Every card on this board lives in object storage — S3, R2 or Tigris. No database, no volume, no server holding state between requests. How it works ↓
How it works
Most real-time apps keep a server holding the authoritative state and a database behind it. This board has neither. Vercel functions start, answer one request, and are thrown away — so the only thing that survives between them is a few objects in a bucket.
You move a card
The browser applies the change immediately and posts the operation to
/api/sync/messages. Some function instance picks it up. It
has no memory of you, so it rebuilds your session, reads the board out
of the bucket, and applies your op.
It lands in the bucket
The instance writes one immutable wal/ object containing the
batch, then compare-and-swaps the _manifest object to point
at it. The swap is what makes the write real — until the manifest names
your segment, nothing reads it.
Your ack comes back on the same request
Not down the event stream. That stream is held open by a different instance, which has no way to reach into this one — so replies ride home in the response to the POST that caused them.
Everyone else finds out
Each open tab has its own instance holding an SSE stream. Four times a second it fetches the manifest — one small read. If the commit counter moved, it pulls the new segments, re-runs the board query, and pushes the difference to that tab.
rooms/demo/_manifest
rooms/demo/wal/writer-a3f-7.jsonl
rooms/demo/wal/writer-91c-2.jsonl
rooms/demo/snap/0-4.jsonThe manifest is small, mutable and the only thing ever overwritten. Everything else is written once and never touched again, which is what makes it safe for many readers at once. When the segment list grows long, a snapshot folds them into one object so a cold board still opens in a couple of reads.
Two people editing at once means two instances writing at once. Each writes its own uniquely-named segment, so they cannot overwrite each other, and then both try to swap the manifest. One wins. The loser is told its version was stale, re-reads, reapplies its change on top and tries again.
That single conditional write is the only thing keeping the board consistent — no lease, no leader election, no coordination service.
Idle costs nothing because there is nothing running: no lease to renew, no connection to keep warm, no instance sitting on the board.