How to avoid a “split-brain” scenario with NameNodes?

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I configure my cluster with HDFS High Availability (HA) using Quorum-Based storage. But I do not implement HDFS Federation.
  
What is the maximum number of NameNodes daemon I should run on my cluster in order to avoid a “split-brain” scenario with my NameNodes?
Jul 11, 2018 in Big Data Hadoop by Meci Matt
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Okay, so let me tell you that what happens in a typical HA cluster is, there are two separate machines configured as NameNodes. At any point in time, one of the NameNodes is in an Active state, and the other is in a Standby state. The Active NameNode is responsible for all client operations in the cluster, while the Standby is simply acting as a slave, maintaining enough state to provide a fast failover if necessary.

The other important point to be noted is that it is vital for the correct operation of an HA cluster that only one of the NameNodes be active at a time. Otherwise, the namespace state would quickly diverge between the two, risking data loss or other incorrect results. In order to ensure this property and prevent the so-called "split-brain scenario," the JournalNodes will only ever allow a single NameNode to be a writer at a time. During a failover, the NameNode which is to become active will simply take over the role of writing to the JournalNodes, which will effectively prevent the other NameNode from continuing in the Active state, allowing the new Active NameNode to safely proceed with failover.

Hope it will answer your query to some extent.
answered Jul 11, 2018 by nitinrawat895
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And what happens when the previously Active Name Node comes online, need to manually failback? If yes how to, considering i want to failback to the original name node.

Hey there! When the previously Active Name Node comes online, it acts as the Standby Name Node (it is in the Standby mode). And to make this as Active Name Node, you will have to make a manual failover. 

The command to initiate a failover between Name Nodes is:

hdfs haadmin -failover
It stays as backup until the current node fails and then they switch places

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