Support for writing Map/Reduce functions in C++
-----------------------------------------------
Key: HADOOP-234
URL: http://issues.apache.org/jira/browse/HADOOP-234
Project: Hadoop
Type: New Feature
Components: mapred
Reporter: Sanjay Dahiya
MapReduce C++ support
Requirements
1. Allow users to write Map and Reduce functions in C++, rest of the
infrastructure already present in Java should be reused.
2. Avoid users having to write both Java and C++ for this to work.
3. Avoid users having to work with JNI methods directly by wrapping them in
helper functions.
4. Use Record IO for describing record format, both MR java framework and C++
should
use the same format to work seemlessly.
5. Allow users to write simple map reduce tasks without learning record IO if
keys and values are
simple strings.
Implementation notes
- If keys and values are simple strings then user passes SimpleNativeMapper in
JobConf and implements
mapper and reducer methods in C++.
- For composite Record IO types user starts with defining a record format using
Record IO DDL.
- User generates Java and C++ classes from the DDL using record IO.
- Users configures JobConf to use the generated Java classes as the MR
input/output, key/value classes.
- User writes Map and Reduce functions in C++ using a standard interface (
given below ) , this interface
makes a serialized record IO format available to the C++ function which
should be deserialized in corrosponding
generated C++ record IO classes.
- User uses the helper functions to pass the serialized format of generated
output key/value pairs to output collector.
Following is a pseudocode for the Mapper ( Reducer can be implemented similarly
) -
Native(JNI) Java proxy for the Mapper :
---------------------------------------
Without Record IO :-
--------------------
public class SimpleNativeMapper extends MapReduceBase implements Mapper {
/**
* Works on simple strings.
**/
public void map(WritableComparable key, Writable value,
OutputCollector output, Reporter reporter) throws
IOException {
mapNative(key.toString().getBytes()
, value.toString().getBytes(), output,
reporter);
}
/**
* Native implementation.
**/
private native void mapNative(byte[] key, byte[] value,
OutputCollector output, Reporter reporter) throws
IOException;
}
With Record IO :-
------------------
public class RecordIONativeMapper extends MapReduceBase implements Mapper {
/**
* Implementation of map method, this acts as a JNI proxy for actual
map
* method implemented in C++. Works for Record IO based records.
* @see map(byte[] , byte[], OutputCollector, Reporter)
*/
public void map(WritableComparable key, Writable value,
OutputCollector output, Reporter reporter) throws
IOException {
byte[] keyBytes = null ;
byte[] valueBytes = null ;
try{
// we need to serialize the key and record and pass the
serialized
// format to C++ / JNI methods so they can interpret it
using appropriate
// record IO classes.
{
ByteArrayOutputStream keyStream = new
ByteArrayOutputStream() ;
BinaryOutputArchive boa = new
BinaryOutputArchive(new DataOutputStream(keyStream)) ;
((Record)key).serialize(boa, "WhatIsTag");
keyBytes = keyStream.toByteArray();
}
{
ByteArrayOutputStream valueStream = new
ByteArrayOutputStream() ;
BinaryOutputArchive boa = new
BinaryOutputArchive(new DataOutputStream(valueStream)) ;
((Record)key).serialize(boa, "WhatIsTag");
valueBytes = valueStream.toByteArray();
}
}catch(ClassCastException e){
// throw better exceptions
throw new IOException("Input record must be of Record
IO Type");
}
// pass the serialized byte[] to C++ implementation.
mapNative(keyBytes, valueBytes, output, reporter);
}
/**
* Implementation in C++.
*/
private native void mapNative(byte[] key, byte[] value,
OutputCollector output, Reporter reporter) throws
IOException;
}
OutputCollector Proxy for C++
------------------------------
public class NativeOutputCollector implements OutputCollector {
// standard method from interface
public void collect(WritableComparable key, Writable value)
throws IOException {
}
// deserializes key and value and calls collect(WritableComparable,
Writable)
public void collectFromNative(byte[]key, byte[]value){
// deserialize key and value to java types ( as configured in
JobConf )
// call actual collect method
}
}
Core Native functions ( helper for user provided Mapper and Reducer )
---------------------------------------------------------------------
#include "org_apache_hadoop_mapred_NativeMapper.h"
#include "UserMapper.h"
/**
* A C++ proxy method, calls actual implementation of the Mapper. This
method
signature is generated by javah.
**/
JNIEXPORT void JNICALL Java_org_apache_hadoop_mapred_NativeMapper_mapNative
(JNIEnv *env, jobject thisObj, jbyteArray key, jbyteArray value,
jobject output_collector, jobject reporter);
{
// convert char* and pass on to user defined map method.
// user's map method should take care of converting it to correct
record IO
// type.
int keyLen = (*env)->GetArrayLength(env, key) ;
int valueLen = (*env)->GetArrayLength(env, valueLen) ;
const char *keyBuf = (*env)->GetByteArrayElements(env,key, keyLen,
JNI_FALSE) ;
const char *valueuf = (*env)->GetByteArrayElements(env,value, valueLen,
JNI_FALSE) ;
// Call User defined method
user_map(keyBuf, valueBuf, output_collector, reporter) ;
(*env)->ReleaseByteArrayElements(env, key, keyBuf, JNI_ABORT) ;
(*env)->ReleaseByteArrayElements(env, value, ValueBuf, JNI_ABORT) ;
}
/**
Helper method, acts as a proxy to OutputCollector in java. key and
value
must be serialized forms of records as specified in JobConf.
**/
void output_collector(const char * key, const char *value,
jobject output_collector, jobject reporter){
// invoke java NativeOutputCollector.collect with key and value.
}
User defined Mapper ( and Reducer )
------------------------------------
/**
implements user defined map operation.
**/
void user_mapper(const char *key, const char *value, jobject collector, jobject
recorder) {
//1. deserialize key/value in the appropriate format using record IO.
//2. process key/value and generate the intermediate key/values in
record IO format.
//3. Deserialize intermediate key/values to intermed_key and
intermed_value
//4. pass intermed_key/intermed_value using helper function -
// output_collector(intermed_key, intermed_value,
collector, recorder);
}
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