[Hadoop Wiki] Update of "ZooKeeper/FailureScenarios" by jeremyhanna

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The "ZooKeeper/FailureScenarios" page has been changed by jeremyhanna.
The comment on this change is: Adding the page content for how zookeeper handles specific failure scenarios..
http://wiki.apache.org/hadoop/ZooKeeper/FailureScenarios

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New page:
= How ZooKeeper Handles Failure Scenarios =

=== Network partitions ===

When there is a network partition that separates members of the !ZooKeeper ensemble and there exists a quorum side and a non-quorum side of the partition:
 * if the leader is in the quorum side of the partition, all read and write requests to the quorum side continue normally.  On the non-quorum side of the partition, there is no leader, so requests to read or write to those leaderless !ZooKeeper servers result in an [[ZooKeeper/ErrorHandling| error]].
 * if the leader is in the non-quorum side of the partition, that side of the partition will recognize that it no longer has a quorum of the ensemble.  The leader will be demoted to being a regular !ZooKeeper server and those nodes will no longer accept reads or writes.  On the quorum-side of the partition, the nodes will recognize that there is not a leader among them and will elect a new leader.  When the partition is resolved, the non-quorum side will join the quorum side and update their data.