@Rushikesh Deshmukh best reference is HBase reference guide https://hbase.apache.org/book.html 67.6. Write Ahead Log (WAL) 67.6.1. Purpose The Write Ahead Log (WAL) records all changes to data in HBase, to file-based storage. Under normal operations, the WAL is not needed because data changes move from the MemStore to StoreFiles. However, if a RegionServer crashes or becomes unavailable before the MemStore is flushed, the WAL ensures that the changes to the data can be replayed. If writing to the WAL fails, the entire operation to modify the data fails. HBase uses an implementation of the WAL interface. Usually, there is only one instance of a WAL per RegionServer. The RegionServer records Puts and Deletes to it, before recording them to the MemStore for the affected [store]. The HLog Prior to 2.0, the interface for WALs in HBase was named HLog . In 0.94, HLog was the name of the implementation of the WAL. You will likely find references to the HLog in documentation tailored to these older versions. The WAL resides in HDFS in the /hbase/WALs/ directory (prior to HBase 0.94, they were stored in /hbase/.logs/), with subdirectories per region. For more general information about the concept of write ahead logs, see the Wikipedia Write-Ahead Log article.
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