Phoenix Thinking

Resumo (pt-BR): Regras para Phoenix/LiveView: nunca fazer queries no mount (é chamado duas vezes). Carregar dados em handle_params. Scopes para autorização; PubSub com tópicos escopados.

Mental shifts for Phoenix applications. These insights challenge typical web framework patterns.

The Iron Law

NO DATABASE QUERIES IN MOUNT

mount/3 is called TWICE (HTTP request + WebSocket connection). Queries in mount = duplicate queries.

def mount(_params, _session, socket) do
  {:ok, assign(socket, posts: [], loading: true)}
end

def handle_params(params, _uri, socket) do
  posts = Blog.list_posts(socket.assigns.scope)
  {:noreply, assign(socket, posts: posts, loading: false)}
end

mount/3 = setup only (empty assigns, subscriptions, defaults). handle_params/3 = data loading (all database queries, URL-driven state). No exceptions.

Scopes: Security-First Pattern (Phoenix 1.8+)

Scopes address Broken Access Control. Authorization context is threaded automatically.

PubSub Topics Must Be Scoped

Unscoped topics = data leaks between tenants. Use e.g. "posts:org:#{org.id}".

External Polling: GenServer, Not LiveView

Single GenServer polls, broadcasts to all via PubSub—not one API call per user.

Components vs LiveViews

Functional components: Display-only, no internal state
LiveComponents: Own state, handle own events
LiveViews: Full page, owns URL, top-level state

Async Data Loading

Use assign_async/3 for data that can load after mount.

Gotchas

terminate/2: Only fires if trapping exits; don't rely on it in LiveView for cleanup—use a GenServer that monitors the LiveView.
start_async duplicate names: Later call wins; use cancel_async/3 first if aborting previous task.
Channel intercept socket: State in handle_out is stale (snapshot at subscription time).
CSS class precedence: Determined by stylesheet order, not HTML order—use variant props instead of class merging.
Upload content-type: User-provided; validate actual file contents (magic bytes).
Webhooks: Read body before Plug.Parsers to verify signatures; don't use preserve_req_body: true for all requests.

Red Flags - STOP and Reconsider

Database query in mount/3
Unscoped PubSub topics in multi-tenant app
LiveView polling external APIs directly
Using terminate/2 for cleanup without trap_exit
start_async with same name without cancel_async first
Relying on socket.assigns in Channel intercepts (stale)
Trusting %Plug.Upload{}.content_type for security

Any of these? Re-read The Iron Law and the Gotchas section.

phoenix-thinking