Phoenix

v1.3.0

Phoenix

Routing

Routers are the main hubs of Phoenix applications. They match HTTP requests to controller actions, wire up real-time channel handlers, and define a series of pipeline transformations for scoping middleware to sets of routes.

The router file that Phoenix generates, lib/hello_web/router.ex, will look something like this one:

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", HelloWeb do
    pipe_through :browser # Use the default browser stack

    get "/", PageController, :index
  end

  # Other scopes may use custom stacks.
  # scope "/api", HelloWeb do
  #   pipe_through :api
  # end
end

The name you gave your application will appear instead of Hello for both the router module and controller name.

The first line of this module, use HelloWeb, :router, simply makes Phoenix router functions available in our particular router.

Scopes have their own section in this guide, so we won’t spend time on the scope "/", HelloWeb do block here. The pipe_through :browser line will get a full treatment in the Pipeline section of this guide. For now, you only need to know that pipelines allow a set of middleware transformations to be applied to different sets of routes.

Inside the scope block, however, we have our first actual route:

  get "/", PageController, :index

get is a Phoenix macro which expands out to define one clause of the match/5 function. It corresponds to the HTTP verb GET. Similar macros exist for other HTTP verbs including POST, PUT, PATCH, DELETE, OPTIONS, CONNECT, TRACE and HEAD.

The first argument to these macros is the path. Here, it is the root of the application, /. The next two arguments are the controller and action we want to have handle this request. These macros may also take other options, which we will see throughout the rest of this guide.

If this were the only route in our router module, the clause of the match/5 function would look like this after the macro is expanded:

  def match(:get, "/", PageController, :index, [])

The body of the match/5 function sets up the connection and invokes the matched controller action.

As we add more routes, more clauses of the match function will be added to our router module. These will behave like any other multi-clause function in Elixir. They will be tried in order from the top, and the first clause to match the parameters given (verb and path) will be executed. After a match is found, the search will stop and no other clauses will be tried.

This means that it is possible to create a route which will never match, based on the HTTP verb and the path, regardless of the controller and action.

If we do create an ambiguous route, the router will still compile, but we will get a warning. Let’s see this in action.

Define this route at the bottom of the scope "/", HelloWeb do block in the router.

get "/", RootController, :index

Then run $ mix compile at the root of your project.

Examining Routes

Phoenix provides a great tool for investigating routes in an application, the mix task phx.routes.

Let’s see how this works. Go to the root of a newly-generated Phoenix application and run $ mix phx.routes. (If you haven’t already done so, you’ll need to run $ mix do deps.get, compile before running the routes task.) You should see something like the following, generated from the only route we currently have:

$ mix phx.routes
page_path  GET  /  HelloWeb.PageController :index

The output tells us that any HTTP GET request for the root of the application will be handled by the index action of the HelloWeb.PageController.

page_path is an example of what Phoenix calls a path helper, and we’ll talk about those very soon.

Resources

The router supports other macros besides those for HTTP verbs like get, post, and put. The most important among them is resources, which expands out to eight clauses of the match/5 function.

Let’s add a resource to our lib/hello_web/router.ex file like this:

scope "/", HelloWeb do
  pipe_through :browser # Use the default browser stack

  get "/", PageController, :index
  resources "/users", UserController
end

For this purpose, it doesn’t matter that we don’t actually have a HelloWeb.UserController.

Then go to the root of your project, and run $ mix phx.routes

You should see something like the following:

user_path  GET     /users           HelloWeb.UserController :index
user_path  GET     /users/:id/edit  HelloWeb.UserController :edit
user_path  GET     /users/new       HelloWeb.UserController :new
user_path  GET     /users/:id       HelloWeb.UserController :show
user_path  POST    /users           HelloWeb.UserController :create
user_path  PATCH   /users/:id       HelloWeb.UserController :update
           PUT     /users/:id       HelloWeb.UserController :update
user_path  DELETE  /users/:id       HelloWeb.UserController :delete

Of course, the name of your project will replace Hello.

This is the standard matrix of HTTP verbs, paths, and controller actions. Let’s look at them individually, in a slightly different order.

  • A GET request to /users will invoke the index action to show all the users.
  • A GET request to /users/:id will invoke the show action with an id to show an individual user identified by that ID.
  • A GET request to /users/new will invoke the new action to present a form for creating a new user.
  • A POST request to /users will invoke the create action to save a new user to the data store.
  • A GET request to /users/:id/edit will invoke the edit action with an ID to retrieve an individual user from the data store and present the information in a form for editing.
  • A PATCH request to /users/:id will invoke the update action with an ID to save the updated user to the data store.
  • A PUT request to /users/:id will also invoke the update action with an ID to save the updated user to the data store.
  • A DELETE request to /users/:id will invoke the delete action with an ID to remove the individual user from the data store.

If we don’t feel that we need all of these routes, we can be selective using the :only and :except options.

Let’s say we have a read-only posts resource. We could define it like this:

resources "/posts", PostController, only: [:index, :show]

Running $ mix phx.routes shows that we now only have the routes to the index and show actions defined.

post_path  GET     /posts      HelloWeb.PostController :index
post_path  GET     /posts/:id  HelloWeb.PostController :show

Similarly, if we have a comments resource, and we don’t want to provide a route to delete one, we could define a route like this.

resources "/comments", CommentController, except: [:delete]

Running $ mix phx.routes now shows that we have all the routes except the DELETE request to the delete action.

comment_path  GET    /comments           HelloWeb.CommentController :index
comment_path  GET    /comments/:id/edit  HelloWeb.CommentController :edit
comment_path  GET    /comments/new       HelloWeb.CommentController :new
comment_path  GET    /comments/:id       HelloWeb.CommentController :show
comment_path  POST   /comments           HelloWeb.CommentController :create
comment_path  PATCH  /comments/:id       HelloWeb.CommentController :update
              PUT    /comments/:id       HelloWeb.CommentController :update

The Phoenix.Router.resources/4 function describes additional options for customizing resource routes.

Forward

The Phoenix.Router.forward/4 macro can be used to send all requests that start with a particular path to a particular plug. Let’s say we have a part of our system that is responsible (it could even be a separate application or library) for running jobs in the background, it could have its own web interface for checking the status of the jobs. We can forward to this admin interface using:

defmodule HelloWeb.Router do
  use HelloWeb, :router

  #...

  scope "/", HelloWeb do
    #...
  end

  forward "/jobs", BackgroundJob.Plug
end

This means that all routes starting with /jobs will be sent to the BackgroundJob.Plug module.

We can even use the forward/4 macro in a pipeline. If we wanted to ensure that the user was authenticated and an administrator in order to see the jobs page, we could use the following in our router.

defmodule HelloWeb.Router do
  use HelloWeb, :router

  #...

  scope "/" do
    pipe_through [:authenticate_user, :ensure_admin]
    forward "/jobs", BackgroundJob.Plug
  end
end

This means that the plugs in the authenticate_user and ensure_admin pipelines will be called before the BackgroundJob.Plug allowing them to send an appropriate response and call halt() .

The opts that are passed to the init/1 callback of a Plug can be passed as a 3rd argument. For example, maybe the background job page lets you set the name of your application to be displayed on the page. This could be passed with:

  forward "/jobs", BackgroundJob.Plug, name: "Hello Phoenix"

There is a fourth router_opts argument that can be passed. These options are outlined in the Phoenix.Router.scope/2 documentation.

Although it is possible to forward to any module plug, it is not advised to forward to another endpoint. This is because plugs defined by your app and the forwarded endpoint would be invoked twice, which may lead to errors.

Writing an actual background job worker is beyond the scope of this guide. However for convenience and to allow you to test the code above, here is the implementation of BackgroundJob.Plug that you can copy into your application:

defmodule BackgroundJob.Plug do
  def init(opts), do: opts
  def call(conn, opts) do
    conn
    |> Plug.Conn.assign(:name, Keyword.get(opts, :name, "Background Job"))
    |> BackgroundJob.Router.call(opts)
  end
end

defmodule BackgroundJob.Router do
  use Plug.Router

  plug :match
  plug :dispatch

  get "/", do: send_resp(conn, 200, "Welcome to #{conn.assigns.name}")
  get "/active", do: send_resp(conn, 200, "5 Active Jobs")
  get "/pending", do: send_resp(conn, 200, "3 Pending Jobs")
  match _, do: send_resp(conn, 404, "Not found")
end

Path Helpers

Path helpers are functions which are dynamically defined on the Router.Helpers module for an individual application. For us, that is HelloWeb.Router.Helpers. Their names are derived from the name of the controller used in the route definition. Our controller is HelloWeb.PageController, and page_path is the function which will return the path to the root of our application.

That’s a mouthful. Let’s see it in action. Run $ iex -S mix at the root of the project. When we call the page_path function on our router helpers with the Endpoint or connection and action as arguments, it returns the path to us.

iex> HelloWeb.Router.Helpers.page_path(HelloWeb.Endpoint, :index)
"/"

This is significant because we can use the page_path function in a template to link to the root of our application. We can then use this helper in our templates:

<a href="<%= page_path(@conn, :index) %>">To the Welcome Page!</a>

The page_path function is imported into our template with use HelloWeb, :view. Please see the View Guide for more information.

This pays off tremendously if we should ever have to change the path of our route in the router. Since the path helpers are built dynamically from the routes, any calls to page_path in our templates will still work.

More on Path Helpers

When we ran the phx.routes task for our user resource, it listed the user_path as the path helper function for each line of output. Here is what that translates to for each action:

iex> import HelloWeb.Router.Helpers
iex> alias HelloWeb.Endpoint
iex> user_path(Endpoint, :index)
"/users"

iex> user_path(Endpoint, :show, 17)
"/users/17"

iex> user_path(Endpoint, :new)
"/users/new"

iex> user_path(Endpoint, :create)
"/users"

iex> user_path(Endpoint, :edit, 37)
"/users/37/edit"

iex> user_path(Endpoint, :update, 37)
"/users/37"

iex> user_path(Endpoint, :delete, 17)
"/users/17"

What about paths with query strings? By adding an optional fourth argument of key value pairs, the path helpers will return those pairs in the query string.

iex> user_path(Endpoint, :show, 17, admin: true, active: false)
"/users/17?admin=true&active=false"

What if we need a full url instead of a path? Just replace _path with _url:

iex(3)> user_url(Endpoint, :index)
"http://localhost:4000/users"

The _url functions will get the host, port, proxy port, and SSL information needed to construct the full URL from the configuration parameters set for each environment. We’ll talk about configuration in more detail in its own guide. For now, you can take a look at config/dev.exs file in your own project to see those values.

Whenever possible prefer to pass a conn in place of an Endpoint.

Nested Resources

It is also possible to nest resources in a Phoenix router. Let’s say we also have a posts resource which has a many-to-one relationship with users. That is to say, a user can create many posts, and an individual post belongs to only one user. We can represent that by adding a nested route in lib/hello_web/router.ex like this:

resources "/users", UserController do
  resources "/posts", PostController
end

When we run $ mix phx.routes now, in addition to the routes we saw for users above, we get the following set of routes:

. . .
user_post_path  GET     /users/:user_id/posts           HelloWeb.PostController :index
user_post_path  GET     /users/:user_id/posts/:id/edit  HelloWeb.PostController :edit
user_post_path  GET     /users/:user_id/posts/new       HelloWeb.PostController :new
user_post_path  GET     /users/:user_id/posts/:id       HelloWeb.PostController :show
user_post_path  POST    /users/:user_id/posts           HelloWeb.PostController :create
user_post_path  PATCH   /users/:user_id/posts/:id       HelloWeb.PostController :update
                PUT     /users/:user_id/posts/:id       HelloWeb.PostController :update
user_post_path  DELETE  /users/:user_id/posts/:id       HelloWeb.PostController :delete

We see that each of these routes scopes the posts to a user ID. For the first one, we will invoke the PostController index action, but we will pass in a user_id. This implies that we would display all the posts for that individual user only. The same scoping applies for all these routes.

When calling path helper functions for nested routes, we will need to pass the IDs in the order they came in the route definition. For the following show route, 42 is the user_id, and 17 is the post_id. Let’s remember to alias our HelloWeb.Endpoint before we begin.

iex> alias HelloWeb.Endpoint
iex> HelloWeb.Router.Helpers.user_post_path(Endpoint, :show, 42, 17)
"/users/42/posts/17"

Again, if we add a key/value pair to the end of the function call, it is added to the query string.

iex> HelloWeb.Router.Helpers.user_post_path(Endpoint, :index, 42, active: true)
"/users/42/posts?active=true"

Scoped Routes

Scopes are a way to group routes under a common path prefix and scoped set of plug middleware. We might want to do this for admin functionality, APIs, and especially for versioned APIs. Let’s say we have user generated reviews on a site, and that those reviews first need to be approved by an admin. The semantics of these resources are quite different, and they might not share the same controller. Scopes enable us to segregate these routes.

The paths to the user facing reviews would look like a standard resource.

/reviews
/reviews/1234
/reviews/1234/edit
. . .

The admin review paths could be prefixed with /admin.

/admin/reviews
/admin/reviews/1234
/admin/reviews/1234/edit
. . .

We accomplish this with a scoped route that sets a path option to /admin like this one. For now, let’s not nest this scope inside of any other scopes (like the scope "/", HelloWeb do one provided for us in a new app).

scope "/admin" do
  pipe_through :browser

  resources "/reviews", HelloWeb.Admin.ReviewController
end

Note also, that the way this scope is currently defined, we need to fully qualify our controller name, HelloWeb.Admin.ReviewController. We’ll fix that in a minute.

Running $ mix phx.routes again, in addition to the previous set of routes we get the following:

. . .
review_path  GET     /admin/reviews           HelloWeb.Admin.ReviewController :index
review_path  GET     /admin/reviews/:id/edit  HelloWeb.Admin.ReviewController :edit
review_path  GET     /admin/reviews/new       HelloWeb.Admin.ReviewController :new
review_path  GET     /admin/reviews/:id       HelloWeb.Admin.ReviewController :show
review_path  POST    /admin/reviews           HelloWeb.Admin.ReviewController :create
review_path  PATCH   /admin/reviews/:id       HelloWeb.Admin.ReviewController :update
             PUT     /admin/reviews/:id       HelloWeb.Admin.ReviewController :update
review_path  DELETE  /admin/reviews/:id       HelloWeb.Admin.ReviewController :delete

This looks good, but there is a problem here. Remember that we wanted both user facing reviews routes /reviews as well as the admin ones /admin/reviews. If we now include the user facing reviews in our router like this:

scope "/", HelloWeb do
  pipe_through :browser
  . . .
  resources "/reviews", ReviewController
  . . .
end

scope "/admin" do
  resources "/reviews", HelloWeb.Admin.ReviewController
end

and we run $ mix phx.routes, we get this output:

. . .
review_path  GET     /reviews                 HelloWeb.ReviewController :index
review_path  GET     /reviews/:id/edit        HelloWeb.ReviewController :edit
review_path  GET     /reviews/new             HelloWeb.ReviewController :new
review_path  GET     /reviews/:id             HelloWeb.ReviewController :show
review_path  POST    /reviews                 HelloWeb.ReviewController :create
review_path  PATCH   /reviews/:id             HelloWeb.ReviewController :update
             PUT     /reviews/:id             HelloWeb.ReviewController :update
review_path  DELETE  /reviews/:id             HelloWeb.ReviewController :delete
. . .
review_path  GET     /admin/reviews           HelloWeb.Admin.ReviewController :index
review_path  GET     /admin/reviews/:id/edit  HelloWeb.Admin.ReviewController :edit
review_path  GET     /admin/reviews/new       HelloWeb.Admin.ReviewController :new
review_path  GET     /admin/reviews/:id       HelloWeb.Admin.ReviewController :show
review_path  POST    /admin/reviews           HelloWeb.Admin.ReviewController :create
review_path  PATCH   /admin/reviews/:id       HelloWeb.Admin.ReviewController :update
             PUT     /admin/reviews/:id       HelloWeb.Admin.ReviewController :update
review_path  DELETE  /admin/reviews/:id       HelloWeb.Admin.ReviewController :delete

The actual routes we get all look right, except for the path helper review_path at the beginning of each line. We are getting the same helper for both the user facing review routes and the admin ones, which is not correct. We can fix this problem by adding an as: :admin option to our admin scope.

scope "/", HelloWeb do
  pipe_through :browser
  . . .
  resources "/reviews", ReviewController
  . . .
end

scope "/admin", as: :admin do
  resources "/reviews", HelloWeb.Admin.ReviewController
end

$ mix phx.routes now shows us we have what we are looking for.

. . .
      review_path  GET     /reviews                        HelloWeb.ReviewController :index
      review_path  GET     /reviews/:id/edit               HelloWeb.ReviewController :edit
      review_path  GET     /reviews/new                    HelloWeb.ReviewController :new
      review_path  GET     /reviews/:id                    HelloWeb.ReviewController :show
      review_path  POST    /reviews                        HelloWeb.ReviewController :create
      review_path  PATCH   /reviews/:id                    HelloWeb.ReviewController :update
                   PUT     /reviews/:id                    HelloWeb.ReviewController :update
      review_path  DELETE  /reviews/:id                    HelloWeb.ReviewController :delete
. . .
admin_review_path  GET     /admin/reviews                  HelloWeb.Admin.ReviewController :index
admin_review_path  GET     /admin/reviews/:id/edit         HelloWeb.Admin.ReviewController :edit
admin_review_path  GET     /admin/reviews/new              HelloWeb.Admin.ReviewController :new
admin_review_path  GET     /admin/reviews/:id              HelloWeb.Admin.ReviewController :show
admin_review_path  POST    /admin/reviews                  HelloWeb.Admin.ReviewController :create
admin_review_path  PATCH   /admin/reviews/:id              HelloWeb.Admin.ReviewController :update
                   PUT     /admin/reviews/:id              HelloWeb.Admin.ReviewController :update
admin_review_path  DELETE  /admin/reviews/:id              HelloWeb.Admin.ReviewController :delete

The path helpers now return what we want them to as well. Run $ iex -S mix and give it a try yourself.

iex(1)> HelloWeb.Router.Helpers.review_path(HelloWeb.Endpoint, :index)
"/reviews"

iex(2)> HelloWeb.Router.Helpers.admin_review_path(HelloWeb.Endpoint, :show, 1234)
"/admin/reviews/1234"

What if we had a number of resources that were all handled by admins? We could put all of them inside the same scope like this:

scope "/admin", as: :admin do
  pipe_through :browser

  resources "/images",  HelloWeb.Admin.ImageController
  resources "/reviews", HelloWeb.Admin.ReviewController
  resources "/users",   HelloWeb.Admin.UserController
end

Here’s what $ mix phx.routes tells us:

. . .
 admin_image_path  GET     /admin/images            HelloWeb.Admin.ImageController :index
 admin_image_path  GET     /admin/images/:id/edit   HelloWeb.Admin.ImageController :edit
 admin_image_path  GET     /admin/images/new        HelloWeb.Admin.ImageController :new
 admin_image_path  GET     /admin/images/:id        HelloWeb.Admin.ImageController :show
 admin_image_path  POST    /admin/images            HelloWeb.Admin.ImageController :create
 admin_image_path  PATCH   /admin/images/:id        HelloWeb.Admin.ImageController :update
                   PUT     /admin/images/:id        HelloWeb.Admin.ImageController :update
 admin_image_path  DELETE  /admin/images/:id        HelloWeb.Admin.ImageController :delete
admin_review_path  GET     /admin/reviews           HelloWeb.Admin.ReviewController :index
admin_review_path  GET     /admin/reviews/:id/edit  HelloWeb.Admin.ReviewController :edit
admin_review_path  GET     /admin/reviews/new       HelloWeb.Admin.ReviewController :new
admin_review_path  GET     /admin/reviews/:id       HelloWeb.Admin.ReviewController :show
admin_review_path  POST    /admin/reviews           HelloWeb.Admin.ReviewController :create
admin_review_path  PATCH   /admin/reviews/:id       HelloWeb.Admin.ReviewController :update
                   PUT     /admin/reviews/:id       HelloWeb.Admin.ReviewController :update
admin_review_path  DELETE  /admin/reviews/:id       HelloWeb.Admin.ReviewController :delete
  admin_user_path  GET     /admin/users             HelloWeb.Admin.UserController :index
  admin_user_path  GET     /admin/users/:id/edit    HelloWeb.Admin.UserController :edit
  admin_user_path  GET     /admin/users/new         HelloWeb.Admin.UserController :new
  admin_user_path  GET     /admin/users/:id         HelloWeb.Admin.UserController :show
  admin_user_path  POST    /admin/users             HelloWeb.Admin.UserController :create
  admin_user_path  PATCH   /admin/users/:id         HelloWeb.Admin.UserController :update
                   PUT     /admin/users/:id         HelloWeb.Admin.UserController :update
  admin_user_path  DELETE  /admin/users/:id         HelloWeb.Admin.UserController :delete

This is great, exactly what we want, but we can make it even better. Notice that for each resource, we needed to fully qualify the controller name by prefixing it with HelloWeb.Admin. That’s tedious and error prone. Assuming that the name of each controller begins with HelloWeb.Admin, then we can add a HelloWeb.Admin option to our scope declaration just after the scope path, and all of our routes will have the correct, fully qualified controller name.

scope "/admin", HelloWeb.Admin, as: :admin do
  pipe_through :browser

  resources "/images",  ImageController
  resources "/reviews", ReviewController
  resources "/users",   UserController
end

Now run $ mix phx.routes again and you can see that we get the same result as above when we qualified each controller name individually.

This doesn’t just apply to nested routes, we can even nest all of the routes for our application inside a scope that simply has an alias for the name of our Phoenix app, and eliminate the duplication of our application name in our controller names.

Phoenix already does this for us in the generated router for a new application (see beginning of this section). Notice here the use of HelloWeb.Router in the defmodule declaration:

defmodule HelloWeb.Router do
  use HelloWeb, :router

  scope "/", HelloWeb do
    pipe_through :browser

    get "/images", ImageController, :index
    resources "/reviews", ReviewController
    resources "/users",   UserController
  end
end

Again $ mix phx.routes tells us that all of our controllers now have the correct, fully-qualified names.

 image_path  GET     /images            HelloWeb.ImageController :index
review_path  GET     /reviews           HelloWeb.ReviewController :index
review_path  GET     /reviews/:id/edit  HelloWeb.ReviewController :edit
review_path  GET     /reviews/new       HelloWeb.ReviewController :new
review_path  GET     /reviews/:id       HelloWeb.ReviewController :show
review_path  POST    /reviews           HelloWeb.ReviewController :create
review_path  PATCH   /reviews/:id       HelloWeb.ReviewController :update
             PUT     /reviews/:id       HelloWeb.ReviewController :update
review_path  DELETE  /reviews/:id       HelloWeb.ReviewController :delete
  user_path  GET     /users             HelloWeb.UserController :index
  user_path  GET     /users/:id/edit    HelloWeb.UserController :edit
  user_path  GET     /users/new         HelloWeb.UserController :new
  user_path  GET     /users/:id         HelloWeb.UserController :show
  user_path  POST    /users             HelloWeb.UserController :create
  user_path  PATCH   /users/:id         HelloWeb.UserController :update
             PUT     /users/:id         HelloWeb.UserController :update
  user_path  DELETE  /users/:id         HelloWeb.UserController :delete

Although technically scopes can also be nested (just like resources), the use of nested scopes is generally discouraged because it can sometimes make our code confusing and less clear. With that said, suppose that we had a versioned API with resources defined for images, reviews and users. Then technically we could setup routes for the versioned API like this:

scope "/api", HelloWeb.Api, as: :api do
  pipe_through :api

  scope "/v1", V1, as: :v1 do
    resources "/images",  ImageController
    resources "/reviews", ReviewController
    resources "/users",   UserController
  end
end

$ mix phx.routes tells us that we have the routes we’re looking for.

 api_v1_image_path  GET     /api/v1/images            HelloWeb.Api.V1.ImageController :index
 api_v1_image_path  GET     /api/v1/images/:id/edit   HelloWeb.Api.V1.ImageController :edit
 api_v1_image_path  GET     /api/v1/images/new        HelloWeb.Api.V1.ImageController :new
 api_v1_image_path  GET     /api/v1/images/:id        HelloWeb.Api.V1.ImageController :show
 api_v1_image_path  POST    /api/v1/images            HelloWeb.Api.V1.ImageController :create
 api_v1_image_path  PATCH   /api/v1/images/:id        HelloWeb.Api.V1.ImageController :update
                    PUT     /api/v1/images/:id        HelloWeb.Api.V1.ImageController :update
 api_v1_image_path  DELETE  /api/v1/images/:id        HelloWeb.Api.V1.ImageController :delete
api_v1_review_path  GET     /api/v1/reviews           HelloWeb.Api.V1.ReviewController :index
api_v1_review_path  GET     /api/v1/reviews/:id/edit  HelloWeb.Api.V1.ReviewController :edit
api_v1_review_path  GET     /api/v1/reviews/new       HelloWeb.Api.V1.ReviewController :new
api_v1_review_path  GET     /api/v1/reviews/:id       HelloWeb.Api.V1.ReviewController :show
api_v1_review_path  POST    /api/v1/reviews           HelloWeb.Api.V1.ReviewController :create
api_v1_review_path  PATCH   /api/v1/reviews/:id       HelloWeb.Api.V1.ReviewController :update
                    PUT     /api/v1/reviews/:id       HelloWeb.Api.V1.ReviewController :update
api_v1_review_path  DELETE  /api/v1/reviews/:id       HelloWeb.Api.V1.ReviewController :delete
  api_v1_user_path  GET     /api/v1/users             HelloWeb.Api.V1.UserController :index
  api_v1_user_path  GET     /api/v1/users/:id/edit    HelloWeb.Api.V1.UserController :edit
  api_v1_user_path  GET     /api/v1/users/new         HelloWeb.Api.V1.UserController :new
  api_v1_user_path  GET     /api/v1/users/:id         HelloWeb.Api.V1.UserController :show
  api_v1_user_path  POST    /api/v1/users             HelloWeb.Api.V1.UserController :create
  api_v1_user_path  PATCH   /api/v1/users/:id         HelloWeb.Api.V1.UserController :update
                    PUT     /api/v1/users/:id         HelloWeb.Api.V1.UserController :update
  api_v1_user_path  DELETE  /api/v1/users/:id         HelloWeb.Api.V1.UserController :delete

Interestingly, we can use multiple scopes with the same path as long as we are careful not to duplicate routes. If we do duplicate a route, we’ll get this familiar warning.

warning: this clause cannot match because a previous clause at line 16 always matches

This router is perfectly fine with two scopes defined for the same path.

defmodule HelloWeb.Router do
  use Phoenix.Router
  . . .
  scope "/", HelloWeb do
    pipe_through :browser

    resources "/users", UserController
  end

  scope "/", AnotherAppWeb do
    pipe_through :browser

    resources "/posts", PostController
  end
  . . .
end

And when we run $ mix phx.routes, we see the following output.

user_path  GET     /users           HelloWeb.UserController :index
user_path  GET     /users/:id/edit  HelloWeb.UserController :edit
user_path  GET     /users/new       HelloWeb.UserController :new
user_path  GET     /users/:id       HelloWeb.UserController :show
user_path  POST    /users           HelloWeb.UserController :create
user_path  PATCH   /users/:id       HelloWeb.UserController :update
           PUT     /users/:id       HelloWeb.UserController :update
user_path  DELETE  /users/:id       HelloWeb.UserController :delete
post_path  GET     /posts           AnotherAppWeb.PostController :index
post_path  GET     /posts/:id/edit  AnotherAppWeb.PostController :edit
post_path  GET     /posts/new       AnotherAppWeb.PostController :new
post_path  GET     /posts/:id       AnotherAppWeb.PostController :show
post_path  POST    /posts           AnotherAppWeb.PostController :create
post_path  PATCH   /posts/:id       AnotherAppWeb.PostController :update
           PUT     /posts/:id       AnotherAppWeb.PostController :update
post_path  DELETE  /posts/:id       AnotherAppWeb.PostController :delete

Pipelines

We have come quite a long way in this guide without talking about one of the first lines we saw in the router - pipe_through :browser. It’s time to fix that.

Remember in the Overview Guide when we described plugs as being stacked and executable in a pre-determined order, like a pipeline? Now we’re going to take a closer look at how these plug stacks work in the router.

Pipelines are simply plugs stacked up together in a specific order and given a name. They allow us to customize behaviors and transformations related to the handling of requests. Phoenix provides us with some default pipelines for a number of common tasks. In turn we can customize them as well as create new pipelines to meet our needs.

A newly generated Phoenix application defines two pipelines called :browser and :api. We’ll get to those in a minute, but first we need to talk about the plug stack in the Endpoint plugs.

The Endpoint Plugs

Endpoints organize all the plugs common to every request, and apply them before dispatching into the router(s) with their underlying :browser, :api, and custom pipelines. The default Endpoint plugs do quite a lot of work. Here they are in order.

  • Plug.Static - serves static assets. Since this plug comes before the logger, serving of static assets is not logged

  • Plug.RequestId - generates a unique request id for each request.

  • Plug.Logger - logs incoming requests

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

  • Plug.Parsers - parses the request body when a known parser is available. By default parsers parse 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

  • Plug.Router - plugs a router into the request cycle

The :browser and :api Pipelines

Phoenix defines two other pipelines by default, :browser and :api. The router will invoke these after it matches a route, assuming we have called pipe_through/1 with them in the enclosing scope.

As their names suggest, the :browser pipeline prepares for routes which render requests for a browser. The :api pipeline prepares for routes which produce data for an api.

The :browser pipeline has five plugs: plug :accepts, ["html"] which defines the request format or formats which will be accepted, :fetch_session, which, naturally, fetches the session data and makes it available in the connection, :fetch_flash which retrieves any flash messages which may have been set, as well as :protect_from_forgery and :put_secure_browser_headers, which protects form posts from cross site forgery.

Currently, the :api pipeline only defines plug :accepts, ["json"].

The router invokes a pipeline on a route defined within a scope. If no scope is defined, the router will invoke the pipeline on all the routes in the router. Although the use of nested scopes is discouraged (see above), if we call pipe_through within a nested scope, the router will invoke all pipe_through’s from parent scopes, followed by the nested one.

Those are a lot of words bunched up together. Let’s take a look at some examples to untangle their meaning.

Here’s another look at the router from a newly generated Phoenix application, this time with the api scope uncommented back in and a route added.

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", HelloWeb do
    pipe_through :browser

    get "/", PageController, :index
  end

  # Other scopes may use custom stacks.
  scope "/api", HelloWeb do
    pipe_through :api

    resources "/reviews", ReviewController
  end
end

When the server accepts a request, the request will always first pass through the plugs in our Endpoint, after which it will attempt to match on the path and HTTP verb.

Let’s say that the request matches our first route: a GET to /. The router will first pipe that request through the :browser pipeline - which will fetch the session data, fetch the flash, and execute forgery protection - before it dispatches the request to the PageController index action.

Conversely, if the request matches any of the routes defined by the resources/2 macro, the router will pipe it through the :api pipeline - which currently does nothing - before it dispatches further to the correct action of the HelloWeb.ReviewController.

If we know that our application only renders views for the browser, we can simplify our router quite a bit by removing the api stuff as well as the scopes:

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipe_through :browser

  get "/", HelloWeb.PageController, :index

  resources "/reviews", HelloWeb.ReviewController
end

Removing all scopes forces the router to invoke the :browser pipeline on all routes.

Let’s stretch these ideas out a little bit more. What if we need to pipe requests through both :browser and one or more custom pipelines? We simply pipe_through a list of pipelines, and Phoenix will invoke them in order.

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end
  ...

  scope "/reviews" do
    # Use the default browser stack.
    pipe_through [:browser, :review_checks, :other_great_stuff]

    resources "/", HelloWeb.ReviewController
  end
end

Here’s another example with two scopes that have different pipelines:

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end
  ...

  scope "/", HelloWeb do
    pipe_through :browser

    resources "/posts", PostController
  end

  scope "/reviews", HelloWeb do
    pipe_through [:browser, :review_checks]

    resources "/", ReviewController
  end
end

In general, the scoping rules for pipelines behave as you might expect. In this example, all routes will pipe through the :browser pipeline. However, only the reviews resources routes will pipe through the :review_checks pipeline. Since we declared both pipes pipe_through [:browser, :review_checks] in a list of pipelines, Phoenix will pipe_through each of them as it invokes them in order.

Creating New Pipelines

Phoenix allows us to create our own custom pipelines anywhere in the router. To do so, we call the pipeline/2 macro with these arguments: an atom for the name of our new pipeline and a block with all the plugs we want in it.

defmodule HelloWeb.Router do
  use HelloWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :review_checks do
    plug :ensure_authenticated_user
    plug :ensure_user_owns_review
  end

  scope "/reviews", HelloWeb do
    pipe_through :review_checks

    resources "/", ReviewController
  end
end

Channel Routes

Channels are a very exciting, real-time component of the Phoenix framework. Channels handle incoming and outgoing messages broadcast over a socket for a given topic. Channel routes, then, need to match requests by socket and topic in order to dispatch to the correct channel. (For a more detailed description of channels and their behavior, please see the Channel Guide.)

We mount socket handlers in our endpoint at lib/hello_web/endpoint.ex. Socket handlers take care of authentication callbacks and channel routes.

defmodule HelloWeb.Endpoint do
  use Phoenix.Endpoint

  socket "/socket", HelloWeb.UserSocket
  ...
end

Next, we need to open our lib/hello_web/channels/user_socket.ex file and use the channel/3 macro to define our channel routes. The routes will match a topic pattern to a channel to handle events. If we have a channel module called RoomChannel and a topic called "rooms:*", the code to do this is straightforward.

defmodule HelloWeb.UserSocket do
  use Phoenix.Socket

  channel "rooms:*", HelloWeb.RoomChannel
  ...
end

Topics are just string identifiers. The form we are using here is a convention which allows us to define topics and subtopics in the same string - “topic:subtopic”. The * is a wildcard character which allows us to match on any subtopic, so "rooms:lobby" and "rooms:kitchen" would both match this route.

Phoenix abstracts the socket transport layer and includes two transport mechanisms out of the box - WebSockets and Long-Polling. If we wanted to make sure that our channel is handled by only one type of transport, we could specify that using the via option, like this.

channel "rooms:*", HelloWeb.RoomChannel, via: [Phoenix.TransportsWebSocket]

Each socket can handle requests for multiple channels.

channel "rooms:*", HelloWeb.RoomChannel, via: [Phoenix.TransportsWebSocket]
channel "foods:*", HelloWeb.FoodChannel

We can mount multiple socket handlers in our endpoint:

socket "/socket", HelloWeb.UserSocket
socket "/admin-socket", HelloWeb.AdminSocket

Summary

Routing is a big topic, and we have covered a lot of ground here. The important points to take away from this guide are:

  • Routes which begin with an HTTP verb name expand to a single clause of the match function.
  • Routes which begin with ‘resources’ expand to 8 clauses of the match function.
  • Resources may restrict the number of match function clauses by using the only: or except: options.
  • Any of these routes may be nested.
  • Any of these routes may be scoped to a given path.
  • Using the as: option in a scope can reduce duplication.
  • Using the helper option for scoped routes eliminates unreachable paths.