Abstract

This ADR describes the outline of the "scene runtime" for Decentraland, it includes a minimum set of required environment functions to run a scene, including the formalization of the RPC protocol to load other parts i.e. the Rendering engine (Renderer from now on).

What is a scene?

Decentraland Explorers (defined in ADR-102 are often compared with operative systems that run programs. A scene is a deployable JavaScript program that controls a set of entities in-world, the user-interface, and also may add functionality to the Explorer. Those programs run in a sandboxed environment exposing a set of functions to enable the scene to communicate with other components like the Rendering engine.

Loading scenes

The deployed scenes MUST comply with the Scene schema defined in ADR-51. And the format used to represent the deployment is the one used in the content servers as defined in ADR-80. Entities can be loaded as scenes if their metadata matches the scene.json schema. For the sake of simplicity in this specification, we are assuming a minimum scene.json in the shape of {"main": "bin/scene.js"} to illustrate how to load and run the code.

1. First the information of the entity is fetched, it includes the list of deployed files. Based on the entity information, the renderer and runtime will know how to resolve assets.
2. Then the runtime will create an isolated runtime environment for the program of the scene.
3. The runtime will use the entity information to fetch the code from bin/scene.js and run it. The mechanism to resolve files based on deployed entities is explained in detail in ADR-79.
4. After that first eval, the scene may load the entities from the EngineAPI module, and it will start a main loop tight after.

Program states & lifecycle

stateDiagram
    [*] --> CreateSceneContext

    state Renderer {
      CreateSceneContext --> LoadInitialScenes("main.crdt")
      LoadInitialScenes("main.crdt") --> CreateRuntime(Entity)
      LoadInitialScenes("main.crdt") --> crdtGetState
    }



    CreateRuntime(Entity) --> FetchCode(Entity)

    crdtGetState --> .OnStart()

    state RuntimeSandbox {
      FetchCode(Entity) --> Eval(code)
      RequireModules --> Eval(code)

    .OnStart()
    Eval(code) --> .OnStart()

      .OnStart() --> .OnUpdate(dt)


      state MainLoop {
        .OnUpdate(dt) --> crdtSendToRenderer
        crdtSendToRenderer --> .OnUpdate(dt)
      }
    }

    MainLoop --> [*]

Exposed functions and objects

The runtime for the SDK7 is compatible with CommonJS's require to load RPC modules. This is so to enable a wide variety of bundlers to create compatible Decentraland scenes.

The exposed RPC modules are defined in the protocol repository.

TODO: define and document naming conventions about code generation for modules

// `require` instantiates a proxy to a RPC module. Every exposed function
// of the module returns a promise.
// require must fail immediately if the moduleName is invalid or unknown,
// and it must return a Module or Proxy synchronously
function require(moduleName: string): Module

// Commonjs-compatible modules
const exports: Object
const module: {
  readonly exports: typeof exports
}

// extra functions
function fetch(requestInit: Request): Promise<Response>
function fetch(url: string, requestInit: Request): Promise<Response>

class WebSocket {}

function setImmediate(fn: Function): void

TODO: Document fetch and WebSocket adaptations for Decentraland Scenes

Loading initial state

It is recommended that scene creators via tooling create a CRDT dump of the initial state of the scene. This is so to enable many optimizations like:

• Enabling rendering of the world before the runtime is ready
• Enabling tooling to create WYSIWYG editors for scenes
• Enabling tooling to create scenes with a minimum of code
• Preventing bloating the scene's code with imperative code only to create the initial state

The runtime will load the initial state of the scene from the main.crdt file (if present). This file is a CRDT dump of the scene's initial state or generated by tooling. The runtime will load the file and apply the CRDT messages to the scene's state. This state will be sent to the scene using the initial EngineApi.crdtGetState RPC call, and if the main.crdt file was loaded, the field EngineApi.crdtGetState().hasEntities will be set to true.

Static entities

The EngineApi.crdtGetState must also return all the initial state of static entities like RootEntity and PlayerEntity. Details about static entities are described in ADR-219.

Note on custom components

The main.crdt file may contain scene-owned components, defined in code. The renderer MAY ignore these components but the runtime MUST NOT. The runtime MUST send the components to the scene, because these are strictly necessary to ensure the well functioning of the scene.

Synchronizing scene's entities with the renderer

The scenes synchronize with the renderer via the EngineApi.crdtSendToRenderer RPC using the CRDT protocol defined in ADR-117. The renderer will keep a local copy of all the entities and components required for rendering. Those components are in their majority serialized using protobuf as defined in ADR-123.

The EngineApi.crdtSendToRenderer response includes a list of CRDT messages to be applied in the local scene, that is used to send information back from the renderer like the position of the player.

sequenceDiagram
  participant S as Scene
  participant K as Runtime
  participant R as Renderer

  note over R: Create SceneContext and load initial entities
  critical load scene
  R->>R: Create SceneContext
  R->>R: Load "main.crdt" initial state
  end

  note over K: Create a RuntimeSandbox for the scene to run
  R-->>K: Create scene VM(ID=1,SceneId,Mappings,...)
  K-->>S: Load the code of the scene and execute it (eval)

  note over S: Initialize scene and load state from renderer

  S->>K: require("~system/EngineApi")
  activate K
  K-->>S: EngineApi
  deactivate K

  note over S: onStart()

  R->>S: EngineApi.crdtGetState() (initial entities travel to the scene)

  opt if `.hasEntities==false`
    S->>S: Load "main.crdt" initial state
  end

  note over S: loop { onUpdate() }
  loop function onUpdate(deltaTime: number)
    S-->>S: engine.update(deltaTime)
    S->>R: EngineApi.crdtSendToRenderer(stateChanges)
    activate R
    R-->>R: Apply patches to the engine owned entities
    R-->>R: Execute queries
    R-->>S: CRDT changes (if any)
    deactivate R
    S-->>S: Apply patches to the scene owned entities
  end

Runtime event handlers

The scene can hook up to certain events by adding functions to the module.exports variable. The functions that can be registered are:

• onStart(): Promise<void> | void is the first function to be called in a scene. It is recommended that all side-effects related to the initialization of a scene are performed inside the onStart function.
• onUpdate(deltaTime: number): Promise<void> | void is called every frame. It is in charge of the scene itself to run the frame and send/receive changes to the renderer

// The following example only illustrates an hypothetic scenario,
// since it is a low-level API and it shouldn't be used this way
let rotation = 0
export async function onUpdate(deltaTimeSeconds: number) {
  const speed = 0.001
  rotation += deltaTimeSeconds * speed
  updateEntityRotation(rotation)
  await sendUpdatesToRenderer()
}

💡 Since the runtime is compatible with CommonJS, the event handler functions can be exported as export function ... and skip the module.exports = ... for convenience.

Pseudocode example of a scene

const engineApi = require("~system/EngineApi")

// this is a lamport timestamp, required by the CRDT rules
let timestamp = 0

const position = Vector3.Zero()
const scale = Vector3.One()
const rotation = Quaternion.Identity()

// entities are now numbers
const entityId = 1234

// component numbers, defined in .proto files
const transformId = 1
const rendererMeshId = 2

const transform = Transform.serialize({ position, rotation, scale })
const mesh = RendererMesh.serialize({ box: {} })

// now we are sending the component messages from the LWW-ElementSet
// this sets the transform & meshRenderer for the entity
const messagesBackFromRenderer = await engineApi.crdtSendToRenderer([
  CRDT.PutMessage(entityId, transformId, transform, timestamp++),
  CRDT.PutMessage(entityId, rendererMeshId, mesh, timestamp++)
])

module.exports.onUpdate = function (deltaTime: number) {
  const transformId = 1
  position.x += deltaTime
  const transform = Transform.serialize({ position, rotation, scale })

  // now we are sending the component messages from the LWW-ElementSet
  // this sets the transform & meshRenderer for the entity
  const messagesBackFromRenderer = await engineApi.crdtSendToRenderer([
    CRDT.PutMessage(entityId, transformId, transform, timestamp++)
  ])
}

License