Unity DOTS / ECS (Entities 1.x)

When (and when not)

DOTS shines with lots of similar things doing lots of similar work: 10k+ units, particle-as-entity sim, deterministic physics for netcode. For typical character-based gameplay (a player, ~10 enemies, UI) classic GameObjects + MonoBehaviour are simpler and fast enough.

Mixing both: hybrid mode — GameObjects for hero/UI, ECS for swarms.

Components

public struct Position : IComponentData { public float3 Value; }
public struct Velocity : IComponentData { public float3 Value; }
public struct Health   : IComponentData { public float Value; }
public struct Enemy    : IComponentData {} // tag

public struct Inventory : IBufferElementData {     // dynamic buffer
    public Entity Item;
    public int    Count;
}

All IComponentData are blittable structs (no reference types directly).

Systems — ISystem (preferred over SystemBase for Burst)

[BurstCompile]
public partial struct MoveSystem : ISystem {
    public void OnUpdate(ref SystemState state) {
        float dt = SystemAPI.Time.DeltaTime;
        new MoveJob { DeltaTime = dt }.ScheduleParallel();
    }
}

[BurstCompile]
public partial struct MoveJob : IJobEntity {
    public float DeltaTime;
    void Execute(ref Position pos, in Velocity vel) {
        pos.Value += vel.Value * DeltaTime;
    }
}

partial struct lets Unity generate the iteration boilerplate at compile time.

Querying

foreach (var (transform, velocity) in
         SystemAPI.Query<RefRW<Position>, RefRO<Velocity>>().WithAll<Enemy>()) {
    transform.ValueRW.Value += velocity.ValueRO.Value * dt;
}

RefRW = read/write, RefRO = read-only (better job scheduling).

Baking — GameObject → Entity

public class EnemyAuthoring : MonoBehaviour {
    public float Speed;
}
public class EnemyBaker : Baker<EnemyAuthoring> {
    public override void Bake(EnemyAuthoring authoring) {
        var e = GetEntity(TransformUsageFlags.Dynamic);
        AddComponent(e, new Velocity { Value = new float3(0, 0, authoring.Speed) });
        AddComponent<Enemy>(e);
    }
}

Designers tweak in Inspector; baking turns Authoring components into Entity components at subscene conversion time.

Sub Scenes

Big static worlds → put in a SubScene asset, baked offline → loaded by streaming. Smaller content/dynamic spawning → spawn at runtime via EntityManager.CreateEntity.

Anti-patterns

Production checklist

• DOTS used only where it beats GameObject perf measurably
• All hot systems Burst-compiled
• EntityCommandBuffer used for structural changes
• SubScenes for static content
• Authoring components reviewed by designers
• Determinism verified if ECS feeds netcode