--- name: scene-runtime-and-crdt description: "Scene runtime internals — V8/ClearScript engine, CRDT protocol, JS module system, scene lifecycle, and threading. Use when implementing or modifying scene runtime modules (require/wrapper/API pattern), working with CRDTProtocol or CRDTWorldSynchronizer, handling MultiThreadSync for ECS access, debugging scene state transitions, or implementing new JS API modules." user-invocable: false --- # Scene Runtime & CRDT ## Sources - `docs/scene-runtime.md` -- SDK7 execution via ClearScript/V8, CRDT bridge, JS modules - `docs/architecture-overview.md` -- Worlds, threading, ECS-async marriage --- ## Scene Lifecycle State Machine **File:** `Explorer/Assets/DCL/Infrastructure/SceneRunner/Scene/SceneState.cs` ```csharp public enum SceneState : byte { NotStarted, // Created but not started Running, // Active update loop EngineError, // Scene communication broken (exception tolerance exceeded) EcsError, // ECS World execution error JavaScriptError, // JS code error (tolerance exceeded) Disposing, // Signaled for disposal Disposed, // Fully disposed } ``` **Transitions:** - `NotStarted` -> `Running` via `SceneFacade.StartUpdateLoopAsync` (calls `SetRunning`) - `Running` -> `EngineError` / `EcsError` / `JavaScriptError` via `SceneExceptionsHandler` when tolerance exceeded - `Running` -> `Disposing` -> `Disposed` via `SceneFacade.DisposeAsync` **`ISceneStateProvider`** exposes `Atomic State`, `TickNumber`, `IsCurrent`, and `EngineStartInfo`. The `Atomic` wrapper provides thread-safe reads/writes since state is mutated from background threads. **`IsNotRunningState()`** returns true for `Disposing`, `Disposed`, `JavaScriptError`, or `EngineError` -- used as a guard to break the update loop. --- ## Threading Model (CRITICAL) > See `docs/scene-runtime.md` -- "the scene lifecycle is thread agnostic and executes in a separate thread." **No thread affinity.** Each scene runs on the thread pool. After every `await`, the continuation may resume on a different thread. You cannot assume thread consistency. ### MultiThreadSync -- Mutex for ECS Access **File:** `Explorer/Assets/DCL/PerformanceAndDiagnostics/Optimization/Multithreading/MultiThreadSync.cs` Arch ECS is not thread-safe. All ECS reads and writes must be serialized through `MultiThreadSync`: ```csharp // Acquire a scope before touching ECS state (from any thread) using MultiThreadSync.Scope mutex = multiThreadSync.GetScope(syncOwner); // Now safe to call crdtWorldSynchronizer.ApplySyncCommandBuffer(...) // or any World read/write ``` - `GetScope(Owner)` blocks until the queue grants access (10s timeout) - `Owner` is a named `ManualResetEventSlim`-based waiter -- create one per logical caller - The scope is a `readonly struct` implementing `IDisposable` -- release is automatic ### SyncedGroup -- Automatic System Synchronization **File:** `Explorer/Assets/DCL/Infrastructure/ECS/Groups/SyncedGroup.cs` Systems in scene worlds use `SyncedGroup` subclasses (`SyncedSimulationSystemGroup`, etc.) that guard `Update`/`BeforeUpdate`/`AfterUpdate` behind a `SceneState.Running` check. This prevents systems from running during disposal: ```csharp public override void Update(in float t, bool throttle) { if (sceneStateProvider.State != SceneState.Running) return; UpdateInternal(in t, throttle); } ``` ### When Explicit Mutex Is Needed - `SyncedGroup` handles synchronization for the normal ECS update loop -- no manual action needed - Use `MultiThreadSync.GetScope()` explicitly for ECS access **outside** the update loop (e.g., async flows in `LoadSystemBase`, `EngineAPIImplementation.ApplySyncCommandBuffer`) --- ## Module Implementation Pattern > See `docs/scene-runtime.md` -- "How to Implement a Module" Each JS API module follows a **4-file pattern**: ### 1. Interface Defines the C# contract. Example: `IEngineApi` ### 2. Implementation Core logic, thread-agnostic. Example: `EngineAPIImplementation` -- receives CRDT bytes, deserializes, reconciles, syncs to ECS, serializes outgoing messages. ### 3. JsApiWrapper Bridges JS calls to C#. Extends `JsApiWrapper` which holds the API impl and a `CancellationTokenSource` for disposal. The wrapper catches exceptions and routes them through `ISceneExceptionsHandler`: ```csharp // From EngineApiWrapper.cs (condensed) public class EngineApiWrapper : JsApiWrapper { public PoolableByteArray CrdtSendToRenderer(ITypedArray data) { if (disposeCts.IsCancellationRequested) return PoolableByteArray.EMPTY; try { instancePoolsProvider.RenewCrdtRawDataPoolFromScriptArray(data, ref lastInput); return api.CrdtSendToRenderer(lastInput.Memory); } catch (Exception e) { if (!disposeCts.IsCancellationRequested) exceptionsHandler.OnEngineException(e); return PoolableByteArray.EMPTY; } } } ``` ### 4. JS Module + Registration - JS module in `Explorer/Assets/StreamingAssets/Js/Modules/EngineApi.js` - Compiled modules loaded via `SceneModuleHub.LoadAndCompileJsModules` - Wrappers registered via `SceneRuntimeImpl.Register(name, wrapper)` and added as host objects - Full module wiring done in `SceneFactory.CreateSceneAsync` which calls `sceneRuntime.RegisterAll(...)` --- ## CRDT Protocol Internals **File:** `Explorer/Assets/DCL/Infrastructure/CRDT/Protocol/CRDTProtocol.cs` ### LWW vs GOVS Processing `ProcessMessage` dispatches by `CRDTMessageType`: - **LWW** (`PUT_COMPONENT`, `DELETE_COMPONENT`, `AUTHORITATIVE_PUT_COMPONENT`): Timestamp comparison. Higher timestamp wins. On tie, byte-level data comparison breaks it. Returns `CRDTReconciliationResult` with effect (`ComponentAdded`, `ComponentModified`, `ComponentDeleted`, `NoChanges`). - **GOVS** (`APPEND_COMPONENT`): Accumulates values in a sorted list per entity+component. Binary search prevents duplicates. Capped at 100 entries per entity-component pair. - **`DELETE_ENTITY`**: Removes all LWW and APPEND data for the entity, tracks version in `deletedEntities`. ### Zero-Allocation Design - State stored in `PooledDictionary` / `PooledList` backed by `ArrayPool.Shared` - `IMemoryOwner` for message data -- disposed returns buffer to pool - Deserialization via `ReadOnlyMemory` advanced forward (no copies) - `ByteUtils` for unsafe memory slicing (`reinterpret_cast`-style) ### State Structure ```csharp internal struct State { // Entity Number -> deleted Entity Version internal readonly PooledDictionary deletedEntities; // ComponentId -> (CRDTEntity -> EntityComponentData) for LWW internal readonly PooledDictionary> lwwComponents; // ComponentId -> (CRDTEntity -> list of EntityComponentData) for APPEND internal readonly PooledDictionary>> appendComponents; internal int messagesCount; } ``` --- ## ECS-CRDT Bridge ### Inbound: Scene -> ECS **File:** `Explorer/Assets/DCL/Infrastructure/CrdtEcsBridge/JsModulesImplementation/EngineAPIImplementation.cs` The flow in `CrdtSendToRenderer` (called from background thread): 1. **Deserialize** -- `crdtDeserializer.DeserializeBatch` into `List` 2. **Reconcile** -- Each message through `crdtProtocol.ProcessMessage` 3. **Buffer** -- `worldSyncBuffer.SyncCRDTMessage` prepares ECS mutations 4. **Apply** -- `ApplySyncCommandBuffer` acquires `MultiThreadSync` scope, then `crdtWorldSynchronizer.ApplySyncCommandBuffer` writes to ECS World 5. **Respond** -- `SerializeOutgoingCRDTMessages` returns buffered outgoing data ### ICRDTWorldSynchronizer **File:** `Explorer/Assets/DCL/Infrastructure/CrdtEcsBridge/WorldSynchronizer/CrdtEcsSynchronizer.cs` - `GetSyncCommandBuffer()` -- Rents a command buffer (semaphore-guarded, one at a time) - `ApplySyncCommandBuffer(buffer)` -- Applies mutations to `World`, releases semaphore - Uses `SemaphoreSlim` (not `Mutex`) because acquire/release may happen on different threads ### Outbound: ECS -> Scene Systems write to `IECSToCRDTWriter` (`PutMessage`, `AppendMessage`, `DeleteMessage`). These are collected by `IOutgoingCRDTMessagesProvider` and serialized back to the scene in `EngineAPIImplementation.SerializeOutgoingCRDTMessages`. Outgoing LWW messages are also synced into the local `CRDTProtocol` state via `EnforceLWWState` to keep timestamps correct. --- ## Gotchas - **`getMutableOrNull` marks the component dirty.** In scene-side TypeScript (SDK), calling `getMutableOrNull` on a component marks it dirty in the CRDT system, which sets `IsDirty = true` on the Explorer side. **Never use `getMutableOrNull` for read-only access** (e.g., reading values to render a UI panel). Use `getOrNull` instead. Calling `getMutableOrNull` every frame in a UI render function causes the component to be permanently dirty, triggering full property re-application and side effects like re-triggering bursts, restarting animations, or redundant material operations. - **V8 engines are not poolable.** Each scene creates a new `V8ScriptEngine`; when disposed, the engine is gone. This creates GC pressure but `ScriptEngine` is not reusable. ClearScript handles unmanaged cleanup. - **No thread affinity.** After any `await` in scene code, the thread may change. Never cache `Thread.CurrentThread` or use thread-local storage. - **API implementations must be thread-agnostic.** Shared resources must be thread-safe. This applies to all `JsApiWrapper` implementations. - **`PoolableByteArray` must be disposed.** It wraps a pooled `byte[]` with a release callback. Failing to dispose leaks the rented array. Use `ReleaseAndDispose()` for wrapper cleanup. - **`SceneExceptionsHandler` has per-minute tolerance.** JS errors (30/min) and engine errors (3/min) before the scene is suspended. Exceptions are tracked with sliding-window timestamps. - **`Atomic`** is required because state transitions happen from background threads. Always use `State.Set()` / `State.Value()`, never direct assignment. --- ## Cross-References - **sdk-component-implementation** -- Outgoing CRDT via `IECSToCRDTWriter`, component registration in `ComponentsContainer` - **async-programming** -- Cancellation token patterns, `SuppressToResultAsync` for exception-free flows - **diagnostics-and-logging** -- `SceneExceptionsHandler` tolerance, `ReportHub.LogException` for scene errors, `ReportCategory.CRDT` / `CRDT_ECS_BRIDGE` / `JAVASCRIPT` --- Source attribution: decentraland Original project: https://github.com/decentraland/unity-explorer/blob/9a03ce0931a819ca98557660d4d7531009ce100c/.claude/skills/scene-runtime-and-crdt/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: decentraland Original project: https://github.com/decentraland/unity-explorer/blob/9a03ce0931a819ca98557660d4d7531009ce100c/.claude/skills/scene-runtime-and-crdt/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Product Management Roles: Backend Developer, DevOps / SRE, Full-Stack Developer, Game Developer, Technical Product Manager Specialties: Backend, DevOps Tools: Clear Script Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 850c42e62e0e0d4c391846866c8769cc892db612b18d8cc757555bf947417833