We cannot be sure that the data will be persistent throughout in Memstore. ASSUME that a PARTICULAR datanode is down. Then the data that resides on that data node’s memory will be LOST.

To OVERCOME this problem, when the request comes from the master it written to WAL as well. WAL is nothing but Write Ahead Logs which resides on the HDFS, a permanent storage. Now we can make sure that even when if the data node is down the data will not be lost I.e. we have the copy of all the ACTIONS that you are supposed to do in the WAL.

We cannot be sure that the data will be persistent throughout in Memstore. Assume that a particular datanode is down. Then the data that resides on that data node’s memory will be lost.

To overcome this problem, when the request comes from the master it written to WAL as well. WAL is nothing but Write Ahead Logs which resides on the HDFS, a permanent storage. Now we can make sure that even when if the data node is down the data will not be lost I.e. we have the copy of all the actions that you are supposed to do in the WAL.