The newly elected leader may not have received all the changes from the old leader before the latter went offline. This is a possibility when replication takes place asynchronously and can cause data loss, which may violate clients’ durability expectations. If external systems depend on the data and certain writes are missing, it can cause failure for these systems. Another twist is the scenario in which the old leader rejoins the cluster. The old leader has writes which weren’t propagated to the new leader and are now suddenly available. Usually, in such a scenario these unreplicated writes on the old leader are simply discarded.