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## Chapter 8. Routing

Cowboy does nothing by default.

To make Cowboy useful, you need to map URIs to Erlang modules that will handle the requests. This is called routing.

Cowboy routes requests using the following algorithm:

- If no configured host matches the request URI, a 400 response is returned.
- Otherwise, the first configured host that matches the request URI will be used. Only the paths configured for this host will be considered.
- If none of the configured paths found in the previous step match the request URI, a 404 response is returned.
- Otherwise, the handler and its initial state are added to the environment and the request continues to be processed.

### Note

It is possible to run into a situation where two hosts match a request URI, but only the paths on the second host match the request URI. In this case the expected result is a 404 response because the only paths used during routing are the paths from the first configured host that matches the request URI.

Routes need to be compiled before they can be used by Cowboy. The result of the compilation is the dispatch rules.

## 8.1. Syntax

The general structure for the routes is defined as follow.

    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.

    Host1 = {HostMatch, PathsList}.
    Host2 = {HostMatch, Constraints, PathsList}.

The list of routes for the path component is defined similar to the list of hosts.

    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 its initial state.

    Path1 = {PathMatch, Handler, InitialState}.
    Path2 = {PathMatch, Constraints, Handler, InitialState}.

Continue reading to learn more about the match syntax and the optional constraints.

## 8.2. 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()`.

    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.

    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.

    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.

    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.

    HostMatch = "ninenines.:_".

Similarly, it is possible to have optional segments. Anything between brackets is optional.

    PathMatch = "/hats/[page/:number]".
    HostMatch = "[www.]ninenines.eu".

You can also have imbricated optional segments.

    PathMatch = "/hats/[page/[:number]]".

While Cowboy does not reject multiple brackets in a route, the behavior may be undefined if the route is under-specified. For example, this route requires constraints to determine what is a chapter and what is a page, since they are both optional:

    PathMatch = "/book/[:chapter]/[:page]".

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.

    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.

    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.

    PathMatch = "/hats/:name/[:name]".

If a binding is defined in both the host and path, then they must also share the same value.

    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.

    PathMatch = '_'.
    HostMatch = '_'.

The second is the special host match `"*"` which will match the wildcard path, generally used alongside the `OPTIONS` method.

    HostMatch = "*".

## 8.3. 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 constraints.

## 8.4. Compilation

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.

To compile routes, just call the appropriate function:

    Dispatch = cowboy_router:compile([
        %% {HostMatch, list({PathMatch, Handler, InitialState})}
        {'_', [{'_', my_handler, #{}}]}
    ]),
    %% Name, TransOpts, ProtoOpts
    cowboy:start_clear(my_http_listener,
        [{port, 8080}],
        #{env => #{dispatch => Dispatch}}
    ).

## 8.5. Using persistent_term

The routes can be stored in `persistent_term` starting from Erlang/OTP 21.2. This may give a performance improvement when there are a large number of routes.

To use this functionality you need to compile the routes, store them in `persistent_term` and then inform Cowboy:

    Dispatch = cowboy_router:compile([
        {'_', [{'_', my_handler, #{}}]}
    ]),
    persistent_term:put(my_app_dispatch, Dispatch),
    cowboy:start_clear(my_http_listener,
        [{port, 8080}],
        #{env => #{dispatch => {persistent_term, my_app_dispatch}}}
    ).

## 8.6. 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:

    Dispatch = cowboy_router:compile(Routes),
    cowboy:set_env(my_http_listener, dispatch, Dispatch).

Note that you need to compile the routes again before updating.

When using `persistent_term` there is no need to call this function, you can simply put the new routes in the storage.

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