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.