Multiplayer Synchronization in Decentraland

Decentraland runs scenes locally in a player's instance of the explorer. By default, players are able to see each other and interact directly, but each player interacts with the environment independently. Changes in the environment aren't shared between players by default.

To sync any changes in the scene state, SDK7 uses CRDT-based synchronization.

Runtime constraint: Decentraland runs in a QuickJS sandbox. No Node.js APIs (fs, http, path, process). Use fetch() and WebSocket for network communication. See the scene-runtime skill for async patterns.

Sync Strategy Decision Tree

Choose the right networking approach based on what you need:

Decision flow:

1. Does every player need to see the same state, including late joiners? --> syncEntity
2. Is it a fire-and-forget event only for players currently in the scene? --> MessageBus
3. Do you need the information to be persisted even after all players leave, or to run secure validations on that information? --> fetch or signedFetch
4. Do you need continuous real-time server communication? --> WebSocket
5. Combine approaches freely: use syncEntity for world state, MessageBus for effects, and fetch for persistence.

syncEntity Essentials

Import and Basic Usage

import { engine, Transform, MeshRenderer, Material } from '@dcl/sdk/ecs'
import { syncEntity } from '@dcl/sdk/network'
import { Vector3, Color4 } from '@dcl/sdk/math'

Signature: syncEntity(entity, componentIds[], syncId?)

• entity — the entity to synchronize
• componentIds[] — array of component IDs to keep in sync (e.g., [Transform.componentId])
• syncId — unique numeric identifier (required for predefined entities, optional for player-spawned entities)

Enum Sync IDs (Predefined Entities)

Every predefined synced entity MUST have a unique numeric ID. Use an enum to avoid collisions:

enum SyncIds {
	DOOR = 1,
	ELEVATOR = 2,
	SCOREBOARD = 3,
}

const door = engine.addEntity()
Transform.create(door, { position: Vector3.create(8, 1, 8) })
MeshRenderer.setBox(door)
syncEntity(
	door,
	[Transform.componentId, MeshRenderer.componentId],
	SyncIds.DOOR
)

Predefined entities (with a sync ID) persist after the creating player leaves. Player-created entities (no sync ID) are removed when the player disconnects.

Best practice — always give singletons a stable sync ID. Auto IDs derive identity from the creating peer + its local engine entity number (which the engine recycles). A singleton synced entity that is destroyed and recreated repeatedly with an auto ID is fragile over real network comms: it may fail to reconcile on remote clients (they see only default component data) even though it works perfectly in local single-process preview. Assign any singleton or small fixed set of well-known synced entities a STABLE explicit sync ID from a reserved enum. Reserve auto IDs for genuinely dynamic, many-instance, create-and-forget entities. Also: never removeEntity a fixed-ID synced entity and recreate it with the same ID in the SAME frame — the internal NetworkEntity survives until a later CRDT flush, so recreating immediately throws id provided is already in use; defer the re-spawn to a later tick. See {baseDir}/references/networking-patterns.md (syncEntity identity section) for the full failure-mode signature, fix, and the optimistic-prediction companion pattern.

Auto-Generated IDs (Player-Spawned Entities)

Entities created at runtime by players do not need an explicit sync ID:

function createProjectile() {
	const projectile = engine.addEntity()
	Transform.create(projectile, { position: Vector3.create(4, 1, 4) })
	MeshRenderer.setSphere(projectile)
	syncEntity(projectile, [Transform.componentId])
	return projectile
}

Some visuals are inherently per-player, not shared. A Billboard (camera-facing) is recomputed locally in each explorer, so every player sees it facing themselves — this is not synced and needs no syncEntity. The exception is a Billboard with a targetEntity: because the target's position is scene state, all players see that billboard oriented the same way. Use targetEntity when you need a shared, consistent orientation (e.g. a sign that points at a shared object). See the player-avatar / sdk-scenes component reference for Billboard details.

Custom Synced Components

Define custom components and sync them between players:

import { engine, Schemas } from '@dcl/sdk/ecs'
import { syncEntity } from '@dcl/sdk/network'

const ScoreBoard = engine.defineComponent('scoreBoard', {
	score: Schemas.Int,
	playerName: Schemas.String,
	lastUpdated: Schemas.Int64,
})

const board = engine.addEntity()
ScoreBoard.create(board, { score: 0, playerName: '', lastUpdated: 0 })
syncEntity(board, [ScoreBoard.componentId])

function addScore(points: number) {
	const data = ScoreBoard.getMutable(board)
	data.score += points
	data.lastUpdated = Date.now()
}

Use Schemas.Int64 for timestamps and other large numbers. Schemas.Number / Schemas.Int corrupt values over 13 digits (like Date.now()) — always store such values in Schemas.Int64 (as lastUpdated above does).

Custom schemas must be deterministic: the same component name must map to the same schema across all clients.

Player-Specific Data

Use PlayerIdentityData to distinguish players:

import { engine, PlayerIdentityData } from '@dcl/sdk/ecs'

engine.addSystem(() => {
	for (const [entity] of engine.getEntitiesWith(PlayerIdentityData)) {
		const data = PlayerIdentityData.get(entity)
		console.log('Player:', data.address, 'Guest:', data.isGuest)
	}
})

Schema Types

Available schema types for custom components:

Example with the less common schema types:

enum Rarity { Common = 0, Rare = 1, Legendary = 2 }

const Loot = engine.defineComponent('game::Loot', {
	rarity: Schemas.EnumNumber<Rarity>(Rarity, Rarity.Common),
	payload: Schemas.OneOf({
		coins: Schemas.Int,
		item: Schemas.String,
	}),
	label: Schemas.Optional(Schemas.String),
})

Parent-Child Sync Relationships

For synced entities with parent-child relationships, use parentEntity() instead of setting Transform.parent:

import {
	syncEntity,
	parentEntity,
	getParent,
	getChildren,
	removeParent,
} from '@dcl/sdk/network'

const parent = engine.addEntity()
const child = engine.addEntity()

syncEntity(parent, [Transform.componentId], 1)
syncEntity(child, [Transform.componentId], 2)

// Use parentEntity() — NOT Transform.parent
parentEntity(child, parent)

const parentRef = getParent(child)
const childrenArray = Array.from(getChildren(parent))

// Remove parent relationship
removeParent(child)

Connection State

Check if the player is connected to the sync room:

import { isStateSyncronized } from '@dcl/sdk/network'

engine.addSystem(() => {
	if (!isStateSyncronized()) return // wait for sync
	// safe to read/write synced state
})

Note: The function is spelled isStateSyncronized (not "Synchronized") in the SDK.

MessageBus

Send custom messages between players (fire-and-forget, no persistence):

import { MessageBus } from '@dcl/sdk/message-bus'

const bus = new MessageBus()

bus.on('hit', (data: { damage: number }) => {
	console.log('Took damage:', data.damage)
})

bus.emit('hit', { damage: 10 })

Authoritative-server scenes: MessageBus is client-only — the headless server runtime does not implement the legacy comms event it relies on, and a module-scope new MessageBus() (as above) fails on the server with RemoteError: not implemented. In those scenes construct it only inside the client branch (if (!isServer())); see the authoritative-server skill.

syncEntity vs MessageBus

• syncEntity: late joiners get current state, automatic conflict resolution — CRDT last-write-wins, so if two players change the same component simultaneously the last write wins. The state persists as long as at least one player remains in the scene
• MessageBus: fire-and-forget, late joiners miss past messages, good for transient effects

Binary MessageBus (Performance Optimization)

The regular MessageBus JSON-encodes every payload before sending. For high-frequency messages or large payloads, there's a lower-level binary alternative that sends raw Uint8Array data directly — faster to process because it skips JSON serialization on both ends. This is the same transport syncEntity uses internally.

Use it when:

• You are emitting many messages per second (e.g., continuous movement streams, particle triggers in tight loops)
• Payload size matters (binary encoding is more compact than JSON)
• You already have binary data (e.g., pre-encoded buffers, CRDT deltas)

Stick with the regular MessageBus for low-frequency events where ergonomics beat performance.

import { sendBinary } from '~system/CommunicationsController'
import { executeTask } from '@dcl/sdk/ecs'

// Send a binary message to all peers (or a specific subset via peerData)
executeTask(async () => {
	const payload = new Uint8Array([1, 2, 3, 4]) // your encoded data
	const response = await sendBinary({
		data: [payload],
		peerData: undefined, // optional: target specific peers
	})
	// response.data is a Uint8Array[] of messages received from other peers
	for (const incoming of response.data) {
		handleBinaryMessage(incoming)
	}
})

You are responsible for encoding/decoding the Uint8Array payloads yourself (e.g., with DataView, TextEncoder/TextDecoder, or protobuf). There's no on/emit/topic layer — sendBinary is a single call that both sends pending outgoing messages and returns incoming ones, so you typically drive it from a system on each tick.

REST API and Signed Fetch

For communicating with your own backend (leaderboards, game state persistence, player auth), use fetch or signedFetch. All network calls must run inside executeTask. signedFetch attaches a cryptographic proof of the player's wallet identity — use it when your server needs to verify who is making the request.

See the scene-runtime skill for full fetch and signedFetch patterns.

WebSocket Connections

For full WebSocket patterns (reconnection, heartbeat, message format), see {baseDir}/references/networking-patterns.md.

Basic Connection

executeTask(async () => {
	const ws = new WebSocket('wss://example.com/ws')

	ws.onopen = () => {
		console.log('Connected to WebSocket')
		ws.send(JSON.stringify({ type: 'join', playerId: 'player123' }))
	}

	ws.onmessage = (event) => {
		const msg = JSON.parse(event.data)
		switch (msg.type) {
			case 'gameState':
				handleGameState(msg)
				break
			case 'playerJoin':
				handlePlayerJoin(msg)
				break
			case 'playerLeave':
				handlePlayerLeave(msg)
				break
		}
	}

	ws.onerror = (error) => console.error('WebSocket error:', error)
	ws.onclose = () => console.log('Disconnected')
})

Player Enter/Leave Events

Detect players entering or leaving the scene:

import { onEnterScene, onLeaveScene } from '@dcl/sdk/src/players'

onEnterScene((player) => {
	console.log('Player entered:', player.userId)
})
onLeaveScene((userId) => {
	console.log('Player left:', userId)
})

SDK Observables (low-level)

The SDK also exposes lower-level observables (onPlayerClickedObservable, onEnterSceneObservable, onLeaveSceneObservable, onRealmChangedObservable, onPlayerExpressionObservable, onProfileChangedObservable) from @dcl/sdk/observables. These are the primitives underlying the onEnterScene/onLeaveScene helpers above. Recent fixes to be aware of:

Multiplayer Testing

Open multiple browser windows to test multiplayer locally. Each window is a separate player.

Offline Mode

For Decentraland Worlds that do not need multiplayer:

{
	"worldConfiguration": {
		"fixedAdapter": "offline:offline"
	}
}

Troubleshooting

Need guaranteed consistency, server-side validation, or anti-cheat? syncEntity and MessageBus are not entirely reliable — if it's important that all players see the same state change, see the authoritative-server skill for the headless server pattern. For a complete competitive game architecture (anti-cheat with server-side proximity validation, checkpoint-only Storage persistence, atomic component splits by change rate), see the Gem Rush reference scene (92,-9-authoritative-server-gem-rush).

Example scenes

Engine-team test scenes exercised against the real engine:

• https://github.com/decentraland/sdk7-test-scenes/tree/main/scenes/88,-13-avatar-masks — serverless syncEntity, the pattern this skill documents: an enum SyncId giving two singletons stable IDs, syncEntity(crateAnchorEntity, [AvatarAttach.componentId], SyncId.CRATE_ANCHOR) and syncEntity(crateEntity, [Transform.componentId], SyncId.CRATE), plus parentEntity/removeParent from @dcl/sdk/network to hand a shared crate between players. Any client may mutate it — there is no validateBeforeChange and no isServer() branch. (The scene's headline feature is emote masks; the sync is the supporting half of it.)

For contrast, the other multiplayer scene in the set is server-authoritative — use it to see how that pattern differs from the serverless one described here:

• https://github.com/decentraland/sdk7-test-scenes/tree/main/scenes/90,-9-authoritative-server-leaderboard — authoritative (NOT serverless): only the server calls syncEntity, and synced components are locked with validateBeforeChange so clients can only read them and send messages. If you instead want any client to mutate shared state directly (the pattern this skill documents), each client calls syncEntity on its own and there is no validateBeforeChange. See the authoritative-server skill for that scene's full breakdown.