Phoenix v1.6.0-rc.1
Phoenix

Routing

Requirement: This guide expects that you have gone through the introductory guides and got a Phoenix application up and running.

Requirement: This guide expects that you have gone through the Request life-cycle guide.

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 scoped to a set 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

    get "/", PageController, :index
  end

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

Both the router and controller module names will be prefixed with the name you gave your application suffixed with Web instead of HelloWeb.

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 "Pipelines" section of this guide. For now, you only need to know that pipelines allow a set of plugs 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 that corresponds to the HTTP verb GET. Similar macros exist for other HTTP verbs including POST, PUT, PATCH, DELETE, OPTIONS, CONNECT, TRACE and HEAD.

Examining routes

Phoenix provides a great tool for investigating routes in an application: mix phx.routes.

Let's see how this works. Go to the root of a newly-generated Phoenix application and run mix phx.routes. You should see something like the following, generated with all routes you currently have:

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

The route above 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. Let's add a resource to our lib/hello_web/router.ex file like this:

scope "/", HelloWeb do
  pipe_through :browser

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

For now it doesn't matter that we don't actually have a HelloWeb.UserController.

Run mix phx.routes once again at the root of your project. 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
...

This is the standard matrix of HTTP verbs, paths, and controller actions. For a while, this was known as RESTful routes, but most consider this a misnomer nowadays. 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/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 GET request to /users/new will invoke the new action to present a form for creating a new user.
  • A GET request to /users/:id will invoke the show action with an id to show an individual user identified by that ID.
  • A POST request to /users will invoke the create action to save a new user to the data store.
  • 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 feel that we don't need all these routes, we can be selective using the :only and :except options to filter certain actions.

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 macro describes additional options for customizing resource routes.

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. The name of each path helper is 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:

<%= link "Welcome Page!", to: Routes.page_path(@conn, :index) %>

The reason we can use Routes.page_path instead of the full HelloWeb.Router.Helpers.page_path name is because HelloWeb.Router.Helpers is aliased as Routes by default in the view_helpers/0 block defined inside lib/hello_web.ex. This definition is made available to our templates through use HelloWeb, :view.

We can, of course, use HelloWeb.Router.Helpers.page_path(@conn, :index) instead, but the convention is to use the aliased version for conciseness. Note that the alias is only set automatically for use in views, controllers and templates - outside these you need either the full name, or to alias it yourself inside the module definition with alias HelloWeb.Router.Helpers, as: Routes.

Using path helpers makes it easy to ensure our controllers, views and templates are linking to pages our router can actually handle.

More on path helpers

When we ran mix phx.routes 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> alias HelloWeb.Router.Helpers, as: Routes
iex> alias HelloWeb.Endpoint

iex> Routes.user_path(Endpoint, :index)
"/users"

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

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

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

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

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

iex> Routes.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> Routes.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> Routes.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 (or @conn in your views) in place of an endpoint module.

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 PostController's 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"

If we hadn't aliased the Helpers module as we did before (remember it is only automatically aliased for views, templates and controllers), and since we are inside iex, we'll have to do it ourselves:

iex> alias HelloWeb.Router.Helpers, as: Routes
iex> alias HelloWeb.Endpoint
iex> Routes.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 plugs. 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 administrator. 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 administration review paths can 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. We can nest this scope inside another scope, but instead let's set it by itself at the root, by adding to lib/hello_web/router.ex the following:

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

  resources "/reviews", ReviewController
end

We define a new scope where all routes are prefixed with /admin and all controllers are under the HelloWeb.Admin namespace.

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 review routes /reviews as well as the admin ones /admin/reviews. If we now include the user facing reviews in our router under the root scope like this:

scope "/", HelloWeb do
  pipe_through :browser

  ...
  resources "/reviews", ReviewController
end

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

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

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

iex> 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", HelloWeb.Admin, as: :admin do
  pipe_through :browser

  resources "/images",  ImageController
  resources "/reviews", ReviewController
  resources "/users",   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. Note how every route, path helper and controller is properly namespaced.

Scopes can also be arbitrarily nested, but you should do it carefully as nesting 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

You can run mix phx.routes to see how these definitions will look like.

Interestingly, we can use multiple scopes with the same path as long as we are careful not to duplicate routes. The following 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

If we do duplicate a route — which means two routes having the same path and the same alias — we'll get this familiar warning:

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

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.

Pipelines are a series of plugs that can be attached to specific scopes. If you are not familiar with plugs, we have an in-depth guide about them.

Routes are defined inside scopes and scopes may pipe through multiple pipelines. Once a route matches, Phoenix invokes all plugs defined in all pipelines associated to that route. For example, accessing / will pipe through the :browser pipeline, consequently invoking all of its plugs.

Phoenix defines two pipelines by default, :browser and :api, which can be used for a number of common tasks. In turn we can customize them as well as create new pipelines to meet our needs.

The :browser and :api pipelines

As their names suggest, the :browser pipeline prepares for routes which render requests for a browser, and 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. Routes outside of a scope have no pipelines. Although the use of nested scopes is discouraged (see above the versioned API example), 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 PageController's 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 no route matches, no pipeline is invoked and a 404 error is raised.

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
    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 [:browser, :review_checks]

    resources "/", ReviewController
  end
end

Note that pipelines themselves are plugs, so we can plug a pipeline inside another pipeline. For example, we could rewrite the review_checks pipeline above to automatically invoke browser, simplifying the downstream pipeline call:

  pipeline :review_checks do
    plug :browser
    plug :ensure_authenticated_user
    plug :ensure_user_owns_review
  end

  scope "/reviews", HelloWeb do
    pipe_through :review_checks

    resources "/", ReviewController
  end

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 HelloWeb.BackgroundJob.Plug module. Inside the plug, you can match on subroutes, such as /pending and /active that shows the status of certain jobs.

We can even mix the forward/4 macro with pipelines. If we wanted to ensure that the user was authenticated and was 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 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 halt the request accordingly.

The opts that are received in the init/1 callback of the Module Plug can be passed as a third argument. For example, maybe the background job 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.

BackgroundJob.Plug can be implemented as any Module Plug discussed in the Plug guide. Note though it is not advised to forward to another Phoenix endpoint. This is because plugs defined by your app and the forwarded endpoint would be invoked twice, which may lead to errors.

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 declared 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.