// Route reads to a follower for scale, but send a user to the leader
// briefly after they write so they never see their own stale data.
function pickReadDB(userId, lastWriteAt) {
  const RECENT_MS = 3000; // longer than typical replication lag
  if (Date.now() - (lastWriteAt[userId] ?? 0) < RECENT_MS) {
    return leader;        // read-your-writes: hit the primary
  }
  return followers[Math.floor(Math.random() * followers.length)];
}