Beginning Apache Hadoop Administration : The First Step towards Hadoop Administration and Management by Prashant Nair

Author:Prashant Nair
Language: eng
Format: azw3
Publisher: Notion Press
Published: 2017-09-07T04:00:00+00:00

Always remember that Fencing gets invoked whenever the Active NameNode is not reachable in-terms of network (heartbeat). To monitor this instance and to invoke Fencing, a watchdog service i.e. ZKFC is used. It is the ZKFC that triggers fencing so that there are no existing split-brain scenarios.

In Hadoop HDFS HA, we can implement two types of Fencing:

SSHfence – This mechanism uses SSH to connect to the old Active NameNode machine and uses fuser command to kill the process. This method of fencing is good if the machine is reachable via the network. But what happens if the system is switched off due to a power failure? What if the network interface card gets burned/damaged? It is obvious that the SSHfence cannot kill the process. This results in no state change and transition occurrences. To avoid this situation, we prefer to use shell as a Fencing method in production setups. Let us understand shell Fencing now.

Shell – The Shell mechanism simply runs the script specified. If the script returns true, the Fencing is considered to be a success and the state transition will occur. This means that whichever NameNode was on standby will become active under any/all conditions. Ideally, if you don’t want to write a script, you can just pass ‘shell(/bin/true)’ string which says that when fencing is initiated by ZKFC, the script will return true resulting in state transition.

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