Phoenix.Router

Parsed documentation:
Defines the Phoenix router.

A router is the heart of a Phoenix application. It has three
main responsibilities:

  * It defines a plug pipeline responsible for handling
    upcoming requests and dispatching those requests to
    controllers and other plugs.

  * It hosts configuration for the router and related
    entities (like plugs).

  * It provides a wrapper for starting and stopping the
    router in a specific web server.

We will explore those responsibilities next.

## Routing

The router provides a set of macros for generating routes
that dispatch to specific controllers and actions. Those
macros are named after HTTP verbs. For example:

    defmodule MyApp.Router do
      use Phoenix.Router

      get "/pages/:page", PageController, :show
    end

The `get/3` macro above accepts a request of format "/pages/VALUE" and
dispatches it to the show action in the `PageController`.

Phoenix's router is extremely efficient, as it relies on Elixir
pattern matching for matching routes and serving requests.

### Helpers

Phoenix automatically generates a module `Helpers` inside your router
which contains named helpers to help developers generate and keep
their routes up to date.

Helpers are automatically generated based on the controller name.
For example, the route:

    get "/pages/:page", PageController, :show

will generate a named helper:

    MyApp.Router.Helpers.page_path(:show, "hello")
    "/pages/hello"

    MyApp.Router.Helpers.page_path(:show, "hello", some: "query")
    "/pages/hello?some=query"

The named helper can also be customized with the `:as` option. Given
the route:

    get "/pages/:page", PageController, :show, as: :special_page

the named helper will be:

    MyApp.Router.Helpers.special_page_path(:show, "hello")
    "/pages/hello"

### Scopes and Resources

The router also supports scoping of routes:

    scope "/api/v1", as: :api_v1 do
      get "/pages/:id", PageController, :show
    end

For example, the route above will match on the path `"/api/v1/pages/:id"
and the named route will be `api_v1_page_path`, as expected from the
values given to `scope/2` option.

Phoenix also provides a `resources/4` macro that allows developers
to generate "RESTful" routes to a given resource:

    defmodule MyApp.Router do
      use Phoenix.Router

      pipe_through :browser

      resources "/pages", PageController, only: [:show]
      resources "/users", UserController, except: [:destroy]
    end

Finally, Phoenix ships with a `mix phoenix.routes` task that nicely
formats all routes in a given router. We can use it to verify all
routes included in the router above:

    $ mix phoenix.routes
    page_path  GET    /pages/:id       PageController.show/2
    user_path  GET    /users           UserController.index/2
    user_path  GET    /users/:id/edit  UserController.edit/2
    user_path  GET    /users/new       UserController.new/2
    user_path  GET    /users/:id       UserController.show/2
    user_path  POST   /users           UserController.create/2
    user_path  PATCH  /users/:id       UserController.update/2
               PUT    /users/:id       UserController.update/2

One can also pass a router explicitly as an argument to the task:

    $ mix phoenix.routes MyApp.Router

Check `scope/2` and `resources/4` for more information.

## Pipelines and plugs

Once a request arrives to the Phoenix router, it performs
a series of transformations through pipelines until the
request is dispatched to a desired end-point.

Such transformations are defined via plugs, as defined
in the [Plug](http://github.com/elixir-lang/plug) specification.
Once a pipeline is defined, it can be piped through per scope.

For example:

    defmodule MyApp.Router do
      use Phoenix.Router

      pipeline :browser do
        plug :fetch_session
        plug :accepts, ~w(html json)
      end

      scope "/" do
        pipe_through :browser

        # browser related routes and resources
      end
    end

`Phoenix.Router` imports functions from both `Plug.Conn` and `Phoenix.Controller`
to help define plugs. In the example above, `fetch_session/2`
comes from `Plug.Conn` while `accepts/2` comes from `Phoenix.Controller`.

By default, Phoenix ships with one pipeline, called `:before`,
that is always invoked before any route matches. All other
pipelines are invoked only after a specific route matches,
but before the route is dispatched to.

### :before pipeline

Those are the plugs in the `:before` pipeline in the order
they are defined. How each plug is configured is defined in
a later sections.

  * `Plug.Static` - serves static assets. Since this plug comes
    before the router, serving of static assets is not logged

  * `Plug.Logger` - logs incoming requests

  * `Plug.Parsers` - parses the request body when a known
    parser is available. By default parsers urlencoded,
    multipart and json (with poison). The request body is left
    untouched when the request content-type cannot be parsed

  * `Plug.MethodOverride` - converts the request method to
    `PUT`, `PATCH` or `DELETE` for `POST` requests with a
    valid `_method` parameter

  * `Plug.Head` - converts `HEAD` requests to `GET` requests and
    strips the response body

  * `Plug.Session` - a plug that sets up session management.
    Note that `fetch_session/2` must still be explicitly called
    before using the session as this plug just sets up how
    the session is fetched

  * `Phoenix.CodeReloader` - a plug that enables code reloading
    for all entries in the `web` directory. It is configured
    directly in the Phoenix application

### Customizing pipelines

You can define new pipelines at any moment with the `pipeline/2`
macro:

    pipeline :api do
      plug :token_authentication
    end

And then in a scope (or at root):

    pipe_through [:api]

## Router configuration

All routers are configured directly in the Phoenix application
environment. For example:

    config :phoenix, YourApp.Router,
      secret_key_base: "kjoy3o1zeidquwy1398juxzldjlksahdk3"

Phoenix configuration is split in two categories. Compile-time
configuration means the configuration is read during compilation
and changing it at runtime has no effect. Most of the compile-time
configuration is related to pipelines and plugs.

On the other hand, runtime configuration is accessed during or
after your application is started and can be read through the
`config/2` function:

    YourApp.Router.config(:port)
    YourApp.Router.config(:some_config, :default_value)

### Compile-time

  * `:session` - configures the `Plug.Session` plug. Defaults to
    `false` but can be set to a keyword list of options as defined
    in `Plug.Session`. For example:

        config :phoenix, YourApp.Router,
          session: [store: :cookie, key: "_your_app_key"]

  * `:parsers` - sets up the request parsers. Accepts a set of options
    as defined by `Plug.Parsers`. If parsers are disabled, parameters
    won't be explicitly fetched before matching a route and functionality
    dependent on parameters, like the `Plug.MethodOverride`, will be
    disabled too. Defaults to:

        [pass: ["*/*"],
         json_decoder: Poison,
         parsers: [:urlencoded, :multipart, :json]]

  * `:static` - sets up static assets serving. Accepts a set of options
    as defined by `Plug.Static`. Defaults to:

        [at: "/",
         from: Mix.Project.config[:app]]

  * `:debug_errors` - when true, uses `Plug.Debugger` functionality for
    debugging failures in the application. Recomended to be set to true
    only in development as it allows listing of the application source
    code during debugging. Defaults to false.

  * `:render_errors` - a module representing a view to render templates
    whenever there is a failure in the application. For example, if the
    application crashes with a 500 error during a HTML request,
    `render("500.html", assigns)` will be called in the view given to
    `:render_errors`. The default view is `MyApp.ErrorsView`.

### Runtime

  * `:http` - the configuration for the http server. Currently uses
    cowboy and accepts all options as defined by `Plug.Adapters.Cowboy`.
    Defaults to false.

  * `:https` - the configuration for the https server. Currently uses
    cowboy and accepts all options as defined by `Plug.Adapters.Cowboy`.
    Defaults to false.

  * `:secret_key_base` - a secret key used as base to generate secrets
    to encode cookies, session and friends. Defaults to nil as it must
    be set per application.

  * `:url` - configuration for generating URLs throughout the app.
    Accepts the host, scheme and port. Defaults to:

        [host: "localhost"]

## Web server

Starting a router as part of a web server can be done by invoking
`YourApp.Router.start/0`. Stopping the router is done with
`YourApp.Router.stop/0`. The web server is configured with the
`:http` and `:https` options defined above.
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