Collaborative Whiteboard Sync: Lease-Based Channel Lifecycles for Reliable State
Use a short-lived, server-authorized lease for every realtime channel, and make reconnect plus replay part of the protocol. That is the dependable choice for a collaborative whiteboard because a browser tab can disappear without saying goodbye, while a whiteboard still needs one ordered view of the document. The deciding constraint is trust. A client may draw a cursor, but it must not mint a…
To ensure reliable state in collaborative whiteboarding, employ a lease-based channel lifecycle with explicit states: requested, active, draining, and closed. Issue an authorization token scoped to a board and role, expiring after a few minutes. The realtime gateway validates this token on connect and every publish, not trusting client-supplied board IDs.
Every accepted operation carries a monotonically increasing server sequence, while clients store the last applied sequence. Upon reconnect, clients provide this cursor, receive a bounded replay, and obtain a snapshot if the cursor exceeds the retention window. This approach establishes a clear boundary: a live stream is an optimization, while a snapshot serves as the recovery contract.
The pseudocode outlines the process, with storage and transport kept generic. After reconnect, the server authenticates a fresh lease, refuses writes with an authorization result, and prompts the client to refresh the lease before reopening. During draining, new publishes halt, but already accepted events complete their fan-out.
Closure records the reason and final sequence, differentiating normal idle timeouts from revoked sessions. Short tokens minimize blast radius but introduce clock and refresh edge cases. Allow a small server-side grace period for skew but never for unbounded retries. Rate-limit refresh attempts by user and board, and ensure the authorization token only permits write operations, not submissions even if the UI accidentally exposes the button.
Log board ID, channel ID, token subject, scope, lease state, last sequence, and close reason, but exclude the bearer token. Correlate client operation IDs with server sequences for support engineers to verify if an operation was applied. For testing, employ property-based testing to generate scenarios like sleep, duplicate connects, token expiry, out-of-order delivery, and snapshots taken between two replays.
Ensure every authorized client ends with the same document hash and no unauthorized operations appear in the log. In a real-world scenario, client A commits sequence 410, then laptop sleep occurs while client B commits 411 through 418. A reconnects with an expired lease, sends its stale cursor, and the gateway authenticates a fresh lease, notices the older sequence, and chooses a snapshot or replay from the cursor.
The server assigns a new sequence after 418 if A publishes before the repair completes, ensuring conflict resolution remains in one place and providing an explainable audit trail.
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