How does the NameNode know DataNodes are available on a cluster running MapReduce v1
(MRv1)

A.
DataNodes listed in the dfs.hosts file. The NameNode uses as the definitive list of available
DataNodes.

B.
DataNodes heartbeat in the master on a regular basis.

C.
The NameNode broadcasts a heartbeat on the network on a regular basis, and DataNodes
respond.

D.
The NameNode send a broadcast across the network when it first starts, and DataNodes
respond.

Explanation:
How NameNode Handles data node failures?
NameNode periodically receives a Heartbeat and a Blockreport from each of the DataNodes in the
cluster. Receipt of a Heartbeat implies that the DataNode is functioning properly. A Blockreport
contains a list of all blocks on a DataNode. When NameNode notices that it has not recieved a
hearbeat message from a data node after a certain amount of time, the data node is marked as
dead. Since blocks will be under replicated the system begins replicating the blocks that were
stored on the dead datanode. The NameNode Orchestrates the replication of data blocks from one
datanode to another. The replication data transfer happens directly between datanodes and the
data never passes through the namenode.
24 Interview Questions & Answers for Hadoop MapReduce developers, How
NameNode Handles data node failures?