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+[[routing]]
+== Routing
+
+Cowboy does nothing by default.
+
+To make Cowboy useful, you need to map URLs to Erlang modules that will
+handle the requests. This is called routing.
+
+When Cowboy receives a request, it tries to match the requested host and
+path to the resources given in the dispatch rules. If it matches, then
+the associated Erlang code will be executed.
+
+Routing rules are given per host. Cowboy will first match on the host,
+and then try to find a matching path.
+
+Routes need to be compiled before they can be used by Cowboy.
+
+=== Structure
+
+The general structure for the routes is defined as follow.
+
+[source,erlang]
+Routes = [Host1, Host2, ... HostN].
+
+Each host contains matching rules for the host along with optional
+constraints, and a list of routes for the path component.
+
+[source,erlang]
+Host1 = {HostMatch, PathsList}.
+Host2 = {HostMatch, Constraints, PathsList}.
+
+The list of routes for the path component is defined similar to the
+list of hosts.
+
+[source,erlang]
+PathsList = [Path1, Path2, ... PathN].
+
+Finally, each path contains matching rules for the path along with
+optional constraints, and gives us the handler module to be used
+along with options that will be given to it on initialization.
+
+[source,erlang]
+Path1 = {PathMatch, Handler, Opts}.
+Path2 = {PathMatch, Constraints, Handler, Opts}.
+
+Continue reading to learn more about the match syntax and the optional
+constraints.
+
+=== Match syntax
+
+The match syntax is used to associate host names and paths with their
+respective handlers.
+
+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.
+
+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 `string()` or a `binary()`.
+
+[source,erlang]
+----
+PathMatch1 = "/".
+PathMatch2 = "/path/to/resource".
+
+HostMatch1 = "cowboy.example.org".
+----
+
+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.
+
+[source,erlang]
+PathMatch2 = "/path/to/resource".
+PathMatch3 = "/path/to/resource/".
+
+Hosts with and without a trailing dot are equivalent for routing.
+Similarly, hosts with and without a leading dot are also equivalent.
+
+[source,erlang]
+HostMatch1 = "cowboy.example.org".
+HostMatch2 = "cowboy.example.org.".
+HostMatch3 = ".cowboy.example.org".
+
+It is possible to extract segments of the host and path and to store
+the values in the `Req` object for later use. We call these kind of
+values bindings.
+
+The syntax for bindings is very simple. A segment that begins with
+the `:` 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.
+
+[source,erlang]
+PathMatch = "/hats/:name/prices".
+HostMatch = ":subdomain.example.org".
+
+If these two end up matching when routing, you will end up with two
+bindings defined, `subdomain` and `name`, each containing the
+segment value where they were defined. For example, the URL
+`http://test.example.org/hats/wild_cowboy_legendary/prices` will
+result in having the value `test` bound to the name `subdomain`
+and the value `wild_cowboy_legendary` bound to the name `name`.
+They can later be retrieved using `cowboy_req:binding/{2,3}`. The
+binding name must be given as an atom.
+
+There is a special binding name you can use to mimic the underscore
+variable in Erlang. Any match against the `_` binding will succeed
+but the data will be discarded. This is especially useful for
+matching against many domain names in one go.
+
+[source,erlang]
+HostMatch = "ninenines.:_".
+
+Similarly, it is possible to have optional segments. Anything
+between brackets is optional.
+
+[source,erlang]
+PathMatch = "/hats/[page/:number]".
+HostMatch = "[www.]ninenines.eu".
+
+You can also have imbricated optional segments.
+
+[source,erlang]
+PathMatch = "/hats/[page/[:number]]".
+
+You can retrieve the rest of the host or path using `[...]`.
+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
+`cowboy_req:host_info/1` and `cowboy_req:path_info/1` respectively.
+They will be represented as a list of segments.
+
+[source,erlang]
+PathMatch = "/hats/[...]".
+HostMatch = "[...]ninenines.eu".
+
+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.
+
+[source,erlang]
+PathMatch = "/hats/:name/:name".
+
+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.
+
+[source,erlang]
+PathMatch = "/hats/:name/[:name]".
+
+If a binding is defined in both the host and path, then they must
+also share the same value.
+
+[source,erlang]
+PathMatch = "/:user/[...]".
+HostMatch = ":user.github.com".
+
+Finally, there are two special match values that can be used. The
+first is the atom `'_'` which will match any host or path.
+
+[source,erlang]
+PathMatch = '_'.
+HostMatch = '_'.
+
+The second is the special host match `"*"` which will match the
+wildcard path, generally used alongside the `OPTIONS` method.
+
+[source,erlang]
+HostMatch = "*".
+
+=== Constraints
+
+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.
+
+The format used for constraints is the same as match functions in
+`cowboy_req`: 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.
+
+Read more about xref:constraints[constraints].
+
+=== Compilation
+
+The structure defined in this chapter needs to be compiled before it is
+passed to Cowboy. This allows Cowboy to efficiently lookup the correct
+handler to run instead of having to parse the routes repeatedly.
+
+This can be done with a simple call to `cowboy_router:compile/1`.
+
+[source,erlang]
+----
+Dispatch = cowboy_router:compile([
+    %% {HostMatch, list({PathMatch, Handler, Opts})}
+    {'_', [{'_', my_handler, []}]}
+]),
+%% Name, NbAcceptors, TransOpts, ProtoOpts
+cowboy:start_http(my_http_listener, 100,
+    [{port, 8080}],
+    [{env, [{dispatch, Dispatch}]}]
+).
+----
+
+Note that this function will return `{error, badarg}` if the structure
+given is incorrect.
+
+=== Live update
+
+You can use the `cowboy:set_env/3` function for updating the dispatch
+list used by routing. This will apply to all new connections accepted
+by the listener.
+
+[source,erlang]
+cowboy:set_env(my_http_listener, dispatch, cowboy_router:compile(Dispatch)).
+
+Note that you need to compile the routes before updating.