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The "ZooKeeper/FAQ" page has been changed by ToddLipcon: http://wiki.apache.org/hadoop/ZooKeeper/FAQ?action=diff&rev1=15&rev2=16 Comment: Added FAQ about session expiration behavior after a cluster downtime In conclusion, DNS RR works as good as a list of ensemble IP arguments except cluster reconfiguration case. It turns out that there is a minor problem with DNS RR. If you are using a tool such as zktop.py, it does not take care of a list of host IP returned by a DNS server. + <<BR>> + <<Anchor(9)>> + '''9. [[#9|What happens to ZK sessions while the cluster is down?]]''' + + Imagine that a client is connected to ZK with a 5 second session timeout, and the administrator brings the entire ZK cluster down for an upgrade. The cluster is down for several minutes, and then is restarted. + + In this scenario, the client is able to reconnect and refresh its session. Because session timeouts are tracked by the leader, the session starts counting down again with a fresh timeout when the cluster is restarted. So, as long as the client connects within the first 5 seconds after a leader is elected, it will reconnect without an expiration, and any ephemeral nodes it had prior to the downtime will be maintained. + + The same behavior is exhibited when the leader crashes and a new one is elected. In the limit, if the leader is flip-flopping back and forth quickly, sessions will never expire since their timers are getting constantly reset. +
