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### MapReduce reduce() method

### MapReduce multivalue_blocks() method

### MapReduce multivalue_block() method

    int MapReduce::reduce(void (*myreduce)(char *, int, char *, int, int *, KeyValue *, void *), void *ptr)

    int MapReduce::multivalue_blocks()

    int MapReduce::multivalue_block(int iblock, char **ptr_multivalue, int **ptr_valuesizes)

This calls the reduce() method of a MapReduce object, passing it a function pointer to a reduce function you write. It operates on a KeyMultiValue object, calling your myreduce function once for each unique key/multi-value pair owned by that processor. A new KeyValue object is created which stores all the key/value pairs generated by your myreduce() function. The method returns the total number of new key/value pairs stored by all processors.

You can give this method a pointer (void *ptr) which will be returned to your myreduce() function. See the Technical Details section for why this can be useful. Just specify a NULL if you don't need this.

In this example the user function is called myreduce() and it must have the following interface, which is the same as that used by the compress() method:

    void myreduce(char *key, int keybytes, char *multivalue, int nvalues, int *valuebytes, KeyValue *kv, void *ptr)

A single key/multi-value pair is passed to your function from the KeyMultiValue object stored by the MapReduce object. The key is typically unique to this reduce task and the multi-value is a list of the nvalues associated with that key in the KeyMultiValue object.

There are two possibilities for the multi-value. The first is that all its values fit in the memory allocated for the MapReduce object, which is the usual case. See the memsize setting for details on memory allocation.

In this case, the char *multivalue argument is a pointer to the beginning of the multi-value which contains all nvalues, packed one after the other. The int *valuebytes argument is an array which stores the length of each value in bytes. If needed, it can be used by your function to compute an offset into char *values for where each individual value begins. Your function is also passed a kv pointer to a new KeyValue object created and stored internally by the MapReduce object.

If the values do not fit in memory, then the meaning of the arguments passed to your function is changed. Your function must call two additional library functions in order to retrieve a block of values that does fit in memory, and process them one block at a time.

In this case, the char *multivalue argument will be NULL, which is how your function can test for this possibility. If you know huge multi-values will not occur or if you don't need to examine the values themselves, then the test is not needed. Nvalues still holds the total number of values in the multi-value. The meaning of the kv and ptr arguments is the same as discussed above. However, the int *valuebytes argument is changed to be a pointer to the MapReduce object. This is to allow you to make the following two kinds of calls back to the library:

    MapReduce *mr = (MapReduce *) valuebytes;
    int nblocks = mr->multivalue_blocks();
    for (int iblock = 0; iblock < nblocks; iblock++) {
      int nv = mr->multivalue_block(iblock,&multivalue,&valuebytes);
      for (int i = 0; i < nv; i++) {
        process each value within the block of values
      }
    }

The call to multivalue_blocks() returns the number of blocks of values in the multi-value. Each call to multivalue_block() retrieves one block of values. The number of values in the block (nv in this case) is returned. The multivalue and valuebytes arguments are pointers to a char * and int * (i.e. a char ** and int **), which will be set to point to the block of values and their lengths respectively, so they can then be used just as the multivalue and valuebytes arguments in the myreduce() callback itself (when the values do not exceed available memory).

Note that in this example we are re-using (and thus overwriting) the original multivalue and valuebytes arguments as local variables.

Also note that your myreduce() function can call multivalue_block() as many times as it wishes and process the blocks of values multiple times or in any order, though looping through blocks in ascending order will typically give the best disk I/O performance.

Your myreduce() function can produce key/value pairs (though this is not required) which it registers with the MapReduce object by calling the add() method of the KeyValue object. The syntax for registration is described on the doc page of the KeyValue add() method. Alternatively, your myreduce() function can write information to an output file.

See the Settings and Technical Details sections for details on the byte-alignment of keys and values that are passed to your myreduce() function and on those you register with the KeyValue add() methods. Note that only the first value of a multi-value (or of each block of values) passed to your myreduce() function will be aligned to the valuealign setting.

This method is an on-processor operation, requiring no communication. When run in parallel, each processor performs a myreduce() on each of the key/value pairs it owns and stores any new key/value pairs it generates.

Related methods: Keyvalue add(), map()