Wednesday, May 16, 2012

HBase basics

--puts--
Doing a put always creates a new version of a cell, at a certain timestamp.

To overwrite an existing value, do a put at exactly the same row, column, and version as that of the cell you would *overshadow*.

HBase internally refuses to distinguish insert vs update -- both are puts.

--column family--
Columns in HBase are grouped into column families. Physically, only and all column-family-1 data are stored in one file dedicated to family-1. 2 column families are usually stored in 2 physical files.

Not mentioned explicitly, but hbase data is, like everyone else, organized into logical tables, consisting of rows and columns. I think this is the logical view end-programmers prefers.

A table often has a few columns belonging to the same column family.

--cells--
A {row, column, version} tuple exactly specifies a cell in HBase.

--versions--
A {row, column, version} tuple exactly specifies a cell in HBase

--deletes--
When deleting an entire row, HBase will internally create a tombstone for each ColumnFamily (i.e., NOT each individual column).

--joins--
simple answer: it doesn't, not in the way that RDBMS' support them (e.g., with equi-joins or outer-joins in SQL).

I believe MR is the join.

--HDFS--
HDFS is a distributed file system that is well suited for the storage of large files. HBase, on the other hand, is built on top of HDFS and provides fast record lookups (and updates) for large tables. Hbase internally puts your data in indexed "StoreFiles" that exist on HDFS.

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