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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.
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.registration_changeset(%User{}, attrs))
|> Multi.run(:welcome_email, fn _repo, %{user: user} ->
MyApp.Mailer.deliver_welcome(user)
{:ok, :sent}
end)
|> Repo.transaction()
end
LiveView Patterns
LiveView module (stateful UI on server)
defmodule MyAppWeb.CounterLive do
use MyAppWeb, :live_view
def mount(_params, _session, socket) do
{:ok, assign(socket, count: 0)}
end
def handle_event("inc", _params, socket) do
{:noreply, update(socket, :count, &(&1 + 1))}
end
def render(assigns) do
~H"""
<div class="space-y-4">
<p class="text-lg">Count: <%= @count %></p>
<button phx-click="inc" class="btn">Increment</button>
</div>
"""
end
end
HEEx tips
Prefer assign_new/3 to lazily compute expensive data only once per connected session.
Use stream/3 for large lists to minimize diff payloads.
Handle params in handle_params/3 for URL-driven state; avoid storing socket state in params.
Live Components
defmodule MyAppWeb.NavComponent do
use MyAppWeb, :live_component
def render(assigns) do
~H"""
<nav>
<%= for item <- @items do %>
<.link navigate={item.href}><%= item.label %></.link>
<% end %>
</nav>
"""
end
end
PubSub-driven LiveView
@impl true
def mount(_params, _session, socket) do
if connected?(socket), do: Phoenix.PubSub.subscribe(MyApp.PubSub, "orders")
{:ok, assign(socket, orders: [])}
end
@impl true
def handle_info({:order_created, order}, socket) do
{:noreply, update(socket, :orders, fn orders -> [order | orders] end)}
end
PubSub, Channels, and Presence
Broadcast changes from contexts
def create_order(attrs) do
with {:ok, order} <- %Order{} |> Order.changeset(attrs) |> Repo.insert() do
Phoenix.PubSub.broadcast(MyApp.PubSub, "orders", {:order_created, order})
{:ok, order}
end
end
Presence for online/typing indicators
defmodule MyAppWeb.RoomChannel do
use Phoenix.Channel
alias Phoenix.Presence
def join("room:" <> room_id, _payload, socket) do
send(self(), :after_join)
{:ok, assign(socket, :room_id, room_id)}
end
def handle_info(:after_join, socket) do
Presence.track(socket, socket.assigns.user_id, %{online_at: System.system_time(:second)})
push(socket, "presence_state", Presence.list(socket))
{:noreply, socket}
end
end
Security: authorize topics in join/3, verify user tokens in params/session, and limit payload size.
Testing Phoenix + LiveView
Use mix test with the generated helpers.
# test/support/conn_case.ex
use MyAppWeb.ConnCase, async: true
test "renders home", %{conn: conn} do
conn = get(conn, "/")
assert html_response(conn, 200) =~ "Welcome"
end
# LiveView test
use MyAppWeb.ConnCase, async: true
import Phoenix.LiveViewTest
test "counter increments", %{conn: conn} do
{:ok, view, _html} = live(conn, "/counter")
view |> element("button", "Increment") |> render_click()
assert render(view) =~ "Count: 1"
end
DataCase: provide sandboxed DB connections; wrap tests in transactions to isolate data.
Fixtures: build factories with ExMachina or simple helper modules under test/support/fixtures.
Performance, Ops, and Deployment
Telemetry: Phoenix exposes events ([:phoenix, :endpoint, ...]). Export via :telemetry_poller, OpentelemetryPhoenix, and OpentelemetryEcto.