Bolts - Getting Started with Storm - page 42

Multiple Anchoring
To use a bolt to join or aggregate streams, you'll need to buffer tuples in memory. In
order to message guarantee in this scenario you have to anchor the stream to more than
one tuple. This is done by calling emit with a List of tuples.
...
List < Tuple > anchors = new ArrayList < Tuple >();
anchors . add ( tuple1 );
anchors . add ( tuple2 );
_collector . emit ( anchors , values );
...
That way, any time a bolt acks or fails, it notifies the tree, and because the stream is
anchored to more than one tuple, all spouts involved are notified.
Using IBasicBolt to Ack Automatically
As you probably noticed, there are lots of use cases in which you need message guar-
antees. To make things easier, Storm provides another interface for bolts called IBasic
Bolt , which encapsulates the pattern of calling ack right after the execute method. An
implementation of this interface, BaseBasicBolt , is used to do the acking automatically.
class SplitSentence extends BaseBasicBolt {
public void execute ( Tuple tuple , BasicOutputCollector collector ) {
String sentence = tuple . getString ( 0 );
for ( String word: sentence . split ( " " )) {
collector . emit ( new Values ( word ));
}
}
public void declareOutputFields ( OutputFieldsDeclarer declarer ) {
declarer . declare ( new Fields ( "word" ));
}
}
Tuples emitted to BasicOutputCollector are automatically anchored to
the input tuple.