@InterfaceAudience.Public @InterfaceStability.Stable public class ChainMapper extends Object implements Mapper
The Mapper classes are invoked in a chained (or piped) fashion, the output of the first becomes the input of the second, and so on until the last Mapper, the output of the last Mapper will be written to the task's output.
The key functionality of this feature is that the Mappers in the chain do not need to be aware that they are executed in a chain. This enables having reusable specialized Mappers that can be combined to perform composite operations within a single task.
Special care has to be taken when creating chains that the key/values output by a Mapper are valid for the following Mapper in the chain. It is assumed all Mappers and the Reduce in the chain use maching output and input key and value classes as no conversion is done by the chaining code.
Using the ChainMapper and the ChainReducer classes is possible to compose
Map/Reduce jobs that look like
[MAP+ / REDUCE MAP*]. And
immediate benefit of this pattern is a dramatic reduction in disk IO.
IMPORTANT: There is no need to specify the output key/value classes for the ChainMapper, this is done by the addMapper for the last mapper in the chain.
ChainMapper usage pattern:
... conf.setJobName("chain"); conf.setInputFormat(TextInputFormat.class); conf.setOutputFormat(TextOutputFormat.class); JobConf mapAConf = new JobConf(false); ... ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class, Text.class, Text.class, true, mapAConf); JobConf mapBConf = new JobConf(false); ... ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class, LongWritable.class, Text.class, false, mapBConf); JobConf reduceConf = new JobConf(false); ... ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class, Text.class, Text.class, true, reduceConf); ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class, LongWritable.class, Text.class, false, null); ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class, LongWritable.class, LongWritable.class, true, null); FileInputFormat.setInputPaths(conf, inDir); FileOutputFormat.setOutputPath(conf, outDir); ... JobClient jc = new JobClient(conf); RunningJob job = jc.submitJob(conf); ...
|Constructor and Description|
|Modifier and Type||Method and Description|
Adds a Mapper class to the chain job's JobConf.
Closes the ChainMapper and all the Mappers in the chain.
Configures the ChainMapper and all the Mappers in the chain.
public static <K1,V1,K2,V2> void addMapper(JobConf job, Class<? extends Mapper<K1,V1,K2,V2>> klass, Class<? extends K1> inputKeyClass, Class<? extends V1> inputValueClass, Class<? extends K2> outputKeyClass, Class<? extends V2> outputValueClass, boolean byValue, JobConf mapperConf)
It has to be specified how key and values are passed from one element of the chain to the next, by value or by reference. If a Mapper leverages the assumed semantics that the key and values are not modified by the collector 'by value' must be used. If the Mapper does not expect this semantics, as an optimization to avoid serialization and deserialization 'by reference' can be used.
For the added Mapper the configuration given for it,
mapperConf, have precedence over the job's JobConf. This
precedence is in effect when the task is running.
IMPORTANT: There is no need to specify the output key/value classes for the ChainMapper, this is done by the addMapper for the last mapper in the chain
job- job's JobConf to add the Mapper class.
klass- the Mapper class to add.
inputKeyClass- mapper input key class.
inputValueClass- mapper input value class.
outputKeyClass- mapper output key class.
outputValueClass- mapper output value class.
byValue- indicates if key/values should be passed by value to the next Mapper in the chain, if any.
mapperConf- a JobConf with the configuration for the Mapper class. It is recommended to use a JobConf without default values using the
JobConf(boolean loadDefaults)constructor with FALSE.
public void configure(JobConf job)
If this method is overriden
super.configure(...) should be
invoked at the beginning of the overwriter method.
public void map(Object key, Object value, OutputCollector output, Reporter reporter) throws IOException
map(...)methods of the Mappers in the chain.
public void close() throws IOException
If this method is overriden
super.close() should be
invoked at the end of the overwriter method.
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