Question-56: What is a NameNode? How many instances of NameNode run on a Hadoop Cluster?

Answer: The NameNode is the centerpiece of an HDFS file system. It keeps the directory tree of all files in the file system, and tracks where across the cluster the file data is kept. It does not store the data of these files itself. 

• There is only One NameNode process run on any hadoop cluster. NameNode runs on its own JVM process. In a typical production cluster, its run on a separate machine.
• The NameNode is a Single Point of Failure for the HDFS Cluster. When the NameNode goes down, the file system goes offline.
• Client applications talk to the NameNode whenever they wish to locate a file, or when they want to add/copy/move/delete a file. The NameNode responds the successful requests by returning a list of relevant DataNode servers where the data lives.

Question-57: What is a DataNode? How many instances of DataNode run on a Hadoop Cluster?

Answer: A DataNode stores data in the Hadoop File System HDFS. There is only One DataNode process run on any hadoop slave node. DataNode runs on its own JVM process. On startup, a DataNode connects to the NameNode. DataNode instances can talk to each other, this is mostly during replicating data. 

Question-58: How the Client communicates with HDFS?

Answer: The Client communication to HDFS happens using Hadoop HDFS API. Client applications talk to the NameNode whenever they wish to locate a file, or when they want to add/copy/move/delete a file on HDFS. The NameNode responds the successful requests by returning a list of relevant DataNode servers where the data lives. Client applications can talk directly to a DataNode, once the NameNode has provided the location of the data.

Question-59: How the HDFS Blocks are replicated?

Answer:  HDFS is designed to reliably store very large files across machines in a large cluster. It stores each file as a sequence of blocks; all blocks in a file except the last block are the same size.

The blocks of a file are replicated for fault tolerance. The block size and replication factor are configurable per file. An application can specify the number of replicas of a file. The replication factor can be specified at file creation time and can be changed later. Files in HDFS are write-once and have strictly one writer at any time.

The NameNode makes all decisions regarding replication of blocks. HDFS uses rack-aware replica placement policy. In default configuration there are total 3 copies of a datablock on HDFS, 2 copies are stored on datanodes on same rack and 3rd copy on a different rack.

Question-60: Can you create HBase table without assigning column family?

Answer:  No, Column family also impact how the data should be stored physically in the HDFS file system, hence there is a mandate that you should always have at least one column family. We can also alter the column families once the table is created.