How do I link Elixir Livebook to a Phoenix application that is already running?

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To link Elixir Livebook to a Phoenix application that is already running, you can follow these steps:

1. Install the Livebook package by adding {:livebook, "~> 0.2.0"} to your mix.exs file and running mix deps.get.

2. In your Phoenix application, create a new Phoenix controller, for example LivebookController, and define an action that will create and start a new Livebook server:

   defmodule MyApp.LivebookController do
    use MyApp.Web, :controller
   
    def livebook(_conn, _params) do
      {:ok, path} = Livebook.new(%{port: 0, storage: :ephemeral})
      {:ok, %{
        port: path.port,
        url: "http://localhost:#{path.port}",
        token: path.token
      }}
    end
   end
   

   This action will create a new Livebook server and return its port, URL, and token as a response.

3. In your Phoenix router (web/router.ex), add a new route that will be handled by the LivebookController:

   scope "/api" do
    post "/livebook", LivebookController, :livebook
   end
   

4. In your frontend application, make an HTTP POST request to the /api/livebook endpoint to start a new Livebook server, for example using the axios library:

   const response = await axios.post('/api/livebook')
   const { port, url, token } = response.data
   

   This will return the port, URL, and token of the Livebook server that was just created.

5. Open the Livebook UI by navigating to the URL returned by the /api/livebook endpoint in your browser, and paste in the token when prompted. You should now be connected to the Livebook server that is linked to your Phoenix application.

You can then use the Livebook server to execute Elixir code and interact with your Phoenix application, for example by making HTTP requests to its endpoints or accessing its database.