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Learning Hbase
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Now, let's see how the read-and-write operation takes place in HBase diagrammatically:

Let's discuss and understand how the read-and-write operation takes place in and from HBase tables. In HBase, the client does not write data to HFile directly; it is first written to WAL and then to HBase MemStore, which is shared by an HStore in the main memory and then flushed to HFile later. Refer to the following figure:

Write-Ahead Logs facilitate the data reliability and reside on HDFS; each RegionServer hosts a single WAL. In the case of a RegionServer crash where MemStore is not flushed, WAL is used to restore the data to a new RegionServer. So, only once data is written successfully to WAL and MemStore, the write operation is said to be successful.
MemStore acts as an in-memory write buffer with a default size of 64 MB. Once data in MemStore reaches the threshold (which is by default 40 percent of the heap size or 64 MB), it is flushed to a new HFile...
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