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From "Adam Warrington (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2965) Implement MultipleTableInputs which is analogous to MultipleInputs in Hadoop
Date Tue, 07 Sep 2010 04:18:32 GMT
Implement MultipleTableInputs which is analogous to MultipleInputs in Hadoop

                 Key: HBASE-2965
                 URL: https://issues.apache.org/jira/browse/HBASE-2965
             Project: HBase
          Issue Type: New Feature
          Components: mapred, mapreduce
            Reporter: Adam Warrington
            Priority: Minor

This feature would be helpful for doing reduce side joins, or even passing similarly structured
data from multiple tables through map reduce. The API I envision would be very similar to
the already existent MultipleInputs, parts of which could be reused.

MultipleTableInputs would have a public api like:

class MultipleTableInputs {
  public static void addInputTable(Job job, Table table, Scan scan, Class<? extends TableInputFormatBase>
inputFormatClass, Class<? extends Mapper> mapperClass);

MultipleTableInputs would build a mapping of Tables to configured TableInputFormats the same
way MultipleInputs builds a mapping between Paths and InputFormats. Since most people will
probably use TableInputFormat.class as the input format class, the MultipleTableInput implementation
will have to replace the TableInputFormatBase's private scan and table members that are configured
when an instance of TableInputFormat is created (from within its setConf() method) by calling
setScan and setHTable with the table and scan that are passed into addInputTable above. MultipleTableInputFormat's
addInputTable() member function would also set the input format for the job to DelegatingTableInputFormat,
described below.

A new class called DelegatingTableInputFormat would be analogous to DelegatingInputFormat,
where getSplits() would return TaggedInputSplits (same TaggedInputSplit object that the Hadoop
DelegatingInputFormat uses), which tag the split with its InputFormat and Mapper. These are
created by looping through the HTable to InputFormat mappings, and calling getSplits on each
input format, and using the split, the input format, and mapper as constructor args to TaggedInputSplits.

The createRecordReader() function in DelegatingTableInputFormat could have the same implementation
as the Hadoop DelegatingInputFormat.

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