HBase Table MapReduce Basics

In this post I am going to explain how we can use HBase tables as source and target for MapReduce program.

For writing MapReduce on HBase tables we should follow below guidelines:

1. Mapper Class

• Mapper class should extend TableMapper.
• Input key to mapper is ImmutableBytesWritable object which has rowkey of HBase table.
• Input value is Result object (org.apache.hadoop.hbase.client.Result) which contains the requested column families (define the required columns/column families in Scan) from HBase table.

2. Reducer Class

• Reducer class should extend TableReducer.
• Output key is NULL.
• Output value is Put (org.apache.hadoop.hbase.client.Put) object.

3. MapReduce Driver

• Configure a Scan (org.apache.hadoop.hbase.client.Scan) object. For this scan object we can define many parameters like:
  • Start row.
  • Stop row.
  • Row filter.
  • Column Familiy(s) to retrieve.
  • Column(s) to retrieve.
• Define input table using TableMapReduceUtil.initTableMapperJob. In this method we can define input table, Mapper, MapOutputKey, MapOutputValue, etc.
• Define output table using TableMapReduceUtil.initTableReducerJob. In this method we can define output table, Reducer and Partitioner.

In my next post I shall give an example MapReduce program using HBase tables as input and output.