# Live Cursors and Presence

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](/actors/docs/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.

| Variant | Starter Code | Transport | Presence Storage |
| --- | --- | --- | --- |
| `cursors` | [GitHub](https://github.com/rivet-dev/rivet/tree/main/examples/cursors) | Typed [actions](/actors/docs/actions) and [events](/actors/docs/events) over the RivetKit connection | `connState` per connection |
| `cursors-raw-websocket` | [GitHub](https://github.com/rivet-dev/rivet/tree/main/examples/cursors-raw-websocket) | Raw [`onWebSocket` handler](/actors/docs/websocket-handler) with a custom JSON message protocol | Socket 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.

| Data | Where It Lives | Why |
| --- | --- | --- |
| Cursor position | `connState` (`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 variants | Canvas 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](/actors/docs/connections) for `connState` and [State](/actors/docs/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](/actors/docs/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.

| Layer | Pattern | Guidance |
| --- | --- | --- |
| Client (smoothness) | Throttle to 20-30Hz | Sample 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 limit | Track 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:

| Direction | Message `type` | Payload |
| --- | --- | --- |
| Client to server | `updateCursor` | `{ userId, x, y }` |
| Client to server | `updateText` | `{ id, userId, text, x, y }` |
| Client to server | `removeText` | `{ id }` |
| Server to client | `init` | `{ cursors, textLabels }` snapshot on connect |
| Server to client | `cursorMoved`, `textUpdated`, `textRemoved`, `cursorRemoved` | The corresponding cursor, label, or ID payload |

## Lifecycle

### cursors (Actions + Events)

```mermaid
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

```mermaid
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](#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.