In Hadoop 1.x NameNode was a single point of failure (SPOF). So, whole Hadoop cluster becomes unavailable as soon as NameNode is down. In other words, High Availability feature of the NameNode that talks about the necessity of a NameNode to be active for serving the requests of Hadoop clients is no more in existence in this scenario. As a result read operation fails.
NameNode High Availability Architecture was introduced to solve this Single Point of Failure problem of NameNode. HA feature was intorduced in Hadoop 2.x where we have two NameNode in our HDFS cluster in an active/passive mode. The Active NameNode is the NameNode that works and runs in the Hadoop cluster. Passive NameNode similar to an active NameNode, it is also known asStandby NameNode. It comes into action only when the active NameNode fails. Whenever the active NameNode fails, the passive NameNode or the standby NameNode replaces the active NameNode, to ensure that the Hadoop cluster is never without a NameNode.The passive NameNode takes over the responsibility of the failed NameNode and keep the HDFS up and running. The passive Namenode takes the edit logs (meta data file) from NameNode and merges it with the FsImage (File system Image) to produce an updated FsImage as well as to prevent the Edit Logs from becoming too large. The read operation continues thereafter as if there was no downtime.