org.apache.nutch.segment
Class SegmentMerger
- java.lang.Object
- org.apache.hadoop.conf.Configured
- org.apache.nutch.segment.SegmentMerger
- All Implemented Interfaces:
- Closeable, AutoCloseable, org.apache.hadoop.conf.Configurable, org.apache.hadoop.mapred.JobConfigurable, org.apache.hadoop.mapred.Mapper
public class SegmentMerger extends org.apache.hadoop.conf.Configured implements org.apache.hadoop.mapred.Mapper<org.apache.hadoop.io.Text,MetaWrapper,org.apache.hadoop.io.Text,MetaWrapper>, org.apache.hadoop.mapred.Reducer<org.apache.hadoop.io.Text,MetaWrapper,org.apache.hadoop.io.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.
- Author:
- Andrzej Bialecki
Nested Class Summary
Nested Classes Modifier and Type Class and Description 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
Constructors Constructor and Description SegmentMerger() SegmentMerger(org.apache.hadoop.conf.Configuration conf)
Method Summary
Methods Modifier and Type Method and Description void close() void configure(org.apache.hadoop.mapred.JobConf conf) static void main(String[] args) void map(org.apache.hadoop.io.Text key,
MetaWrapper value,
org.apache.hadoop.mapred.OutputCollector void merge(org.apache.hadoop.fs.Path out,
org.apache.hadoop.fs.Path[] segs,
boolean filter,
boolean normalize,
long slice) void reduce(org.apache.hadoop.io.Text key,
Iterator
NOTE: in selecting the latest version we rely exclusively on the segment name (not all segment data contain time information).
void setConf(org.apache.hadoop.conf.Configuration conf)
-
Methods inherited from class org.apache.hadoop.conf.Configured
getConf
-
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Constructor Detail
-
SegmentMerger
public SegmentMerger()
-
SegmentMerger
public SegmentMerger(org.apache.hadoop.conf.Configuration conf)
Method Detail
-
setConf
public void setConf(org.apache.hadoop.conf.Configuration conf)
- Specified by:
- <code>setConf</code> in interface <code>org.apache.hadoop.conf.Configurable</code>
- Overrides:
- <code>setConf</code> in class <code>org.apache.hadoop.conf.Configured</code>
-
close
public void close()
throws IOException
- Specified by:
- <code>close</code> in interface <code>Closeable</code>
- Specified by:
- <code>close</code> in interface <code>AutoCloseable</code>
- Throws:
- <code>IOException</code>
-
configure
public void configure(org.apache.hadoop.mapred.JobConf conf)
- Specified by:
- <code>configure</code> in interface <code>org.apache.hadoop.mapred.JobConfigurable</code>
-
map
public void map(org.apache.hadoop.io.Text key,
MetaWrapper value,
org.apache.hadoop.mapred.OutputCollector<org.apache.hadoop.io.Text,MetaWrapper> output,
org.apache.hadoop.mapred.Reporter reporter)
throws IOException
- Specified by:
- <code>map</code> in interface <code>org.apache.hadoop.mapred.Mapper<org.apache.hadoop.io.text,metawrapper,org.apache.hadoop.io.text,metawrapper></org.apache.hadoop.io.text,metawrapper,org.apache.hadoop.io.text,metawrapper></code>
- Throws:
- <code>IOException</code>
-
reduce
public void reduce(org.apache.hadoop.io.Text key,
Iterator<MetaWrapper> values,
org.apache.hadoop.mapred.OutputCollector<org.apache.hadoop.io.Text,MetaWrapper> output,
org.apache.hadoop.mapred.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:
- <code>reduce</code> in interface <code>org.apache.hadoop.mapred.Reducer<org.apache.hadoop.io.text,metawrapper,org.apache.hadoop.io.text,metawrapper></org.apache.hadoop.io.text,metawrapper,org.apache.hadoop.io.text,metawrapper></code>
- Throws:
- <code>IOException</code>
-
merge
public void merge(org.apache.hadoop.fs.Path out,
org.apache.hadoop.fs.Path[] segs,
boolean filter,
boolean normalize,
long slice)
throws Exception
- Throws:
- <code>Exception</code>
-
main
public static void main(String[] args) throws Exception
- Parameters:
- <code>args</code> -
- Throws:
- <code>Exception</code>
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