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<h1 class="lined-header"><span>Routing</span></h1>
<div class="paragraph"><p>Cowboy does nothing by default.</p></div>
<div class="paragraph"><p>To make Cowboy useful, you need to map URIs to Erlang modules that will
handle the requests. This is called routing.</p></div>
<div class="paragraph"><p>When Cowboy receives a request, it tries to match the requested host and
path to the configured routes. When there’s a match, the route’s
associated handler is executed.</p></div>
<div class="paragraph"><p>Routes need to be compiled before they can be used by Cowboy.
The result of the compilation is the dispatch rules.</p></div>
<div class="sect1">
<h2 id="_syntax">Syntax</h2>
<div class="sectionbody">
<div class="paragraph"><p>The general structure for the routes is defined as follow.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">Routes</span> <span style="color: #990000">=</span> [<span style="color: #009900">Host1</span>, <span style="color: #009900">Host2</span>, <span style="color: #990000">...</span> <span style="color: #009900">HostN</span>]<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Each host contains matching rules for the host along with optional
constraints, and a list of routes for the path component.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">Host1</span> <span style="color: #990000">=</span> {<span style="color: #009900">HostMatch</span>, <span style="color: #009900">PathsList</span>}<span style="color: #990000">.</span>
<span style="color: #009900">Host2</span> <span style="color: #990000">=</span> {<span style="color: #009900">HostMatch</span>, <span style="color: #009900">Constraints</span>, <span style="color: #009900">PathsList</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The list of routes for the path component is defined similar to the
list of hosts.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">PathsList</span> <span style="color: #990000">=</span> [<span style="color: #009900">Path1</span>, <span style="color: #009900">Path2</span>, <span style="color: #990000">...</span> <span style="color: #009900">PathN</span>]<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Finally, each path contains matching rules for the path along with
optional constraints, and gives us the handler module to be used
along with its initial state.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">Path1</span> <span style="color: #990000">=</span> {<span style="color: #009900">PathMatch</span>, <span style="color: #009900">Handler</span>, <span style="color: #009900">InitialState</span>}<span style="color: #990000">.</span>
<span style="color: #009900">Path2</span> <span style="color: #990000">=</span> {<span style="color: #009900">PathMatch</span>, <span style="color: #009900">Constraints</span>, <span style="color: #009900">Handler</span>, <span style="color: #009900">InitialState</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Continue reading to learn more about the match syntax and the optional
constraints.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_match_syntax">Match syntax</h2>
<div class="sectionbody">
<div class="paragraph"><p>The match syntax is used to associate host names and paths with their
respective handlers.</p></div>
<div class="paragraph"><p>The match syntax is the same for host and path with a few subtleties.
Indeed, the segments separator is different, and the host is matched
starting from the last segment going to the first. All examples will
feature both host and path match rules and explain the differences
when encountered.</p></div>
<div class="paragraph"><p>Excluding special values that we will explain at the end of this section,
the simplest match value is a host or a path. It can be given as either
a <code>string()</code> or a <code>binary()</code>.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">PathMatch1</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/"</span><span style="color: #990000">.</span>
<span style="color: #009900">PathMatch2</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/path/to/resource"</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch1</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"cowboy.example.org"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>As you can see, all paths defined this way must start with a slash
character. Note that these two paths are identical as far as routing
is concerned.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">PathMatch2</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/path/to/resource"</span><span style="color: #990000">.</span>
<span style="color: #009900">PathMatch3</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/path/to/resource/"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Hosts with and without a trailing dot are equivalent for routing.
Similarly, hosts with and without a leading dot are also equivalent.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">HostMatch1</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"cowboy.example.org"</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch2</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"cowboy.example.org."</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch3</span> <span style="color: #990000">=</span> <span style="color: #FF0000">".cowboy.example.org"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>It is possible to extract segments of the host and path and to store
the values in the <code>Req</code> object for later use. We call these kind of
values bindings.</p></div>
<div class="paragraph"><p>The syntax for bindings is very simple. A segment that begins with
the <code>:</code> character means that what follows until the end of the segment
is the name of the binding in which the segment value will be stored.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/hats/:name/prices"</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">":subdomain.example.org"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If these two end up matching when routing, you will end up with two
bindings defined, <code>subdomain</code> and <code>name</code>, each containing the
segment value where they were defined. For example, the URL
<code>http://test.example.org/hats/wild_cowboy_legendary/prices</code> will
result in having the value <code>test</code> bound to the name <code>subdomain</code>
and the value <code>wild_cowboy_legendary</code> bound to the name <code>name</code>.
They can later be retrieved using <code>cowboy_req:binding/{2,3}</code>. The
binding name must be given as an atom.</p></div>
<div class="paragraph"><p>There is a special binding name you can use to mimic the underscore
variable in Erlang. Any match against the <code>_</code> binding will succeed
but the data will be discarded. This is especially useful for
matching against many domain names in one go.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"ninenines.:_"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Similarly, it is possible to have optional segments. Anything
between brackets is optional.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/hats/[page/:number]"</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"[www.]ninenines.eu"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can also have imbricated optional segments.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/hats/[page/[:number]]"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can retrieve the rest of the host or path using <code>[...]</code>.
In the case of hosts it will match anything before, in the case
of paths anything after the previously matched segments. It is
a special case of optional segments, in that it can have
zero, one or many segments. You can then find the segments using
<code>cowboy_req:host_info/1</code> and <code>cowboy_req:path_info/1</code> respectively.
They will be represented as a list of segments.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/hats/[...]"</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"[...]ninenines.eu"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If a binding appears twice in the routing rules, then the match
will succeed only if they share the same value. This copies the
Erlang pattern matching behavior.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/hats/:name/:name"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>This is also true when an optional segment is present. In this
case the two values must be identical only if the segment is
available.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/hats/:name/[:name]"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If a binding is defined in both the host and path, then they must
also share the same value.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"/:user/[...]"</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">":user.github.com"</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Finally, there are two special match values that can be used. The
first is the atom <code>'_'</code> which will match any host or path.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">PathMatch</span> <span style="color: #990000">=</span> <span style="color: #FF6600">'_'</span><span style="color: #990000">.</span>
<span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF6600">'_'</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The second is the special host match <code>"*"</code> which will match the
wildcard path, generally used alongside the <code>OPTIONS</code> method.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">HostMatch</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"*"</span><span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_constraints">Constraints</h2>
<div class="sectionbody">
<div class="paragraph"><p>After the matching has completed, the resulting bindings can be tested
against a set of constraints. Constraints are only tested when the
binding is defined. They run in the order you defined them. The match
will succeed only if they all succeed. If the match fails, then Cowboy
tries the next route in the list.</p></div>
<div class="paragraph"><p>The format used for constraints is the same as match functions in
<code>cowboy_req</code>: they are provided as a list of fields which may have
one or more constraints. While the router accepts the same format,
it will skip fields with no constraints and will also ignore default
values, if any.</p></div>
<div class="paragraph"><p>Read more about <a href="../constraints">constraints</a>.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_compilation">Compilation</h2>
<div class="sectionbody">
<div class="paragraph"><p>The routes must be compiled before Cowboy can use them. The compilation
step normalizes the routes to simplify the code and speed up the
execution, but the routes are still looked up one by one in the end.
Faster compilation strategies could be to compile the routes directly
to Erlang code, but would require heavier dependencies.</p></div>
<div class="paragraph"><p>To compile routes, just call the appropriate function:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">Dispatch</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_router:compile</span></span>([
<span style="font-style: italic"><span style="color: #9A1900">%% {HostMatch, list({PathMatch, Handler, InitialState})}</span></span>
{<span style="color: #FF6600">'_'</span>, [{<span style="color: #FF6600">'_'</span>, <span style="color: #FF6600">my_handler</span>, #{}}]}
]),
<span style="font-style: italic"><span style="color: #9A1900">%% Name, NbAcceptors, TransOpts, ProtoOpts</span></span>
<span style="font-weight: bold"><span style="color: #000000">cowboy:start_clear</span></span>(<span style="color: #FF6600">my_http_listener</span>,
[{<span style="color: #FF6600">port</span>, <span style="color: #993399">8080</span>}],
#{<span style="color: #0000FF">env</span> <span style="color: #990000">=></span> #{<span style="color: #0000FF">dispatch</span> <span style="color: #990000">=></span> <span style="color: #009900">Dispatch</span>}}
)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_live_update">Live update</h2>
<div class="sectionbody">
<div class="paragraph"><p>You can use the <code>cowboy:set_env/3</code> function for updating the dispatch
list used by routing. This will apply to all new connections accepted
by the listener:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">Dispatch</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_router:compile</span></span>(<span style="color: #009900">Routes</span>),
<span style="font-weight: bold"><span style="color: #000000">cowboy:set_env</span></span>(<span style="color: #FF6600">my_http_listener</span>, <span style="color: #FF6600">dispatch</span>, <span style="color: #009900">Dispatch</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Note that you need to compile the routes again before updating.</p></div>
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