Phoenix Liveview

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.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.
• Assets: mix assets.deploy runs npm install, esbuild, tailwind (if configured), and digests.
• Releases: MIX_ENV=prod mix release. Configure runtime env in config/runtime.exs. Start with PHX_SERVER=true _build/prod/rel/my_app/bin/my_app start.
• Clustering: add libcluster with DNS/epmd strategy for horizontal scale; use distributed PubSub/Presence.
• Caching: use ETS/Cachex for hot paths; prefer short TTLs and invalidate on write.
• Background jobs: Oban for retries/backoff; supervise it in application tree.
• Hot path checks: enable :telemetry metrics, check LiveView diff sizes, avoid large assigns; prefer streams.

---

Common Pitfalls

• Forgetting to subscribe LiveViews to PubSub after connected?/1 check — events will be missed on initial render.
• Doing heavy work inside LiveView render; move to contexts and precompute assigns.
• Not using Ecto.Multi for multi-step writes; failures leave partial state.
• Blocking BEAM schedulers with long NIFs or heavy CPU work; offload to ports/Oban jobs.
• Overusing global ETS without supervision or limits; leak memory.

---

Reference Commands

• mix phx.routes — list routes and LiveView paths.
• mix phx.gen.live Accounts User users email:string confirmed_at:naive_datetime — generate LiveView CRUD (review context boundaries afterward).
• mix format && mix credo --strict — formatting and linting.
• mix test --seed 0 --max-failures 1 — deterministic failures; pair with mix test.watch.

Phoenix + LiveView excels when domain logic stays in contexts, LiveViews stay thin, and the BEAM supervises every component for resilience.