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<title>Nine Nines: Connection</title>
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<h1 class="lined-header"><span>Connection</span></h1>
<div class="paragraph"><p>This chapter describes how to open, monitor and close
a connection using the Gun client.</p></div>
<div class="sect1">
<h2 id="_gun_connections">Gun connections</h2>
<div class="sectionbody">
<div class="paragraph"><p>Gun is designed with the HTTP/2 and Websocket protocols in mind.
They are built for long-running connections that allow concurrent
exchange of data, either in the form of request/responses for
HTTP/2 or in the form of messages for Websocket.</p></div>
<div class="paragraph"><p>A Gun connection is an Erlang process that manages a socket to
a remote endpoint. This Gun connection is owned by a user
process that is called the <em>owner</em> of the connection, and is
managed by the supervision tree of the <code>gun</code> application.</p></div>
<div class="paragraph"><p>The owner process communicates with the Gun connection
by calling functions from the module <code>gun</code>. All functions
perform their respective operations asynchronously. The Gun
connection will send Erlang messages to the owner process
whenever needed.</p></div>
<div class="paragraph"><p>When the remote endpoint closes the connection, Gun attempts
to reconnect automatically.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_opening_a_new_connection">Opening a new connection</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <code>gun:open/{2,3}</code> function must be used to open a connection.</p></div>
<div class="listingblock">
<div class="title">Opening a connection to example.org on port 443</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #FF6600">ok</span>, <span style="color: #009900">ConnPid</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">gun:open</span></span>(<span style="color: #FF0000">"example.org"</span>, <span style="color: #993399">443</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If the port given is 443, Gun will attempt to connect using
SSL. The protocol will be selected automatically using the
ALPN extension for TLS. By default Gun supports HTTP/2
and HTTP/1.1 when connecting using SSL.</p></div>
<div class="paragraph"><p>For any other port, Gun will attempt to connect using TCP
and will use the HTTP/1.1 protocol.</p></div>
<div class="paragraph"><p>The transport and protocol used can be overriden using
options. The manual documents all available options.</p></div>
<div class="paragraph"><p>Options can be provided as a third argument, and take the
form of a map.</p></div>
<div class="listingblock">
<div class="title">Opening an SSL connection to example.org on port 8443</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #FF6600">ok</span>, <span style="color: #009900">ConnPid</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">gun:open</span></span>(<span style="color: #FF0000">"example.org"</span>, <span style="color: #993399">8443</span>, #{<span style="color: #0000FF">transport</span><span style="color: #990000">=></span><span style="color: #FF6600">ssl</span>})<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_waiting_for_the_connection_to_be_established">Waiting for the connection to be established</h2>
<div class="sectionbody">
<div class="paragraph"><p>When Gun successfully connects to the server, it sends a
<code>gun_up</code> message with the protocol that has been selected
for the connection.</p></div>
<div class="paragraph"><p>Gun provides the functions <code>gun:await_up/{1,2,3}</code> that wait
for the <code>gun_up</code> message. They can optionally take a monitor
reference and/or timeout value. If no monitor is provided,
one will be created for the duration of the function call.</p></div>
<div class="listingblock">
<div class="title">Synchronous opening of a connection</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #FF6600">ok</span>, <span style="color: #009900">ConnPid</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">gun:open</span></span>(<span style="color: #FF0000">"example.org"</span>, <span style="color: #993399">443</span>),
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Protocol</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">gun:await_up</span></span>(<span style="color: #009900">ConnPid</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_handling_connection_loss">Handling connection loss</h2>
<div class="sectionbody">
<div class="paragraph"><p>When the connection is lost, Gun will send a <code>gun_down</code>
message indicating the current protocol, the reason the
connection was lost and two list of stream references.</p></div>
<div class="paragraph"><p>The first list indicates open streams that <em>may</em> have been
processed by the server. The second list indicates open
streams that the server did not process.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_monitoring_the_connection_process">Monitoring the connection process</h2>
<div class="sectionbody">
<div class="paragraph"><p>@todo Gun should detect the owner process being killed</p></div>
<div class="paragraph"><p>Because software errors are unavoidable, it is important to
detect when the Gun process crashes. It is also important
to detect when it exits normally. Erlang provides two ways
to do that: links and monitors.</p></div>
<div class="paragraph"><p>Gun leaves you the choice as to which one will be used.
However, if you use the <code>gun:await/{2,3}</code> or <code>gun:await_body/{2,3}</code>
functions, a monitor may be used for you to avoid getting
stuck waiting for a message that will never come.</p></div>
<div class="paragraph"><p>If you choose to monitor yourself you can do it on a permanent
basis rather than on every message you will receive, saving
resources. Indeed, the <code>gun:await/{3,4}</code> and <code>gun:await_body/{3,4}</code>
functions both accept a monitor argument if you have one already.</p></div>
<div class="listingblock">
<div class="title">Monitoring the connection process</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #FF6600">ok</span>, <span style="color: #009900">ConnPid</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">gun:open</span></span>(<span style="color: #FF0000">"example.org"</span>, <span style="color: #993399">443</span>)<span style="color: #990000">.</span>
<span style="color: #009900">MRef</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">monitor</span></span>(<span style="font-weight: bold"><span style="color: #000080">process</span></span>, <span style="color: #009900">ConnPid</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>This monitor reference can be kept and used until the connection
process exits.</p></div>
<div class="listingblock">
<div class="title">Handling <code>DOWN</code> messages</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #0000FF">receive</span></span>
<span style="font-style: italic"><span style="color: #9A1900">%% Receive Gun messages here...</span></span>
{<span style="color: #FF6600">'DOWN'</span>, <span style="color: #009900">Mref</span>, <span style="font-weight: bold"><span style="color: #000080">process</span></span>, <span style="color: #009900">ConnPid</span>, <span style="color: #009900">Reason</span>} <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">error_logger:error_msg</span></span>(<span style="color: #FF0000">"Oops!"</span>),
<span style="font-weight: bold"><span style="color: #000080">exit</span></span>(<span style="color: #009900">Reason</span>);
<span style="font-weight: bold"><span style="color: #0000FF">end</span></span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>What to do when you receive a <code>DOWN</code> message is entirely up to you.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_closing_the_connection_abruptly">Closing the connection abruptly</h2>
<div class="sectionbody">
<div class="paragraph"><p>The connection can be stopped abruptly at any time by calling
the <code>gun:close/1</code> function.</p></div>
<div class="listingblock">
<div class="title">Immediate closing of the connection</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">gun:close</span></span>(<span style="color: #009900">ConnPid</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The process is stopped immediately without having a chance to
perform the protocol’s closing handshake, if any.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_closing_the_connection_gracefully">Closing the connection gracefully</h2>
<div class="sectionbody">
<div class="paragraph"><p>The connection can also be stopped gracefully by calling the
<code>gun:shutdown/1</code> function.</p></div>
<div class="listingblock">
<div class="title">Graceful shutdown of the connection</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">gun:shutdown</span></span>(<span style="color: #009900">ConnPid</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Gun will refuse any new requests or messages after you call
this function. It will however continue to send you messages
for existing streams until they are all completed.</p></div>
<div class="paragraph"><p>For example if you performed a GET request just before calling
<code>gun:shutdown/1</code>, you will still receive the response before
Gun closes the connection.</p></div>
<div class="paragraph"><p>If you set a monitor beforehand, you will receive a message
when the connection has been closed.</p></div>
</div>
</div>
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<h3>
Gun
1.0
User Guide
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