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<title>Accumulo Combiners</title>
<link rel='stylesheet' type='text/css' href='documentation.css' media='screen'/>
<h1>Apache Accumulo Documentation : Combiners</h1>
<p>Accumulo supports on the fly lazy aggregation of data using Combiners. Aggregation is done at compaction and scan time. No lookup is done at insert time, which` greatly speeds up ingest.
<p>Combiners are easy to use. You use the setiters command to configure a combiner for a table. Allowing a Combiner to apply to a whole column family is an interesting twist that gives the user great flexibility. The example below demonstrates this flexibility.
Shell - Apache Accumulo Interactive Shell
- version: 1.5.0-SNAPSHOT
- instance id: 863fc0d1-3623-4b6c-8c23-7d4fdb1c8a49
- type 'help' for a list of available commands
user@instance&gt; createtable perDayCounts
user@instance perDayCounts&gt; setiter -t perDayCounts -p 10 -scan -minc -majc -n daycount -class org.apache.accumulo.core.iterators.user.SummingCombiner
TypedValueCombiner can interpret Values as a variety of number encodings (VLong, Long, or String) before combining
----------&gt; set SummingCombiner parameter columns, &lt;col fam&gt;[:&lt;col qual&gt;]{,&lt;col fam&gt;[:&lt;col qual&gt;]} escape non aplhanum chars using %&lt;hex&gt;.: day
----------&gt; set SummingCombiner parameter type, &lt;VARNUM|LONG|STRING&gt;: STRING
user@instance perDayCounts&gt; insert foo day 20080101 1
user@instance perDayCounts&gt; insert foo day 20080101 1
user@instance perDayCounts&gt; insert foo day 20080103 1
user@instance perDayCounts&gt; insert bar day 20080101 1
user@instance perDayCounts&gt; insert bar day 20080101 1
user@instance perDayCounts&gt; scan
bar day:20080101 [] 2
foo day:20080101 [] 2
foo day:20080103 [] 1
<p>Implementing a new Combiner is a snap. Simply write some Java code that extends <a href='apidocs/org/apache/accumulo/core/iterators/Combiner.html'>org.apache.accumulo.core.iterators.Combiner</a>. A good place to look for examples is the <a href='apidocs/org/apache/accumulo/core/iterators/user/package-summary.html'>org.apache.accumulo.core.iterators.user</a> package. Also look at the example StatsCombiner.
<p>To deploy a new aggregator, jar it up and put the jar in accumulo/lib/ext. To see an example look at <a href='examples/README.combiner'>README.combiner</a>
<p>If you would like to see what iterators a table has you can use the config command like in the following example.
user@instance perDayCounts&gt; config -t perDayCounts -f iterator
table | table.iterator.majc.daycount .............. | 10,org.apache.accumulo.core.iterators.user.SummingCombiner
table | table.iterator.majc.daycount.opt.columns .. | day
table | table.iterator.majc.daycount.opt.type ..... | STRING
table | table.iterator.majc.vers .................. | 20,org.apache.accumulo.core.iterators.VersioningIterator
table | table.iterator.majc.vers.opt.maxVersions .. | 1
table | table.iterator.minc.daycount .............. | 10,org.apache.accumulo.core.iterators.user.SummingCombiner
table | table.iterator.minc.daycount.opt.columns .. | day
table | table.iterator.minc.daycount.opt.type ..... | STRING
table | table.iterator.minc.vers .................. | 20,org.apache.accumulo.core.iterators.VersioningIterator
table | table.iterator.minc.vers.opt.maxVersions .. | 1
table | table.iterator.scan.daycount .............. | 10,org.apache.accumulo.core.iterators.user.SummingCombiner
table | table.iterator.scan.daycount.opt.columns .. | day
table | table.iterator.scan.daycount.opt.type ..... | STRING
table | table.iterator.scan.vers .................. | 20,org.apache.accumulo.core.iterators.VersioningIterator
table | table.iterator.scan.vers.opt.maxVersions .. | 1
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