Chat Room - Rivet — ContextQMD

Patterns for building a chat room backend with RivetKit: room-scoped actors, persistent message history, and realtime delivery over WebSocket connections.

Starter Code

Start with the working example on GitHub and adapt it. The backend is a single chatRoom actor; the frontend is a React app using @rivetkit/react (see the React quickstart).

Topic	Summary
Room model	One `chatRoom` actor per room key. The frontend defaults the key to `general`; typing a different room name connects to a different actor.
History	SQLite `messages` table created in `db({ onMigrate })`, read back with `ORDER BY id ASC`.
Delivery	`sendMessage` inserts the row, then broadcasts a typed `newMessage` event to every connected client.
Identity	None in the example. `sender` is a plain action argument; production should bind identity to the connection.

Room-Per-Actor Model

Each room is one Rivet Actor instance, addressed by key. The client calls useActor({ name: "chatRoom", key: [roomId] }), which gets-or-creates the actor for that room. This gives you:

Isolation: each room's history and connections are fully scoped to its key. Switching the room input re-keys the hook and connects to a different actor with separate history.
A single serialized writer: all sendMessage calls for one room run through one actor, so message ordering is consistent without locks. The SQLite AUTOINCREMENT id is the canonical order, which is why getHistory sorts by id rather than by timestamp.
Natural scaling: rooms spread across the cluster independently. A hot room does not slow down other rooms.

Message History Storage

This example stores history in the actor's SQLite database, not in JSON state. Pick based on history size and query needs:

Approach	Use When	Implementation Guidance
SQLite (what this example uses)	Large or long-lived history that needs ordering, caps, pagination, or search	Create the `messages` table in `db({ onMigrate })`, insert with parameterized queries (`c.db.execute("INSERT ... VALUES (?, ?, ?)", ...)`), and read with `ORDER BY id ASC`. History survives actor sleep and scales past what you want in memory.
JSON state	Small recent history, for example the last 50 to 100 messages	Push onto a `messages` array in actor state and trim to a cap on every send. Simplest option, but the whole history lives in memory and there is no query layer, so it only fits bounded recent-history use cases.

Broadcast Delivery

New messages reach connected clients through a typed event:

The actor declares events: { newMessage: event<Message>() }, where Message is { sender, text, timestamp }.
The sendMessage action builds the message with a server-side Date.now() timestamp, inserts it into the messages table, then calls c.broadcast("newMessage", message) and returns the message to the caller.
Each client subscribes with useEvent("newMessage", ...) and appends to its local list. The sender renders its own message through the same broadcast path as everyone else, so all clients stay on one code path.
History load is connection-gated: once the connection is ready, the client calls getHistory() once to render the backlog, then relies on events for everything after.

Use c.broadcast(...) for room-wide messages. For private or per-recipient payloads (such as DMs inside a room), send on the individual connection instead, which is a recommended extension beyond this example.

Typing Indicators And Presence (Extension)

The example does not implement typing indicators, presence, or join/leave handling of any kind. There is no createConnState, onConnect, or onDisconnect in the code. If you need them, add them as ephemeral connection behavior:

Keep it ephemeral: store the username and typing flag in per-connection state, never in SQLite or persisted actor state. Presence is derived from live connections and should disappear with them.
Broadcast on change only: emit a typing event when a user starts or stops typing, and a presence event from onConnect / onDisconnect, rather than polling or ticking.
Expire on the client: clear a typing indicator after a short client-side timeout so a dropped connection never leaves a stuck "is typing" row.

Per-User Inbox (Extension)

For offline delivery, DMs, unread counts, or notification fanout, add a userInbox[userId] actor per user. This is an extension beyond the example:

The room actor forwards each message to the inbox actor of every member via actor-to-actor calls, so users who are not connected to the room still accumulate messages.
The inbox actor owns per-user unread state and serves it when the user comes online, independent of which rooms they are in.
DMs become a degenerate room: either a chatRoom keyed by the sorted pair of user ids, or direct inbox-to-inbox delivery if you do not need shared history semantics.

Actors

Key: chatRoom[roomId]
Responsibility: Owns one chat room. Persists the room's message history in its SQLite database and broadcasts each new message to every connected client.
Actions
- sendMessage
- getHistory
Queues
- None
Events
- newMessage
State
- SQLite
- messages table: id (autoincrement primary key), sender, text, timestamp

</Accordion> </AccordionGroup>

Lifecycle

mermaid

sequenceDiagram
	participant A as Client A
	participant B as Client B
	participant R as chatRoom

	A->>R: connect with key [roomId]
	Note over R: every start runs onMigrate (CREATE TABLE IF NOT EXISTS messages)
	A->>R: getHistory()
	R-->>A: Message[] ordered by id
	B->>R: connect with key [roomId]
	B->>R: getHistory()
	R-->>B: Message[] ordered by id
	A->>R: sendMessage(sender, text)
	Note over R: INSERT row with server timestamp
	R-->>A: newMessage (broadcast)
	R-->>B: newMessage (broadcast)
	A->>R: disconnect
	Note over R: history stays in SQLite for the next connection

Security Checklist

The example is intentionally minimal and skips all of the following. Add them before production:

Auth before join: any client can join any room by knowing its name, and sender is arbitrary client input on every call. Validate a token during connection auth, bind identity to connection state, and check room membership before serving history. Never trust a sender name passed as an action argument.
Message length clamps: the example accepts empty messages and has no length limit. Trim server-side, reject empty text, and clamp to a maximum length.
Per-connection rate limiting: rate limit sendMessage per connection to stop spam and broadcast amplification.
Server-side timestamps and ids: the example already does this correctly. timestamp comes from Date.now() inside the action and id from SQLite AUTOINCREMENT. Keep it that way; never accept client-supplied timestamps or ids.
History caps: getHistory returns every row with no limit. Add a LIMIT plus pagination, and prune or archive old rows so a long-lived room cannot grow unbounded.
Parameterized queries: the example already inserts with ? placeholders. Keep all user-supplied text out of SQL string interpolation.