phoenix-thinking
This skill should be used when the user asks to "add a LiveView page", "create a form", "handle real-time updates", "broadcast changes to users", "add a new route", "create an API endpoint", "fix this LiveView bug", "why is mount called twice?", or mentions handle_event, handle_info, handle_params, mount, channels, controllers, components, assigns, sockets, or PubSub. Essential for avoiding duplicate queries in mount.
What this skill does
# Phoenix Thinking
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.
```elixir
def mount(_params, _session, socket) do
# NO database queries here! Called twice.
{:ok, assign(socket, posts: [], loading: true)}
end
def handle_params(params, _uri, socket) do
# Database queries here - once per navigation
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:** Don't query "just this one small thing" in mount. Don't "optimize later". LiveView lifecycle is non-negotiable.
## Scopes: Security-First Pattern (Phoenix 1.8+)
Scopes address OWASP #1 vulnerability: Broken Access Control. Authorization context is threaded automatically—no more forgetting to scope queries.
```elixir
def list_posts(%Scope{user: user}) do
Post |> where(user_id: ^user.id) |> Repo.all()
end
```
## PubSub Topics Must Be Scoped
```elixir
def subscribe(%Scope{organization: org}) do
Phoenix.PubSub.subscribe(@pubsub, "posts:org:#{org.id}")
end
```
Unscoped topics = data leaks between tenants.
## External Polling: GenServer, Not LiveView
**Bad:** Every connected user makes API calls (multiplied by users).
**Good:** Single GenServer polls, broadcasts to all via PubSub.
## Components Receive Data, LiveViews Own Data
- **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:
```elixir
def mount(_params, _session, socket) do
{:ok, assign_async(socket, :user, fn -> {:ok, %{user: fetch_user()}} end)}
end
```
## Gotchas from Core Team
### LiveView terminate/2 Requires trap_exit
`terminate/2` only fires if you're trapping exits—which you shouldn't do in LiveView.
**Fix:** Use a separate GenServer that monitors the LiveView process via `Process.monitor/1`, then handle `:DOWN` messages to run cleanup.
### start_async Duplicate Names: Later Wins
Calling `start_async` with the same name while a task is in-flight: the **later one wins**, the previous task's result is ignored.
**Fix:** Call `cancel_async/3` first if you want to abort the previous task.
### Channel Intercept Socket State is Stale
The socket in `handle_out` intercept is a snapshot from subscription time, not current state.
**Why:** Socket is copied into fastlane lookup at subscription time for performance.
**Fix:** Use separate topics per role, or fetch current state explicitly.
### CSS Class Precedence is Stylesheet Order
When merging classes on components, precedence is determined by **stylesheet order**, not HTML order. If `btn-primary` appears later in the compiled CSS than `bg-red-500`, it wins regardless of HTML order.
**Fix:** Use variant props instead of class merging.
### Upload Content-Type Can't Be Trusted
The `:content_type` in `%Plug.Upload{}` is user-provided. Always validate actual file contents (magic bytes) and rewrite filename/extension.
### Read Body Before Plug.Parsers for Webhooks
To verify webhook signatures, you need the raw body. But Plug.Parsers consumes it.
```elixir
{:ok, body, conn} = Plug.Conn.read_body(conn)
verify_signature!(conn, body)
%{conn | body_params: JSON.decode!(body)}
```
Don't use `preserve_req_body: true`—it keeps the entire body in memory 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 (won't fire without trap_exit)
- Calling start_async with same name without cancel_async first
- Relying on socket.assigns in Channel intercepts (stale!)
- CSS class merging for component customization (use variants)
- Trusting `%Plug.Upload{}.content_type` for security
**Any of these? Re-read The Iron Law and the Gotchas section.**
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