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composable-architecture

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Apply functional and compositional architecture patterns when designing systems, modules, and components. Use when creating architecture definitions, designing system boundaries, defining interfaces, or building modular systems.

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Composable Architecture Patterns

Core Principles

Functional Core, Imperative Shell

  • Pure core: Business logic as pure functions (no side effects)
  • Imperative shell: I/O pushed to system edges
  • Unit test core, integration test shell

Composition Over Inheritance

  • Build from small, composable functions
  • Prefer data transformation over stateful mutation
  • Use pipelines and modifier chaining

Hexagonal Architecture (Ports & Adapters)

[External] → [Adapter] → [Port] → [Core] → [Port] → [Adapter] → [External]
  • Core: Pure business logic, no external dependencies
  • Ports: Interfaces defining capabilities
  • Adapters: Implementations connecting to external systems

C4 Model Hierarchy

  1. System Context: Boundaries and external actors
  2. Module: Major architectural components (map 1:1 to directories)
  3. Component: Internal module structure
  4. Code: Implementation details

Design Process

New system:

  1. Define boundaries (inside vs outside)
  2. Identify ports (capabilities needed/provided)
  3. Design pure core (business rules as functions)
  4. Create adapters (connect to external world)

Refactoring:

  1. Extract pure functions from mixed logic
  2. Define interfaces for dependencies
  3. Move I/O to edges
  4. Replace inheritance with composition

Anti-patterns

  • Mixing business logic with I/O
  • Circular dependencies between modules
  • Using inheritance for code reuse
  • Coupling core logic to specific technologies
  • "God" modules with multiple responsibilities

Remember: Build from simple, pure, testable pieces. Keep complex logic in pure core, push messy reality to edges.