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A real-time whiteboard needs more than a WebSocket connection: it needs a message format, room membership, a rule for ordering drawing operations, and a plan for reconnecting clients. WebSockets provide the persistent, two-way connection; your Go application defines what a drawing event means and which connected users may send or receive it.
A practical first version can use a single Go server to validate each operation, assign it a room sequence number, and broadcast it to that room. Give each connection one reader and one writer, keep outbound queues bounded, and decide how the board is saved before relying on it for durable work.
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What WebSockets do—and what your whiteboard must define
RFC 6455 describes a protocol that begins with an opening handshake and then carries messages over a bidirectional TCP connection. It does not define drawing events, room membership, board storage, or conflict resolution. As the RFC puts it, “The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code.”
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Choose a small, explicit event protocol
Represent each operation as a typed message rather than sending unstructured coordinates. Give each room, session, and persistent drawing object a stable identifier. If the server assigns ordering, include a sequence number on accepted events; clients can use it to detect gaps and request recovery.
| Event | Purpose | Typical data |
|---|---|---|
stroke.begin |
Start a stroke | Stroke ID, color, width, initial point |
stroke.point |
Add points to an active stroke | Stroke ID and one or more points |
stroke.end |
Finish a stroke | Stroke ID |
shape.add |
Create a shape | Object ID, shape type, geometry, style |
object.delete |
Delete an object | Object ID |
presence.update |
Show transient user activity | Session ID and cursor or selection state |
For example, a client request might look like this:
{
"type": "stroke.point",
"roomId": "team-sketch",
"clientOpId": "op-82",
"objectId": "stroke-19",
"points": [{"x": 142, "y": 96}, {"x": 145, "y": 99}]
}Treat client-provided identifiers and coordinates as input, not as proof of permission or correctness. The server should derive the authenticated user and authorized room membership from its own session state, validate the envelope and payload, then emit an accepted event that includes the server-assigned sequence.
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Build the connection and room flow in Go
A small deployment can start with an in-memory room hub. The hub owns room membership and serializes accepted operations for each room. A read loop receives and validates messages; the hub applies room authorization, assigns the next sequence, and fans out the accepted event. Each connection’s write loop is the only code that writes to its socket.
- Accept a connection. Serve a WebSocket endpoint such as
/ws?room=team-sketch. Authenticate the HTTP request, check that the user may join the requested room, and allow only trusted browser origins in the upgrader’s origin check. Use WSS outside local development. - Register the connection. Add it to the authorized room and arrange for a bounded outbound queue and a writer loop. Do not let each incoming handler write directly to every socket.
- Read and validate. Decode a message, enforce a maximum message size, check the event type and payload, and check authorization for that specific action.
- Order and broadcast. Have one room owner or other serialized mechanism assign the next sequence number and send the accepted event to room members. Decide how the sender receives confirmation and how duplicate client retries are handled.
- Remove disconnected clients. On read or write failure, unregister the connection, stop its loops, and release its resources. Also define what happens to a user’s unfinished stroke.
Gorilla WebSocket’s documentation states, “Connections support one concurrent reader and one concurrent writer.” A common design is therefore one read loop and one write loop per connection. The following excerpt illustrates a bounded per-client send queue; it is a building block, not a complete server:
type Client struct {
conn *websocket.Conn
send chan []byte // bounded when the client is created
}
func (c *Client) writePump() {
defer c.conn.Close()
for message := range c.send {
if err := c.conn.WriteMessage(websocket.TextMessage, message); err != nil {
return
}
}
}When the hub broadcasts, it should enqueue rather than write to a socket. If a client’s queue is full, do not block the whole room indefinitely: mark the client for disconnection or recovery and let its writer loop exit cleanly. The queue capacity and the policy for a slow client are deployment decisions, not values specified by WebSocket.
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Effective Go explains goroutines and channels as tools for concurrent work and synchronization, while warning that unbounded goroutine creation can consume unlimited resources. Apply the same discipline to connection counts, room sizes, queued messages, and shutdown: bound resources, and ensure goroutines stop when a client disconnects or the server shuts down.
Keep the browser responsive
The browser WebSocket API has no built-in backpressure. MDN warns that if messages arrive faster than the application can process them, memory use can grow or the application can become unresponsive.
- Batch points from a fast pointer stream into fewer messages instead of sending every raw input independently.
- Coalesce transient cursor or presence updates so that a newer position can replace an older queued one.
- Bound server-side outbound queues and choose a clear slow-client policy, such as disconnecting and requiring a resynchronization.
- Keep durable drawing operations distinct from ephemeral presence. Dropping an old cursor update may be acceptable; silently dropping a committed stroke operation is not.
These are application strategies for dealing with the API’s flow-control limitation; they are not guarantees supplied by WebSockets.
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Decide ordering, persistence, and reconnect behavior
For an initial single-server whiteboard, a server-ordered event stream is a straightforward design: the server validates an operation, assigns its place in the room’s sequence, and broadcasts that accepted version. It gives clients a common order for applying events, but the application still has to specify deduplication, replay, and recovery.
Server-ordered operations
Keep a monotonically increasing sequence per room or board. On reconnect, a client can report the last sequence it applied; the server can replay later operations if they remain available, or tell the client to load a snapshot. Define how retries are identified so a reconnecting client does not accidentally apply the same user action twice.
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Snapshots plus an operation log
Persist board state periodically as a snapshot and retain subsequent operations in a log. This can make recovery faster than replaying the board’s entire history, but it adds decisions about storage, snapshot versions, log retention, compaction, and what happens if persistence fails. An in-memory hub alone loses its state when the process stops.
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CRDTs or operational transformation
Consider a conflict-handling model such as a CRDT or operational transformation when offline edits or concurrent multi-writer changes are product requirements. Those approaches require choices about data representation and merge behavior; a WebSocket connection by itself does not provide them. They are not automatically necessary for every whiteboard.
Secure the socket and every operation
OWASP’s WebSocket Security Cheat Sheet recommends secure WebSocket transport, an explicit trusted-origin allowlist, authentication, authorization checks for each action, input validation, reasonable message-size limits, rate limiting, session-expiry handling, and connection cleanup.
- Use WSS in deployed environments and reject browser origins that are not on your allowlist.
- Authenticate the connection and check room access before joining; continue checking authorization for operations rather than treating an open socket as blanket permission.
- Validate message types, required fields, coordinate ranges, object ownership rules, and payload sizes before applying or broadcasting events.
- Set rate limits appropriate to your application and disconnect or otherwise handle clients that exceed them.
- Handle session expiry by closing or re-authenticating the connection, and always remove closed clients from room state.
- Avoid putting credentials in query strings and avoid logging complete message contents or secrets.
The exact limits and authorization rules depend on the application and deployment. The important point is that a WebSocket transports messages; it does not make them trustworthy.
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A useful first implementation boundary
Start with one authoritative Go process, a small set of typed operations, one ordering rule, bounded per-client queues, and a defined reconnect path. Keep persistence behind a clear boundary so an in-memory prototype can later store snapshots and operations without changing the browser’s basic event vocabulary. Add offline multi-writer conflict handling only if the product needs it.
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