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https://issues.apache.org/jira/browse/HDFS-2354?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13136002#comment-13136002
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Justin Joseph commented on HDFS-2354:
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The neutral state is not totally a new way of handling network partitions. For
example, consider the quorum concept supported by Pacemaker to handle split
brain scenarios.
http://www.woodwose.net/thatremindsme/2011/04/stonith-and-quorum-in-pacemaker/
{quote}Whenever quorum is present, pacemaker will go with the majority vote on
important decisions. When quorum is lost (i.e. the cluster is separated into
groups where no group has a majority of votes) the behavior of the pool is
determined by the *no-quorum-policy* property:
* _ignore_ - Do nothing when quorum is lost.
* _stop_ (default) - stop all resources in the affected cluster partition.
* _freeze_ - continue running existing resources, but don’t start any
stopped ones.
* _suicide_ - fence all nodes in the affected partition.{quote}
Neutral state is a modification of the stop option in the list above. When
quorum is lost, instead of stopping the Namenode, it is turned into a state
where it is neither Active nor Standby. When quorum is available again, it is
turned to either Active or Standby state. The advantage of keeping the process
in neutral state, compared to stopping the process, is about the time to start
servicing the requests after network connection is restored. The down time
will be very less in case of going to neutral state.
Another approach is to turn the Namenode to Standby state when quorum is lost.
In case of network partition, this may not be of any help since Standby
Namenode won’t be able to ping the Active Namenode & it will just keep
retrying. After network is restored, if this Namenode (which is turned to
Standby when quorum is lost) is elected as active again, the time to turn the
Namenode from Standby state to Active state will be higher depending on what is
the timeout configured for RPC calls.
When Zookeeper is used as the distributed coordinator, Neutral state is an
effective way of handling the network partitions. In future, it may be
supported by Pacemaker as an option for *no-quorum-policy*.
I hope the approach is simple & clear. No one gets elected to Active state or
continues to be in Active state unless a quorum decides so. If quorum is not
available, it relinquishes the role & remains neutral. Being in neutral state
helps to come back to Active state faster after quorum is available again.
> Generalize the HAServiceProtocol interface
> ------------------------------------------
>
> Key: HDFS-2354
> URL: https://issues.apache.org/jira/browse/HDFS-2354
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: name-node
> Reporter: Justin Joseph
> Assignee: Justin Joseph
> Attachments: HAService_fw_Class_Diagram.JPG, HDFS-2354.1.patch,
> HDFS-2354.patch, Namenode HA using Backup Namenode as Hot Standby.pdf
>
>
> This JIRA intends to revisit the patches committed for HADOOP-7455 and
> HDFS-1974 & to provide more generic interfaces which allows alternative HA
> implementations to co-exist complying with HAServiceProtocol.
> Some of the considerations are
> 1) Support life cycle methods (start*() and stop() APIs) in HAServiceProtocol
> 2) Support custom states in HAServiceProtocol
> 3) As per the patch submitted for HDFS-1974, Namenode implements HAService
> interface. This needs to be reconsidered.
> I will elaborate on these points, in the form of comments below.
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