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Hadoop, mail # user - RecordReader and non thread safe JNI libraries

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RecordReader and non thread safe JNI libraries
Saptarshi Guha 2009-03-02, 04:07
My RecordReader subclass reads from object X. To parse this object and
emit records, i need the use of a C library and a JNI wrapper.

public boolean next(LongWritable key, BytesWritable value) throws IOException {
   if (leftover == 0) return false;
   long wi = pos + split.getStart();
   value.readFields(X.at( wi);
   pos ++; leftover --;
   return true;

X.at uses the JNI lib to read a record number wi

My question is who running this?
1) For a given job, is one instance of this running on each
tasktracker? reading records and feeding to the mappers on its
2) as I have mapred.tasktracker.map.tasks.maximum == 7, does each jvm
launched have one RecordReader running feeding records to the maps its
jvm is running.

If it's either (1) or (2), I guess I'm safe from threading issues.

Please correct me if i'm totally wrong.

Saptarshi Guha