Phoenix + LiveView (Elixir/BEAM)

Phoenix builds on Elixir and the BEAM VM to deliver fault-tolerant, real-time web applications with minimal JavaScript. LiveView keeps UI state on the server while streaming HTML diffs over WebSockets. The BEAM provides lightweight processes, supervision trees, hot code upgrades, and soft-realtime scheduling.

Key ideas

• OTP supervision keeps web, data, and background processes isolated and restartable.
• Contexts encode domain boundaries (e.g., Accounts, Billing) around Ecto schemas and queries.
• LiveView renders HTML on the server, syncing UI state over WebSockets with minimal client code.
• PubSub + Presence enable fan-out updates, tracking, and collaboration features.

Environment and Project Setup

# Erlang + Elixir via asdf (recommended)
asdf install erlang 27.0
asdf install elixir 1.17.3
asdf global erlang 27.0 elixir 1.17.3

# Install Phoenix generator
mix archive.install hex phx_new

# Create project with LiveView + Ecto + esbuild
mix phx.new my_app --live
cd my_app
mix deps.get
mix ecto.create
mix phx.server

Project layout (key pieces):

• lib/my_app/application.ex — OTP supervision tree (Repo, Endpoint, Telemetry, PubSub, Oban, etc.)
• lib/my_app_web/endpoint.ex — Endpoint, plugs, sockets, LiveView config
• lib/my_app_web/router.ex — Pipelines, scopes, routes, LiveSessions
• lib/my_app/ — Contexts (domain modules) and Ecto schemas
• test/support/{conn_case,data_case}.ex — Testing helpers for Ecto + Phoenix

BEAM + OTP Essentials

Supervision tree (application.ex): keep short, isolated children.

def start(_type, _args) do
  children = [
    MyApp.Repo,
    {Phoenix.PubSub, name: MyApp.PubSub},
    MyAppWeb.Endpoint,
    {Oban, Application.fetch_env!(:my_app, Oban)},
    MyApp.Metrics
  ]

  Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
end

GenServer pattern: wrap stateful services.

defmodule MyApp.Counter do
  use GenServer

  def start_link(initial \\ 0), do: GenServer.start_link(__MODULE__, initial, name: __MODULE__)
  def increment(), do: GenServer.call(__MODULE__, :inc)

  @impl true
  def handle_call(:inc, _from, state) do
    new_state = state + 1
    {:reply, new_state, new_state}
  end
end

BEAM principles

• Prefer many small processes; processes are cheap and isolated.
• Supervise everything with clear restart strategies.
• Use message passing (GenServer.cast/send) to avoid shared state.
• Use ETS/Cachex for in-memory caches; keep them supervised.

Phoenix Anatomy and Routing

Pipelines and scopes (router.ex): keep browser/api concerns separated.

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

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

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

  scope "/", MyAppWeb do
    pipe_through :browser
    live "/", HomeLive
    resources "/users", UserController
  end

  scope "/api", MyAppWeb do
    pipe_through :api
    resources "/users", Api.UserController, except: [:new, :edit]
  end
end

Plugs: composable request middleware. Keep plugs pure and short; prefer pipeline plugs over controller plugs when cross-cutting.

Contexts and Ecto

Schema + changeset

defmodule MyApp.Accounts.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    field :email, :string
    field :hashed_password, :string
    field :confirmed_at, :naive_datetime
    timestamps()
  end

  def registration_changeset(user, attrs) do
    user
    |> cast(attrs, [:email, :password])
    |> validate_required([:email, :password])
    |> validate_format(:email, ~r/@/)
    |> validate_length(:password, min: 12)
    |> unique_constraint(:email)
    |> put_password_hash()
  end

  defp put_password_hash(%{valid?: true} = changeset),
    do: put_change(changeset, :hashed_password, Argon2.hash_pwd_salt(get_change(changeset, :password)))
  defp put_password_hash(changeset), do: changeset
end

Context API

defmodule MyApp.Accounts do
  import Ecto.Query, warn: false
  alias MyApp.{Repo, Accounts.User}

  def list_users, do: Repo.all(User)
  def get_user!(id), do: Repo.get!(User, id)

  def register_user(attrs) do
    %User{}
    |> User.registration_changeset(attrs)
    |> Repo.insert()
  end
end

Transactions with Ecto.Multi

alias Ecto.Multi

def register_and_welcome(attrs) do
  Multi.new()
  |> Multi.insert(:user, User