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<title>Nine Nines: Migrating from Cowboy 2.0 to 2.1</title>
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<h1 class="lined-header"><span>Migrating from Cowboy 2.0 to 2.1</span></h1>
<div class="paragraph"><p>Cowboy 2.1 focused on adding features that were temporarily
removed in Cowboy 2.0. A number of bugs found in the 2.0
release were also fixed.</p></div>
<div class="sect1">
<h2 id="_features_added">Features added</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
It is now possible to obtain the client TLS certificate
and the local IP/port for the connection from the Req object.
</p>
</li>
<li>
<p>
Informational responses (1XX responses) can now be sent.
They must be sent before initiating the final response.
</p>
</li>
<li>
<p>
The <code>expect: 100-continue</code> header is now handled
automatically. The 100 response will be sent on the
first <code>cowboy_req:read_body/2,3,4</code> call. This only applies
when using the default <code>cowboy_stream_h</code> stream handler.
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_experimental_features_added">Experimental features added</h2>
<div class="sectionbody">
<div class="paragraph"><p>Experimental features are previews of features that will be
added in a future release. They are not documented and their
interface may change at any time. You are welcome to try them
and provide feedback.</p></div>
<div class="ulist"><ul>
<li>
<p>
The <code>cowboy_metrics_h</code> stream handler can be used to
extract metrics out of Cowboy. It must be used first in
the list of stream handlers, and will record all events
related to requests, responses and spawned processes.
When the stream terminates it will pass this information
to a user-defined callback.
</p>
</li>
<li>
<p>
The <code>cowboy_tracer_h</code> stream handler can be used to setup
automatic tracing of specific requests. You can conditionally
enable tracing based on a function, header, path or any other
element from the request and the trace will apply to the
entire connection and any processes created by it. This is
meant to be used for debugging both in tests and production.
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_changed_behaviors">Changed behaviors</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
The <code>cowboy_rest</code> handler now implements a mechanism for
switching to a different type of handler from any callback
where <code>stop</code> is also allowed. Switch by returning
<code>{switch_handler, Module}</code> or <code>{switch_handler, Module, Opts}</code>.
This is especially useful for switching to <code>cowboy_loop</code>
for streaming the request or response body.
</p>
</li>
<li>
<p>
REST callbacks that do not allow <code>stop</code> as a return value
are now explicitly listed in the documentation.
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_new_functions">New functions</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
The function <code>cowboy_req:sock/1</code> returns the IP/port
of the local socket.
</p>
</li>
<li>
<p>
The function <code>cowboy_req:cert/1</code> returns the client
TLS certificate or <code>undefined</code> if it isn’t available.
</p>
</li>
<li>
<p>
The function <code>cowboy_req:inform/2,3</code> sends an
informational response.
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_bugs_fixed">Bugs fixed</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
Ensure HTTP/2 connections are not closed prematurely
when the user code does not read the request body.
</p>
</li>
<li>
<p>
Ensure HTTP/1.1 streams are not terminated too early.
Their behavior is now consistent with the HTTP/2 code
where the stream handler is only terminated when the
<code>stop</code> command is returned.
</p>
</li>
<li>
<p>
Sending zero-sized data from stream handlers or from
<code>cowboy_req:stream_body/3</code> could lead to issues with
HTTP/1.1. This has been fixed.
</p>
</li>
<li>
<p>
The final chunk sent by Cowboy when it terminates a
chunked body after the handler process exits was not
passed through stream handlers, which could lead to
issues when <code>cowboy_compress_h</code> was being used. This
is now corrected.
</p>
</li>
<li>
<p>
The stream handler state was discarded in some cases
where Cowboy had to send a response or response data
automatically when ending a stream. This has now
been corrected.
</p>
</li>
<li>
<p>
The stream handler callback <code>terminate/3</code> will now be
called when switching to another protocol using the
command <code>switch_protocol</code>. This doesn’t apply when
doing upgrades to HTTP/2 as those occur before the
stream is initialized.
</p>
</li>
<li>
<p>
Cowlib has been updated to 2.0.1 to fix an issue with
Websocket compression when using Erlang/OTP 20.1. Note
that at the time of writing all 20.1 versions (from
20.1 to 20.1.4) have issues when compression is enabled.
It is expected to work properly from 20.1.5 onward. In
the meantime it is recommended to run the plain 20.1
release and disable Websocket compression, or use a
release before 20.1.
</p>
</li>
<li>
<p>
Cowboy will no longer crash when the <code>cowboy_clock</code>
process is not running. This can happen when Cowboy
is being restarted during upgrades, for example.
</p>
</li>
</ul></div>
</div>
</div>
<nav style="margin:1em 0">
<a style="float:left" href="https://ninenines.eu/docs/en/cowboy/2.4/guide/migrating_from_2.1/">
Migrating from Cowboy 2.1 to 2.2
</a>
<a style="float:right" href="https://ninenines.eu/docs/en/cowboy/2.4/guide/migrating_from_1.0/">
Migrating from Cowboy 1.0 to 2.0
</a>
</nav>
</div>
<div class="span3 sidecol">
<h3>
Cowboy
2.4
User Guide
</h3>
<ul>
<li><a href="/docs/en/cowboy/2.4/guide">User Guide</a></li>
<li><a href="/docs/en/cowboy/2.4/manual">Function Reference</a></li>
</ul>
<h4 id="docs-nav">Navigation</h4>
<h4>Version select</h4>
<ul>
<li><a href="/docs/en/cowboy/2.4/guide">2.4</a></li>
<li><a href="/docs/en/cowboy/2.3/guide">2.3</a></li>
<li><a href="/docs/en/cowboy/2.2/guide">2.2</a></li>
<li><a href="/docs/en/cowboy/2.1/guide">2.1</a></li>
<li><a href="/docs/en/cowboy/2.0/guide">2.0</a></li>
<li><a href="/docs/en/cowboy/1.0/guide">1.0</a></li>
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