Elixir Patterns

Overview

Start with pure functions. Escalate to processes only when you need runtime concerns — fault isolation, parallelism, or state across calls. Separate domain logic from temporal logic.

Pattern Escalation

What are you solving?
  Transform data, no side effects      → Level 0 (pure functions)
  Operations that can fail              → Level 1 (tagged tuples, with)
  Validating external input             → Level 2 (Ecto.Changeset)
  Need state across calls               → Level 3 (GenServer, consider ETS first)
  Process might crash                   → Level 4 (Supervision)
  Many dynamic processes to coordinate  → Level 5 (Registry, Task, DynamicSupervisor)
  Organizing modules into domains       → Level 6 (Contexts, Protocols, Behaviours)

Common Mistakes

• •Reaching for GenServer too early: Most business logic is pure functions. A GenServer adds memory, serialization, and complexity. Only justified by runtime benefits.
• •Single Global Process on multiple nodes: GenServer-as-cache diverges silently across nodes. Default to the database for consistency.
• •Unbounded queues: Every queue must be bounded. Unbounded queues are a latent memory leak.
• •Circuit breakers as default resilience: They convert partial failures into complete failures. Prefer token bucket retries.
• •Returning error tuples nobody can act on: If nothing actionable exists, raise instead.

Reference Files

• •escalation-ladder.md — Full Pattern Escalation Ladder (Levels 0-6 with code examples and decision triggers)
• •otp-patterns.md — GenServer, Supervisor, Registry, Task, Protocol, SGP anti-pattern, init guarantees, BEAM nuances
• •overload-management.md — Back-pressure, load-shedding, circuit breaker critique, token bucket retries, adaptive concurrency
• •domain-patterns.md — Phoenix contexts, code quality patterns, pattern matching, tagged tuples, changesets
• •references/contexts.md — Full context design patterns, boundaries, anti-patterns, testing

Related Skills

• •distributed-systems: Multi-node clustering, consensus, CRDTs
• •production-quality: Testing, security, observability
• •cognitive-complexity: Ousterhout principles, deep modules
• •phoenix-liveview: LiveView-specific patterns