Tap sink = new Hfs(new SequenceFile(new Fields("key", "a_sum", "b_sum", "a_b_sum", "a_b_sum_doubled")), OUTPUT_PATH);
Pipe pipe = new Pipe("pipe");
pipe = Combiner.assembly(pipe,
new MultiExactAggregator(
new ExactAggregatorDefinition(new Fields("a"), new Fields("a_sum"), new Aggregator()),
new ExactAggregatorDefinition(new Fields("b"), new Fields("b_sum"), new Aggregator()),
new ExactAggregatorDefinition(new Fields("a", "b"), new Fields("a_b_sum"), new OtherAggregator())),
new Fields("key"),
new Fields("a", "b"),
new Fields("a_sum", "b_sum", "a_b_sum", "a_b_sum_doubled"));
CascadingUtil.get().getFlowConnector().connect(source, sink, pipe).complete();