Lag timeline with barrier and fallback
Lag has no bound: barrier below, fallback above
one write, one lagging follower, three routed reads, and the stall zone with no upper bound
stall zone
no upper bound
leader
v7 applied at t0, LSN 7
follower B
stale: serves v5
caught up: serves v7
router
r1 refused: 5 < 7
r2 admitted: 7 >= 7
r3 unbounded: leader fallback
t0 write v7
t1 B applies LSN 7
t2 stall begins
barrier rule: serve only replicas at LSN 7 or more, and every read names its version
fallback rule: no fresh follower means read from the leader, never a fixed sleep
primary routing covers own-writes, stickiness covers repeats, durability covers failover