Class SegmentMerger

  extended by org.apache.hadoop.conf.Configured
      extended by org.apache.nutch.segment.SegmentMerger
All Implemented Interfaces:
Closeable, Configurable, JobConfigurable, Mapper<Text,MetaWrapper,Text,MetaWrapper>, Reducer<Text,MetaWrapper,Text,MetaWrapper>

public class SegmentMerger
extends Configured
implements Mapper<Text,MetaWrapper,Text,MetaWrapper>, Reducer<Text,MetaWrapper,Text,MetaWrapper>

This tool takes several segments and merges their data together. Only the latest versions of data is retained.

Optionally, you can apply current URLFilters to remove prohibited URL-s.

Also, it's possible to slice the resulting segment into chunks of fixed size.

Important Notes

Which parts are merged?

It doesn't make sense to merge data from segments, which are at different stages of processing (e.g. one unfetched segment, one fetched but not parsed, and one fetched and parsed). Therefore, prior to merging, the tool will determine the lowest common set of input data, and only this data will be merged. This may have some unintended consequences: e.g. if majority of input segments are fetched and parsed, but one of them is unfetched, the tool will fall back to just merging fetchlists, and it will skip all other data from all segments.

Merging fetchlists

Merging segments, which contain just fetchlists (i.e. prior to fetching) is not recommended, because this tool (unlike the Generator doesn't ensure that fetchlist parts for each map task are disjoint.

Duplicate content

Merging segments removes older content whenever possible (see below). However, this is NOT the same as de-duplication, which in addition removes identical content found at different URL-s. In other words, running DeleteDuplicates is still necessary.

For some types of data (especially ParseText) it's not possible to determine which version is really older. Therefore the tool always uses segment names as timestamps, for all types of input data. Segment names are compared in forward lexicographic order (0-9a-zA-Z), and data from segments with "higher" names will prevail. It follows then that it is extremely important that segments be named in an increasing lexicographic order as their creation time increases.

Merging and indexes

Merged segment gets a different name. Since Indexer embeds segment names in indexes, any indexes originally created for the input segments will NOT work with the merged segment. Newly created merged segment(s) need to be indexed afresh. This tool doesn't use existing indexes in any way, so if you plan to merge segments you don't have to index them prior to merging.

Andrzej Bialecki

Nested Class Summary
static class SegmentMerger.ObjectInputFormat
          Wraps inputs in an MetaWrapper, to permit merging different types in reduce and use additional metadata.
static class SegmentMerger.SegmentOutputFormat
Constructor Summary
SegmentMerger(Configuration conf)
Method Summary
 void close()
 void configure(JobConf conf)
static void main(String[] args)
 void map(Text key, MetaWrapper value, OutputCollector<Text,MetaWrapper> output, Reporter reporter)
 void merge(Path out, Path[] segs, boolean filter, boolean normalize, long slice)
 void reduce(Text key, Iterator<MetaWrapper> values, OutputCollector<Text,MetaWrapper> output, Reporter reporter)
          NOTE: in selecting the latest version we rely exclusively on the segment name (not all segment data contain time information).
 void setConf(Configuration conf)
Methods inherited from class org.apache.hadoop.conf.Configured
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public SegmentMerger()


public SegmentMerger(Configuration conf)
Method Detail


public void setConf(Configuration conf)
Specified by:
setConf in interface Configurable
setConf in class Configured


public void close()
           throws IOException
Specified by:
close in interface Closeable


public void configure(JobConf conf)
Specified by:
configure in interface JobConfigurable


public void map(Text key,
                MetaWrapper value,
                OutputCollector<Text,MetaWrapper> output,
                Reporter reporter)
         throws IOException
Specified by:
map in interface Mapper<Text,MetaWrapper,Text,MetaWrapper>


public void reduce(Text key,
                   Iterator<MetaWrapper> values,
                   OutputCollector<Text,MetaWrapper> output,
                   Reporter reporter)
            throws IOException
NOTE: in selecting the latest version we rely exclusively on the segment name (not all segment data contain time information). Therefore it is extremely important that segments be named in an increasing lexicographic order as their creation time increases.

Specified by:
reduce in interface Reducer<Text,MetaWrapper,Text,MetaWrapper>


public void merge(Path out,
                  Path[] segs,
                  boolean filter,
                  boolean normalize,
                  long slice)
           throws Exception


public static void main(String[] args)
                 throws Exception
args -

Copyright © 2006 The Apache Software Foundation