callb</em> to avoid data races.</p>
+</div>
+<h2 id="destroy-handle"><a class="header" href="#destroy-handle">Destroy Handle</a></h2>
+<pre><code class="language-c">void nng_aio_free(nng_aio *aio);
+void nng_aio_reap(nng_aio *aio);
+</code></pre>
+<p>The <a name="a006"></a><code>nng_aio_free</code> handle destroys the handle <em>aio</em>, waiting for any operations
+and associated callbacks to complete before doing so.</p>
+<p>The <a name="a007"></a><code>nng_aio_reap</code> handle destroys the handle <em>aio</em> asynchronously, using a <em>reaper</em>
+<a href="/api/thread.html">thread</a> to do so. It does not wait for the object to be destroyed. Thus this function
+is safe to call from <em>aio</em>’s own callback.</p>
+<div class="mdbook-alerts mdbook-alerts-note">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ note
+</p>
+<p>The <code>nng_aio_free</code> function must never be called from an <em>aio</em> callback.
+Use <code>nng_aio_reap</code> instead if an object must be destroyed from a callback.</p>
+</div>
+<h2 id="cancellation"><a class="header" href="#cancellation">Cancellation</a></h2>
+<pre><code class="language-c">void nng_aio_abort(nng_aio *aio, int err);
+void nng_aio_cancel(nng_aio *aio);
+void nng_aio_stop(nng_aio *aio);
+</code></pre>
+<p>These functions are used to stop a previously submitted asynchronous
+I/O operation. The operation may be canceled, or may continue to
+completion. If no operation is in progress (perhaps because it has
+already completed), then these operations have no effect.
+If the operation is successfully canceled or aborted, then the callback
+will still be called.</p>
+<p>The <a name="a008"></a><code>nng_aio_abort</code> function aborts the operation associated with <em>aio</em>
+and returns immediately without waiting. If cancellation was successful,
+then <a href="/api/api.html#result-of-operation"><code>nng_aio_result</code></a> will return <em>err</em>.</p>
+<p>The <a name="a009"></a><code>nng_aio_cancel</code> function acts like <code>nng_aio_abort</code>, but uses the error code
+<a href="/api/errors.html#NNG_ECANCELED"><code>NNG_ECANCELED</code></a><a name="a010"></a>.</p>
+<p>The <a name="a011"></a><code>nng_aio_stop</code> function aborts the <em>aio</em> operation with <a href="/api/errors.html#NNG_ECANCELED"><code>NNG_ECANCELED</code></a>,
+and then waits the operation and any associated callback to complete.
+This function also marks <em>aio</em> itself permanently stopped, so that any
+new operations scheduled by I/O providers using <a href="/TODO.html"><code>nng_aio_begin</code></a>
+return false. Thus this function should be used to teardown operations.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>When multiple asynchronous I/O handles are in use and need to be
+deallocated, it is safest to stop all of them using <code>nng_aio_stop</code>,
+before deallocating any of them with <a href="/api/aio.html#destroy-handle"><code>nng_aio_free</code></a>,
+particularly if the callbacks might attempt to reschedule further operations.</p>
+</div>
+<h2 id="set-timeout"><a class="header" href="#set-timeout">Set Timeout</a></h2>
+<pre><code class="language-c">void nng_aio_set_timeout(nng_aio *aio, nng_duration timeout);
+void nng_aio_set_expire(nng_aio *aio, nng_time expiration);
+</code></pre>
+<p>The <code>nng_aio_set_timeout</code> function sets a <a name="a012"></a><em>timeout</em>
+for the asynchronous operation associated with <em>aio</em>.
+This causes a timer to be started when the operation is actually started.
+If the timer expires before the operation is completed, then it is
+<a href="/api/aio.html#cancellation">aborted</a> with an error of <code>NNG_ETIMEDOUT</code>.
+The <em>timeout</em> <a href="/api/time.html">duration</a> is specified as a relative number of milliseconds.</p>
+<p>If the timeout is <a href="/api/time.html#duration-type"><code>NNG_DURATION_INFINITE</code></a>, then no timeout is used.
+If the timeout is <a href="/api/time.html#duration-type"><code>NNG_DURATION_DEFAULT</code></a>, then a “default” or socket-specific
+timeout is used.
+(This is frequently the same as <a href="/api/time.html#duration-type"><code>NNG_DURATION_INFINITE</code></a>.)</p>
+<p>The <a name="a013"></a><code>nng_aio_set_expire</code> function is similar to <code>nng_aio_set_timeout</code>, but sets
+an expiration time based on the system clock. The <em>expiration</em>
+<a href="/api/time.html">time</a> is a clock timestamp, such as would be returned by <a href="/api/time.html#get-the-current-time"><code>nng_clock</code></a>.</p>
+<h2 id="wait-for-completion"><a class="header" href="#wait-for-completion">Wait for Completion</a></h2>
+<pre><code class="language-c">void nng_aio_wait(nng_aio *aio);
+</code></pre>
+<p>The <a name="a014"></a><code>nng_aio_wait</code> function waits for an asynchronous I/O operation to complete.
+If the operation has not been started, or has already completed, then it returns immediately.</p>
+<p>If a callback was set with <em>aio</em> when it was allocated, then this
+function will not be called until the callback has completed.</p>
+<div class="mdbook-alerts mdbook-alerts-important">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ important
+</p>
+<p>The <code>nng_aio_wait</code> function should never be called from a function that itself
+is a callback of an <a href="/TODO.html"><code>nng_aio</code></a>, either this one or any other.
+Doing so may result in a deadlock.</p>
+</div>
+<h2 id="test-for-completion"><a class="header" href="#test-for-completion">Test for Completion</a></h2>
+<pre><code class="language-c">bool nng_aio_busy(nng_aio *aio);
+</code></pre>
+<p>The <a name="a015"></a><code>nng_aio_busy</code> function returns <code>true</code> if the <em>aio</em> is currently busy performing an
+operation or is executing a completion callback. Otherwise it return <code>false</code>.
+This is the same test used internally by <a href="/api/api.html#wait-for-completion"><code>nng_aio_wait</code></a>.</p>
+<div class="mdbook-alerts mdbook-alerts-important">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ important
+</p>
+<p>The caller is responsible for coordinating any use of this with any reuse of the <em>aio</em>.
+Because the <em>aio</em> can be reused use of this function can be racy.</p>
+</div>
+<h2 id="result-of-operation"><a class="header" href="#result-of-operation">Result of Operation</a></h2>
+<pre><code class="language-c">int nng_aio_result(nng_aio *aio);
+size_t nng_aio_count(nng_aio *aio);
+</code></pre>
+<p>The <a name="a016"></a><code>nng_aio_result</code> function returns the result of the operation associated
+with the handle <em>aio</em>.
+If the operation was successful, then 0 is returned.
+Otherwise a non-zero <a href="/api/errors.html">error</a> code, such as <a href="/api/errors.html#NNG_ECANCELED"><code>NNG_ECANCELED</code></a> or <a href="/api/errors.html#NNG_ETIMEDOUT"><code>NNG_ETIMEDOUT</code></a>, is returned.</p>
+<p>For operations that transfer data, <a name="a017"></a><code>nng_aio_count</code> returns the
+number of bytes transferred by the operation associated with the handle <em>aio</em>.
+Operations that do not transfer data, or do not keep a count, may return zero for this function.</p>
+<div class="mdbook-alerts mdbook-alerts-note">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ note
+</p>
+<p>The return value from these functions is undefined if the operation has not completed yet.
+Either call these from the handle’s completion callback, or after waiting for the
+operation to complete with <a href="/api/api.html#wait-for-completion"><code>nng_aio_wait</code></a>.</p>
+</div>
+<h2 id="messages"><a class="header" href="#messages">Messages</a></h2>
+<pre><code class="language-c">nng_msg *nng_aio_get_msg(nng_aio *aio);
+void nng_aio_set_msg(nng_aio *aio, nng_msg *msg);
+</code></pre>
+<p>The <a name="a018"></a><code>nng_aio_get_msg</code> and <a name="a019"></a><code>nng_aio_set_msg</code> functions retrieve and store a <a href="/api/msg.html">message</a>
+in <em>aio</em>.
+For example, if a function to receive data is called, that function can generally be expected
+to store a message on the asssociated <em>aio</em>, for the application to retrieve with
+<code>nng_aio_get_msg</code>.
+Conversely an application desiring to send a message <em>msg</em> will store it in the <em>aio</em> using
+<code>nng_aio_set_msg</code>. The function implementing the send operation will retrieve the message
+and arrange for it to be sent.</p>
+<h3 id="message-ownership"><a class="header" href="#message-ownership">Message Ownership</a></h3>
+<p>For send or transmit operations, the rule of thumb is that implementation of the operation
+is responsible for taking ownership of the message (and releasing resources when it is complete),
+if it will return success. If the operation will end in error, then the message will be
+retained and it is the consuming application’s responsibility to dispose of the message.
+This allows an application the opportunity to reuse the message to try again, if it so desires.</p>
+<p>For receive operations, the implementation of the operation will set the message on the <em>aio</em>
+on success, and the consuming application hasa responsibility to retrieve and dispose of the
+message. Failure to do so will leak the message. If the operation does not complete successfully,
+then no message is stored on the <em>aio</em>.</p>
+<h2 id="io-vector"><a class="header" href="#io-vector">I/O Vector</a></h2>
+<pre><code class="language-c">typedef struct nng_iov {
+ void * iov_buf;
+ size_t iov_len;
+};
+
+int nng_aio_set_iov(nng_aio *aio, unsigned int niov, nng_iov *iov);
+</code></pre>
+<p>For some operations, the unit of data transferred is not a <a href="/api/msg.html">message</a>, but
+rather a stream of bytes.</p>
+<p>For these operations, an array of <em>niov</em> <a name="a020"></a><code>nng_iov</code> structures can be passed to
+the <a name="a021"></a><code>nng_aio_set_iov</code> function to provide a scatter/gather array of
+elements describing the location (<code>iov_buf</code>) and length (<code>iov_len</code>) of data,
+to transfer.</p>
+<p>The <em>iov</em> vector is copied into storage in the <em>aio</em> itself, so that callers may use stack allocated <code>nng_iov</code> structures.
+The values pointed to by the <code>iov_buf</code> members are <em>not</em> copied by this function though.</p>
+<p>A maximum of four (4) <code>nng_iov</code> members may be supplied.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>Most functions using <code>nng_iov</code> do not guarantee to transfer all of the data that they
+are requested to. To be sure that correct amount of data is transferred, as well as to
+start an attempt to complete any partial transfer, check the amount of data transferred by
+calling <a href="/api/api.html#result-of-operation"><code>nng_aio_count</code></a>.</p>
+</div>
+<h2 id="inputs-and-outputs"><a class="header" href="#inputs-and-outputs">Inputs and Outputs</a></h2>
+<pre><code class="language-c">void nng_aio_set_input(nng_aio *aio, unsigned int index, void *param);
+void *nng_aio_get_output(nng_aio *aio, unsigned int index);
+</code></pre>
+<p>Asynchronous operations can take additional input parameters, and
+provide additional result outputs besides the <a href="/api/api.html#result-of-operation">result</a> code.</p>
+<p>The <code>nng_aio_set_input</code> function sets the input parameter at <em>index</em>
+to <em>param</em> for the operation associated with <em>aio</em>.</p>
+<p>The <code>nng_aio_get_output</code> function returns the output result at <em>index</em>
+for the operation associated with <em>aio</em>.</p>
+<p>The type and semantics of input parameters and output results are determined by specific
+operations. The documentation for the operation should provide details.</p>
+<p>The valid values of <em>index</em> range from zero (0) to three (3), as no operation
+currently defined can accept more than four parameters or return more than four additional
+results.</p>
+<div class="mdbook-alerts mdbook-alerts-note">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ note
+</p>
+<p>If the <em>index</em> does not correspond to a defined input for the operation,
+then <code>nng_aio_set_input</code> will have no effect, and <code>nng_aio_get_output</code> will
+return <code>NULL</code>.</p>
+</div>
+<div class="mdbook-alerts mdbook-alerts-important">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ important
+</p>
+<p>It is an error to call this function while the <em>aio</em> is currently
+in use by an active asynchronous operation.</p>
+</div>
+<h2 id="see-also"><a class="header" href="#see-also">See Also</a></h2>
+<p><a href="/api/synch.html">Synchronization</a>,
+<a href="/api/thread.html">Threads</a>,
+<a href="/api/time.html">Time</a></p>
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