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<h1>nng_req(7) Manual Page</h1>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_synopsis">SYNOPSIS</a></li>
<li><a href="#_description">DESCRIPTION</a>
<ul class="sectlevel2">
<li><a href="#_socket_operations">Socket Operations</a></li>
<li><a href="#_protocol_versions">Protocol Versions</a></li>
<li><a href="#_protocol_options">Protocol Options</a></li>
<li><a href="#_protocol_headers">Protocol Headers</a></li>
</ul>
</li>
<li><a href="#_see_also">SEE ALSO</a></li>
</ul>
</div>
<h2>NAME</h2>
<div class="sectionbody">
<p>nng_req - request protocol</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="content">
<pre class="pygments highlight"><code data-lang="c"><span></span><span class="tok-cp">#include</span> <span class="tok-cpf"><nng/protocol/reqrep0/req.h></span><span class="tok-cp"></span>
<span class="tok-kt">int</span> <span class="tok-nf">nng_req0_open</span><span class="tok-p">(</span><span class="tok-n">nng_socket</span> <span class="tok-o">*</span><span class="tok-n">s</span><span class="tok-p">);</span></code></pre>
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</div>
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</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The <em>nng_req</em> protocol is one half of a request/reply pattern.
In this pattern, a requester sends a message to one replier, who
is expected to reply. The request is resent if no reply arrives,
until a reply is received or the request times out.</p>
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This protocol is useful in setting up RPC-like services. It
is also "reliable", in that a the requester will keep retrying until
a reply is received.
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</tr>
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Because requests are resent, it is important that they be idempotent
to ensure predictable and repeatable behavior even in the face of duplicated
requests, which can occur (for example if a reply message is lost for
some reason.)
</td>
</tr>
</table>
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<div class="paragraph">
<p>The requester generally only has one outstanding request at a time unless
in "raw" mode (via <code>NNG_OPT_RAW</code>), and it will generally attempt to spread
work requests to different peer repliers.</p>
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This property, when combined with a <a href="nng_device.html">device</a> can
help provide a degree of load-balancing.
</td>
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</table>
</div>
<div class="paragraph">
<p>The <em>nng_req</em> protocol is the requester side, and the
<a href="nng_rep.html">nng_rep(7)</a> protocol is the replier side.</p>
</div>
<div class="sect2">
<h3 id="_socket_operations">Socket Operations</h3>
<div class="paragraph">
<p>The <code>nng_req0_open()</code> call creates a requester socket. This socket
may be used to send messages (requests), and then to receive replies. Generally
a reply can only be received after sending a request. (Attempts to receive
a message will result in <code>NNG_ESTATE</code> if there is no outstanding request.)</p>
</div>
<div class="paragraph">
<p>Requests may be canceled by sending a different request. This will
cause the requester to discard any reply from the earlier request,
but it will not stop a replier
from processing a request it has already received or terminate a request
that has already been placed on the wire.</p>
</div>
<div class="paragraph">
<p>Attempts to receive on a socket with no outstanding requests will result
in <code>NNG_ESTATE</code>.</p>
</div>
<div class="paragraph">
<p>Raw mode sockets (set with <code>NNG_OPT_RAW</code>) ignore all these restrictions.</p>
</div>
</div>
<div class="sect2">
<h3 id="_protocol_versions">Protocol Versions</h3>
<div class="paragraph">
<p>Only version 0 of this protocol is supported. (At the time of writing,
no other versions of this protocol have been defined.)</p>
</div>
</div>
<div class="sect2">
<h3 id="_protocol_options">Protocol Options</h3>
<div class="paragraph">
<p>The following protocol-specific options are available.</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>NNG_OPT_REQ_RESENDTIME</code></dt>
<dd>
<p>This read/write option is a duration (32-bit unsigned integer) representing
a relative number of milliseconds.
When a new request is started, a timer of this duration is also started.
If no reply is received before this timer expires, then the request will
be resent. (Requests are also automatically resent if the peer to whom
the original request was sent disconnects, or if a peer becomes available
while the requester is waiting for an available peer.)</p>
</dd>
<dt class="hdlist1"><code>NNG_OPT_MAXTTL</code></dt>
<dd>
<p>Maximum time-to-live. This option is an integer value
between 0 and 255,
inclusive, and is the maximum number of "hops" that a message may
pass through until it is discarded. The default value is 8. A value
of 0 may be used to disable the loop protection, allowing an infinite
number of hops.</p>
</dd>
</dl>
</div>
</div>
<div class="sect2">
<h3 id="_protocol_headers">Protocol Headers</h3>
<div class="paragraph">
<p>This protocol uses a <em>backtrace</em> in the header. This
form uses a "stack" of 32-bit big-endian identifiers. There <strong>must</strong> be
at least one identifier, the <em>request ID</em>, which will be the last
element in the array, and <strong>must</strong> have the most significant bit set.</p>
</div>
<div class="paragraph">
<p>There may be additional <em>peer ID</em>s preceeding the request ID. These
will be distinguishable from the request ID by having their most
significant bit clear.</p>
</div>
<div class="paragraph">
<p>When a request message is received by a forwarding node (see
<a href="nng_device.html">nng_device(3)</a>), the forwarding node prepends a
32-bit peer ID (which <strong>must</strong> have the most significant bit clear),
which is the forwarder’s way of identifying the directly connected
peer from which it received the message. (This peer ID, except for the
most significant bit, has meaning only to the forwarding node itself.)</p>
</div>
<div class="paragraph">
<p>It may help to think of prepending a peer ID as "pushing" a peer ID onto the
front of the stack of headers for the message. (It will use the peer ID
it popped from the front to determine the next intermediate destination
for the reply.)</p>
</div>
<div class="paragraph">
<p>When a reply message is created, it is created using the same headers
that the request contained.</p>
</div>
<div class="paragraph">
<p>A forwarding node can "pop" the peer ID it originally pushed on the
message, stripping it from the front of the message as it does so.</p>
</div>
<div class="paragraph">
<p>When the reply finally arrives back at the initiating requestor, it
should have only a single element in the message, which will be the
request ID it originally used for the request.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="#nng_device(3)<mark>">nng_device(3)</a>,
<a href="#nng</mark>">nng(7)</a>,
<a href="nng_rep.html">nng_rep(7)</a></p>
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