The components of a Region Server are :
WAL : Write Ahead Log (WAL) is a file attached to every Region Server inside the distributed environment. The WAL stores the new data that hasn’t been persisted or committed to the permanent storage.
Block Cache : Block Cache resides in the top of Region Server. It stores the frequently read data in the memory.
MemStore : It is the write cache. It stores all the incoming data before committing it to the disk or permanent memory. There is one MemStore for each column family in a region.
HFile : HFile is stored in HDFS. It stores the actual cells on the disk.
