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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Update of "ZooKeeper/FailureScenarios" by jeremyhanna
Date Thu, 09 Dec 2010 23:45:24 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Hadoop Wiki" for change notification.

The "ZooKeeper/FailureScenarios" page has been changed by jeremyhanna.
The comment on this change is: Clarified failure scenario based on info from Henry R..
http://wiki.apache.org/hadoop/ZooKeeper/FailureScenarios?action=diff&rev1=1&rev2=2

--------------------------------------------------

  
  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.
+  * 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.  For writes, you need a quorum
in order to commit a change so all writes will fail.  For reads, there may be a narrow window
where reads are returned with stale data - note that no consistency guarantees are violated
here because it's legal to serve a stale value as long as you yourself haven't overwritten
it.  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.
  

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