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live-cursors

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by rivet-dev · part of rivet-dev/skills

Live cursors and multiplayer presence with Rivet Actors: per-connection cursor state, realtime updates over events or raw WebSockets, and throttling.

🧩 One of 7 skills in the rivet-dev/skills package — works on its own, and pairs well with its siblings.

This is the playbook your agent receives when the skill activates — you don't need to read it to use the skill, but it's here to audit before installing.

Live Cursors and Presence

IMPORTANT: Before doing anything, you MUST read BASE_SKILL.md in this skill's directory. It contains essential guidance on debugging, error handling, state management, deployment, and project setup. Those rules and patterns apply to all RivetKit work. Everything below assumes you have already read and understood it.

Working Examples

If you need a reference implementation, read the raw working example code in these templates:

Patterns for building live cursors, multiplayer presence, and realtime cursor sharing with RivetKit. One room actor fans cursor positions out to every connected client, keyed per room with actor keys.

Starter Code

Start with one of the two working variants on GitHub. Both implement the same collaborative cursor canvas with persistent text labels; they differ only in transport.

VariantStarter CodeTransportPresence Storage
cursorsGitHubTyped actions and events over the RivetKit connectionconnState per connection
cursors-raw-websocketGitHubRaw onWebSocket handler with a custom JSON message protocolSocket map in createVars

Use cursors by default: typed actions, typed events, and automatic connection tracking cover most apps with less code. Use cursors-raw-websocket when you need full control of the wire format, for example a custom JSON or binary protocol, or clients that do not use the RivetKit client library.

Connection State vs Persistent State

Presence is ephemeral by definition. A cursor position is only meaningful while its connection is alive, so it belongs in per-connection storage, not in persistent actor state. Persistent state is reserved for data that must survive disconnects and actor restarts.

DataWhere It LivesWhy
Cursor positionconnState (cursors) or the createVars socket map (cursors-raw-websocket)Scoped to one connection and discarded with it. Stale presence cannot accumulate in storage.
Text labels (textLabels)Persistent actor state in both variantsCanvas content must survive disconnects and actor restarts.

In the cursors variant, updateCursor writes c.conn.state.cursor and getRoomState rebuilds the presence snapshot by iterating c.conns.values(), so the cursor map is always derived from live connections rather than stored. See Connections for connState and State for persistence semantics.

Presence Lifecycle

  • Join: The cursors-raw-websocket variant pushes an init message with the current { cursors, textLabels } snapshot as soon as a socket connects. The cursors variant has no explicit join broadcast; the client calls the getRoomState action once after connecting to seed its local maps, and peers first see a new user on that user's first cursorMoved broadcast.
  • Move: Every updateCursor call writes the connection's presence entry, then broadcasts cursorMoved to all connections, including the sender.
  • Leave: The cursors variant handles leave in onDisconnect, broadcasting cursorRemoved with the connection's last cursor. The raw variant does the same from the socket close listener, then deletes the session from the vars.websockets map. Clients delete that user from their local cursor map, so stale cursors disappear the moment a tab closes.

See Lifecycle for onDisconnect and createVars.

Update Throttling

Neither example throttles. Both frontends send a cursor update on every raw mousemove event with no debounce or interval cap. That is fine for a demo, but a fast mouse on a high-refresh display can emit hundreds of events per second per user. The patterns below are recommended production hardening on top of the starter code, not something the examples implement.

LayerPatternGuidance
Client (smoothness)Throttle to 20-30HzSample the latest pointer position every 33-50ms and send only that. Drop intermediate moves, but always flush the final position so cursors settle at the true location. Interpolate between received positions on the rendering side.
Server (enforcement)Per-connection rate limitTrack the last accepted update timestamp per connection and drop or coalesce updates arriving faster than your cap. Client throttles are cooperative; the actor is the enforcement boundary.

Actors

  • Key: cursorRoom[roomId] (the frontend defaults roomId to "general")

  • Responsibility: Holds per-connection cursor presence in connState, persists shared text labels in actor state, and broadcasts cursor and text updates to all connections.

  • Actions

    • updateCursor
    • updateText
    • removeText
    • getRoomState

  • Events

    • cursorMoved
    • cursorRemoved
    • textUpdated
    • textRemoved

  • Queues

    • None

  • State

    • JSON
    • textLabels (persistent)
    • connState.cursor per connection (ephemeral)

  • Key: cursorRoom[roomId] (resolved via client.cursorRoom.getOrCreate(roomId))

  • Responsibility: Exposes a raw WebSocket endpoint, tracks live sockets and their cursors in a createVars map keyed by a sessionId query parameter, persists text labels, and manually fans JSON frames out to every socket.

  • Actions

    • getOrCreate (stub returning { status: "ok" }; the frontend resolves the actor ID with the client handle's getOrCreate(roomId).resolve(), which creates the actor without dispatching this action)
    • getRoomState

  • Queues

    • None

  • State

    • JSON
    • textLabels (persistent)
    • vars.websockets map of sessionId to socket and cursor (in-memory, lost on restart)

The raw variant defines no RivetKit events. Its message names are type fields on raw JSON frames:

DirectionMessage typePayload
Client to serverupdateCursor{ userId, x, y }
Client to serverupdateText{ id, userId, text, x, y }
Client to serverremoveText{ id }
Server to clientinit{ cursors, textLabels } snapshot on connect
Server to clientcursorMoved, textUpdated, textRemoved, cursorRemovedThe corresponding cursor, label, or ID payload

Lifecycle

cursors (Actions + Events)

sequenceDiagram
	participant A as Client A
	participant R as cursorRoom
	participant B as Other Clients

	A->>R: connect via useActor (cursorRoom[roomId])
	A->>R: getRoomState()
	R-->>A: {cursors, textLabels}
	loop every mouse move
		A->>R: updateCursor(userId, x, y)
		Note over R: write c.conn.state.cursor
		R-->>B: cursorMoved (broadcast)
	end
	A->>R: updateText(id, userId, text, x, y)
	Note over R: upsert persistent state.textLabels
	R-->>B: textUpdated (broadcast)
	Note over A: tab closes
	Note over R: onDisconnect reads conn.state.cursor
	R-->>B: cursorRemoved (broadcast)

cursors-raw-websocket

sequenceDiagram
	participant A as Client A
	participant R as cursorRoom
	participant B as Other Clients

	A->>R: getOrCreate(roomId).resolve()
	R-->>A: actorId
	A->>R: open WebSocket /gateway/{actorId}/websocket?sessionId=...
	Note over R: close 1008 if sessionId is missing
	Note over R: store socket in vars.websockets
	R-->>A: init {cursors, textLabels}
	loop every mouse move
		A->>R: {type: "updateCursor"} frame
		Note over R: update session cursor in vars
		R-->>B: cursorMoved frame
	end
	Note over A: socket closes
	R-->>B: cursorRemoved frame
	Note over R: delete session from vars.websockets

Security Checklist

Both examples ship without authentication so the presence pattern stays readable. Everything below is recommended hardening for production, not behavior the examples implement.

  • Identity: Bind presence identity to the connection (c.conn.id in the actions variant, a server-generated session ID in the raw variant). Never trust a client-supplied userId; in the examples it is a random client-generated string, so any client can impersonate or remove any cursor.
  • Authorization: Authorize label mutations by owner. In the examples, updateText accepts arbitrary id and userId arguments and removeText accepts an arbitrary id, so any client can edit or delete any label.
  • Input validation: Clamp x and y to canvas bounds, cap text label length, and cap the total textLabels count so persistent state cannot grow unbounded.
  • Rate limiting: Enforce a per-connection cap on updateCursor (for example 30Hz) and on label writes, as described in Update Throttling.
  • Protocol strictness (raw variant): Validate message shape before use and close the socket on malformed JSON instead of logging and continuing. Reject duplicate sessionId values rather than silently overwriting another session's socket entry.

Reference Map

Actors

Cli

Clients

Cookbook

Deploy

General

Self Hosting