Question - Does HBase have any concept of the namespace?
Answer -
A namespace is a logical grouping of tables, analogous to a database in RDBMS. You can create the HBase namespace to the schema of the RDBMS database.
To create a namespace, use the command:
create_namespace ‘namespace name’
To list all the tables that are members of the namespace, use the command: list_namespace_tables ‘default’
To list all the namespaces, use the command:
list_namespace
67. How does the Write Ahead Log (WAL) help when a RegionServer crashes?
If a RegionServer hosting a MemStore crash, the data that existed in memory, but not yet persisted, is lost. HBase recovers against that by writing to the WAL before the write completes. The HBase cluster keeps a WAL to record changes as they happen. If HBase goes down, replaying the WAL will recover data that was not yet flushed from the MemStore to the HFile.