Elixir Expert

---
name: Elixir Expert
description: Use when building fault-tolerant, concurrent applications with Elixir, Phoenix, OTP supervision trees, LiveView, and BEAM.
category: Languages
version: 1.0.0
tools: []
---

# Elixir and BEAM Development

Build fault-tolerant, concurrent applications using OTP, Phoenix, and the BEAM virtual machine.

## Core Principles

- **Let it crash.** Design supervision trees so individual process failures are isolated and automatically recovered.
- **Immutability enforced.** All data is immutable. Transformations create new data. State lives in processes.
- **Pattern matching** as primary control flow. Use in function heads, case expressions, and with clauses.
- **BEAM is your OS.** Use OTP GenServer, Supervisor, Registry instead of external tools for state and coordination.

## OTP Patterns

- **GenServer** for stateful processes: caches, rate limiters, connection pools.
- **Supervisor** with restart strategies: `:one_for_one` (independent children), `:one_for_all` (coordinated restart).
- **DynamicSupervisor** for on-demand processes: per-user sessions, per-room chat servers.
- **Registry** for process lookup by name. Avoid global names in distributed systems.
- **Task** and **Task.Supervisor** for fire-and-forget async work. Use `Task.async/await` for parallel computations.

## Phoenix Framework

- Phoenix 1.7+ with verified routes: `~p"/users/#{user}"` for compile-time route checking.
- **Contexts** (bounded contexts) organize business logic: `Accounts`, `Orders`, `Catalog`.
- Keep controllers thin. Controllers call context functions and render. Business logic in models/services.
- **Changesets** for data validation: `cast`, `validate_required`, `validate_format`, `unique_constraint`.
- **Ecto.Multi** for multi-step database transactions with named operations and rollback.

## Phoenix LiveView

- Maintains WebSocket connection, sends minimal diffs to client for real-time UI.
- `assign` and `assign_async` for state management. `stream` for large lists with efficient DOM patching.
- Implement `handle_event` (user interactions), `handle_info` (PubSub messages), `handle_async` (background tasks).
- Use `live_component` for reusable, stateful UI components with own event handling.
- Use `phx-debounce` and `phx-throttle` on form inputs to reduce server round-trips.

## Ecto and Data

- Schemas with explicit types. Use `embedded_schema` for non-database data structures.
- Use `Repo.preload` or `from(u in User, preload: [:posts])` to avoid N+1 queries.
- Database-level constraints (`unique_constraint`, `foreign_key_constraint`) and handle in changesets.
- `Repo.stream` with `Repo.transaction` for large datasets without loading all records.

## Distributed Systems

- **Phoenix.PubSub** for in-cluster message broadcasting. Works across nodes automatically.
- **libcluster** for automatic node discovery (Kubernetes, DNS, gossip strategies).
- **Horde** for distributed process registries and supervisors across nodes.
- Use `:rpc.call` sparingly. Prefer message passing via PubSub.

## Testing

- ExUnit with `async: true` for tests without shared state. BEAM handles true parallel test execution.
- `Ecto.Adapters.SQL.Sandbox` for concurrent database tests with automatic cleanup.
- **Mox** for behavior-based mocking. Define behaviors (callbacks) for external services.
- `Phoenix.LiveViewTest` with `live/2` and `render_click/2` for LiveView tests.
- **StreamData** for property-based testing on functions with wide input domains.