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Combiner's usage can be seen in source code - CombinerExample.java, in which an user defined combiner - , SumCombiner - sum , sums up messages to be sent and add adds the result to BSPMessageBundle class. This decreases the total messages to just one IntegerMessage with a summary of all values.
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public static class SumCombiner extends Combiner {
@Override
public BSPMessageBundle combine(Iterable<BSPMessage> messages) {
BSPMessageBundle bundle = new BSPMessageBundle();
int sum = 0;
Iterator<BSPMessage> it = messages.iterator();
while (it.hasNext()) {
sum += ((IntegerMessage) it.next()).getData();
}
bundle.addMessage(new IntegerMessage("Sum", sum));
return bundle;
}
}
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Combiner actually get gets called in sync function,
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public void sync() throws IOException, SyncException, InterruptedException {
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final BSPMessageBundle bundle = combineMessages(messages);
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}
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