4. Operating a NameNode HA cluster

While operating an HA cluster, the Active NameNode cannot commit a transaction if it cannot write successfully to a quorum of the JournalNodes.
When restarting an HA cluster, the steps for initializing JournalNodes and NN2 can be skipped.
Start the services in the following order:
1. JournalNodes
2. NameNodes
  Note
  Verify that the ZKFailoverController (ZKFC) process on each node is running so that one of the NameNodes can be converted to active state.
3. DataNodes

	Note
Verify that the ZKFailoverController (ZKFC) process on each node is running so that one of the NameNodes can be converted to active state.