Why failover-based implementations are not enough

Distributed locks with Redis – Redis https://redis.io/topics/distlock

There is an obvious race condition with this model:

1. Client A acquires the lock in the master.
2. The master crashes before the write to the key is transmitted to the slave.
3. The slave gets promoted to master.
4. Client B acquires the lock to the same resource A already holds a lock for. SAFETY VIOLATION!