HBase Write Mechanism

1. When a client issues a put request, it will write the data to the WAL.

Write Ahead Log (WAL) is a file used to store new data that is yet to be put on permanent storage. It is used for recovery in the case of failure.

1. Once data is written to the WAL, it is then copied to the MemStore

MemStore is the write cache that stores new data that has not yet been written to disk. There is one MemStore per column family per region.

1. Once the data is placed in MemStore, the client then receives the ACK (acknowledgement)

2. When the MemStore reaches the threshold, it dumps or commits the data into a HFile.

Hfiles store store the rows of data as stored KeyValue on disk.