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MapReduce open() method

MapReduce close() method

void MapReduce::open()
void MapReduce::open(int addflag)
uint64_t MapReduce::close() 

These call the open() and close() methods of a MapReduce object. This is only necessary when you will be performing a map() or reduce() that generates key/value pairs, and you wish to add pairs not only to the MapReduce object which is invoking the map() and reduce(), but also to one or more other MapReduce objects. In order to do this, you need to invoke the open() and close() methods on the other MapReduce object(s), so that they can accumulate new key/value pairs properly. The close() method returns the total number of key/value pairs in the KeyValue object.

Here is an example of how this is done:

MapReduce *mr = new MapReduce()
MapReduce *mr2 = new MapReduce()
void mymap(int itask, KeyValue *kv, void *ptr) {
  KeyValue *kv2 = (KeyValue *) ptr;

The mymap() function is being called from the "mr" MapReduce object, and can add key/value pairs to "mr" in the usual way, via the kv->add() function call. But it can also add key/value pairs to the "mr2" MapReduce object via the kv2->add() function call. To do this, 3 things were necessary:

The second bullet point was accomplishsed by passing mr2->kv directly to the map() method, but other variations are possible. For example, a pointer to a data structure could be passed, which contains pointers to several other MapReduce objects. In this case, the open() and close() methods for each of the other MapReduce objects would need to be called appropriately before and after the map() method, assuming they would each have key/value pairs added to them by the mymap() function.

You can call open() and close() as many times as needed, but note calls to open() and close() should always come in pairs. You should not call close() when an open() has not been invoked. And you should not open() a second time without calling close() first.

Related methods: map(), reduce