dots-spawning-patterns

Spawning Patterns — Senior Patterns

Spawning N entities efficiently is the difference between a 60fps shooter and a 6fps slideshow. The correct pattern is batched Instantiate from a baked prefab Entity, recorded into an ECB when called from a job, with a deterministic seed.

Intent

Create entities in bulk, deterministically, without main-thread cost in hot paths.

Senior pattern — main-thread spawner (one batch per trigger)

public partial struct SpawnSystem : ISystem
{
    uint m_UpdateCounter;

    [BurstCompile]
    public void OnCreate(ref SystemState state)
    {
        // Don't run until a Spawner singleton exists (set by baking).
        state.RequireForUpdate<Spawner>();
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        var spinningCubesQuery = SystemAPI.QueryBuilder().WithAll<RotationSpeed>().Build();

        // Only spawn when none exist — bursty, not per-frame.
        if (!spinningCubesQuery.IsEmpty) return;

        var prefab = SystemAPI.GetSingleton<Spawner>().Prefab;

        // Batched: instantiate 500 in one call, return a temp NativeArray of entities.
        var instances = state.EntityManager.Instantiate(prefab, 500, Allocator.Temp);

        // Deterministic random: CreateFromIndex hashes the seed so consecutive
        // seeds don't produce visually-similar results.
        var rng = Random.CreateFromIndex(m_UpdateCounter++);

        foreach (var entity in instances)
        {
            var transform = SystemAPI.GetComponentRW<LocalTransform>(entity);
            transform.ValueRW.Position = (rng.NextFloat3() - new float3(0.5f, 0, 0.5f)) * 20f;
        }
        // Allocator.Temp auto-frees at end of frame. No manual Dispose.
    }
}

Senior pattern — spawn from inside a parallel job

When spawn cadence depends on per-entity state (e.g. each turret fires when its cooldown expires):

public partial struct TurretFireSystem : ISystem
{
    [BurstCompile]
    public void OnCreate(ref SystemState state) {
        state.RequireForUpdate<Spawner>();
        state.RequireForUpdate<BeginSimulationEntityCommandBufferSystem.Singleton>();
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        var prefab = SystemAPI.GetSingleton<Spawner>().BulletPrefab;
        var ecb = SystemAPI.GetSingleton<BeginSimulationEntityCommandBufferSystem.Singleton>()
                           .CreateCommandBuffer(state.WorldUnmanaged)
                           .AsParallelWriter();

        new FireJob {
            Ecb = ecb,
            Prefab = prefab,
            Dt = SystemAPI.Time.DeltaTime,
        }.ScheduleParallel();
    }
}

[BurstCompile]
partial struct FireJob : IJobEntity
{
    public EntityCommandBuffer.ParallelWriter Ecb;
    public Entity Prefab;
    public float Dt;

    void Execute([ChunkIndexInQuery] int chunkIndex,
                 ref TurretCooldown cd,
                 in LocalTransform t)
    {
        cd.TimeLeft -= Dt;
        if (cd.TimeLeft > 0) return;
        cd.TimeLeft = cd.Interval;

        // Record the spawn; ECB plays back next frame.
        var bullet = Ecb.Instantiate(chunkIndex, Prefab);
        Ecb.SetComponent(chunkIndex, bullet, LocalTransform.FromPosition(t.Position));
    }
}

Deterministic randomness

// Per-frame seed; the same frame number gives the same sequence — replay-safe.
var rng = Random.CreateFromIndex(updateCounter);

// Per-entity seed (so two entities created on the same frame diverge):
var rng = Random.CreateFromIndex((uint)entity.Index ^ updateCounter);

Banned:

• UnityEngine.Random — managed, not Burst-compatible, not deterministic across runs
• new Random() without seed — non-deterministic
• Random.CreateFromIndex(0) — CreateFromIndex rejects 0; will throw

Anti-patterns

• ❌ state.EntityManager.Instantiate(prefab) called in a loop. Always use the batched overload that takes a count or a destination array.
• ❌ Spawning from a job using state.EntityManager.Instantiate directly. Must go through Ecb.Instantiate with [ChunkIndexInQuery].
• ❌ Loading the prefab via Resources.Load or Addressables in OnUpdate. The prefab Entity is baked at conversion — fetch it once from a singleton.
• ❌ Spawning N entities per frame "just in case". Gate with RequireForUpdate<T> and intent components; spawn bursty, not continuous.
• ❌ Using Allocator.Persistent for the temp entity array from Instantiate. Allocator.Temp is right — it auto-frees at frame end.
• ❌ Forgetting RequireForUpdate<Spawner>() in OnCreate. Without it, GetSingleton<Spawner>() throws if no spawner exists.

Failure modes

Symptom	Likely cause
Spawn cost spikes in the profiler	Per-entity Instantiate instead of batched
"InvalidOperationException: Singleton does not exist"	Missing RequireForUpdate<Spawner>
Spawn position is always (0,0,0)	Forgot to set LocalTransform after Instantiate; prefab's transform applies but per-entity position not written
Replays diverge	UnityEngine.Random or unseeded Random
Crash on Random.CreateFromIndex(0)	Seed must be non-zero; use `(updateCounter
Bullets spawn one frame late	Expected — ECB playback. If you need same-frame visibility, the reader must run after the ECB phase you recorded into

Runtime verification

• Static: every Instantiate(prefab) call should use the batched overload or be wrapped in Ecb.Instantiate. Grep for EntityManager.Instantiate( with no second argument inside OnUpdate.
• Runtime: spawn N entities, assert query count rises by exactly N within the expected number of frames (1 for main-thread, 2 for ECB-deferred). Run twice with the same seed; assert positions are bit-identical.

Performance notes

• Batched Instantiate(prefab, 500, Allocator.Temp) is roughly the cost of one structural change, not 500. Always batch.
• The baked prefab Entity has all components fully constructed; spawn copies them. Minimize prefab archetype size if spawning thousands per frame — every byte multiplies.
• Spawning a million entities at once is feasible; spawning ten per frame for a hundred thousand frames is not. Pick burst spawn over sustained drip.

Compile / editor safety

• The prefab Entity reference is produced by a Baker (see dots-baking-patterns). Don't try to load it at runtime.
• Random here is Unity.Mathematics.Random (using Unity.Mathematics;) — Burst-friendly value type. UnityEngine.Random looks similar but is managed and banned in hot paths.

Entities version notes (1.4.x)

• state.EntityManager.Instantiate(prefab, count, allocator) is the current batched overload.
• Random.CreateFromIndex(seed) is current. Old code used new Random((uint)Time.frameCount) — non-deterministic across builds, refuse it.
• Allocator.Temp for per-system, single-frame arrays. Allocator.TempJob only when an array is passed into a job that may outlive the calling method.

See also

• dots-baking-patterns — how the prefab Entity ref gets made
• dots-ecb-orchestration — phase selection for in-job spawn
• dots-entity-lifecycle — destruction is the matching half