Data-Oriented Architecture Patterns

When To Use This Skill

Activate this skill when:

• Encountering switch statements or if/else chains dispatching on entity/object type
• Designing systems with multiple variants of similar entities
• Refactoring code where adding new types requires changes in multiple locations
• Building plugin systems, handler registries, or factory patterns
• Noticing the "expression problem" (hard to add new types AND new operations)

Core Principle

Separate data from behavior, dispatch via registry.

Entity = Pure Data (what it IS) + type discriminator
Definition = Bundled Behavior (what it DOES)
Registry = Type → Definition mapping (HOW to dispatch)

Pattern 1: Registry-Based Polymorphism

Problem

Switch statements scattered throughout codebase:

// Scattered in rendering.ts
switch (entity.type) {
  case 'typeA': renderA(entity); break;
  case 'typeB': renderB(entity); break;
}

// Scattered in update.ts
switch (entity.type) {
  case 'typeA': updateA(entity); break;
  case 'typeB': updateB(entity); break;
}

// Adding new type = edit N files

Solution

Single registry bundling all type-specific behavior:

// definitions.ts - ONE location for all type-specific code
const DEFS: Record<EntityType, Definition> = {
  typeA: { render: renderA, update: updateA, ... },
  typeB: { render: renderB, update: updateB, ... },
};

// Consumers dispatch generically
DEFS[entity.type].render(entity, ctx);
DEFS[entity.type].update(entity, dt);

// Adding new type = ONE registry entry, ZERO consumer changes

Implementation Checklist

1. Define base Definition interface with all operations
2. Create DEFS: Record<Type, Definition> registry
3. Export getDef(type): Definition helper
4. Replace all switches with getDef(entity.type).operation()
5. Use language features for exhaustiveness (TypeScript Record, Rust match)

Pattern 2: Capability Composition

Problem

Not all entities need all behaviors. Deep inheritance or marker interfaces create coupling.

Solution

Optional capability configs with type guards:

interface Definition<T> {
  // Required for all
  create(): T;
  render(): void;

  // Optional capabilities - entities opt-in
  collision?: CollisionConfig;
  physics?: PhysicsConfig;
  persistence?: PersistenceConfig;
}

// Type guard for safe access
function hasCollision(def): def is Definition & { collision: CollisionConfig } {
  return def.collision !== undefined;
}

// Consumer checks capability
if (hasCollision(def)) {
  collisionSystem.register(entity, def.collision);
}

Benefits

• Entities opt-in to behaviors they need
• No inheritance hierarchies
• Capability presence is runtime-checkable
• Systems ignore entities without relevant capabilities

Pattern 3: Layered Definition Interfaces

BaseDefinition           (create, render, layer)
    ↓ extends
DomainDefinition         (domain-specific: AI, weapons)
    ↓ implemented by
ConcreteDefinitions      (typeA, typeB, typeC)

Implementation

// Base - works for any domain
interface EntityDefinition<TState, TType> {
  type: TType;
  create(pos): TState;
  update?(entity, ctx): void;
  render(entity, ctx): void;
  collision?: CollisionConfig;
  physics?: PhysicsConfig;
}

// Domain-specific extension
interface EnemyDefinition extends EntityDefinition<EnemyState, EnemyType> {
  aiStrategy: AIStrategy;
  weapons: WeaponConfig[];
}

// Another domain
interface PickupDefinition extends EntityDefinition<PickupState, PickupType> {
  onCollect(collector): void;
  floatAnimation: AnimationConfig;
}

Pattern 4: Context Objects

Problem

Functions with many parameters, hard to extend.

Solution

Bundle related parameters into context objects:

// Bad - hard to extend
function update(entity, dt, playerPos, playerVel, gravity, time) { ... }

// Good - extensible
interface UpdateContext {
  dt: number;
  playerPos: Vec2;
  playerVel: Vec2;
  // Easy to add fields without breaking signatures
}

function update(entity, ctx: UpdateContext) { ... }

Context Inheritance

interface BaseContext { dt: number; }
interface AIContext extends BaseContext { playerPos: Vec2; threats: Entity[]; }
interface RenderContext { graphics: Graphics; screenPos: Vec2; scale: number; }

Pattern 5: Infrastructure-First Development

Order of Implementation

1. Generic infrastructure first (dispatcher, event bus, registry helpers)
2. Base interfaces (EntityDefinition, capability configs)
3. First domain implementation (proves the pattern)
4. Second domain validates pattern (confirms generality)
5. Retrofit existing systems (migrate incrementally)

Rule

If writing a switch statement on entity type, infrastructure is missing.

Anti-Patterns To Avoid

1. Scattered Switches

Adding new type requires editing N files. Fix: Consolidate into registry.

2. Deep Inheritance

SpecialEnemy extends FlyingEnemy extends Enemy extends Entity Fix: Capability composition.

3. Optional Fields Instead of Capabilities

interface Entity {
  weapon?: Weapon;  // null checks everywhere
}

Fix: Separate capability with type guard.

4. Premature Abstraction

Creating registry for 1 type. Fix: Wait for second type to validate pattern.

5. God Objects

Definition with 50 fields for every possible behavior. Fix: Required base + optional capabilities.

Exhaustiveness Enforcement

Use language features to ensure all types are handled:

// TypeScript - Record requires all keys
const DEFS: Record<EntityType, Definition> = {
  // Compiler error if type missing
};

// Helper for switch exhaustiveness
function assertNever(x: never): never {
  throw new Error(`Unexpected: ${x}`);
}

Summary Checklist

When designing entity systems:

• Entities are pure data with type discriminator field
• Definitions bundle ALL type-specific behavior
• Single registry maps type → definition
• Consumers dispatch via registry lookup (no switches)
• Capabilities are optional configs with type guards
• Context objects bundle related parameters
• Language features enforce exhaustiveness
• Adding new type = one registry entry, zero system changes