Direct stream example

gistfile1.txt

public class DirectBoltExample implements IBasicBolt {

    @Override
    public void prepare(Map conf, TopologyContext context) {
    }

    @Override
    public void execute(Tuple tuple, BasicOutputCollector collector) {
        int out = tuple.getInteger(0);
        
        // We will use this map to keep track of how many tuples we've sent to each 
        // task. After we finish emitting tuples, we will emit to each task how many tuples
        // we sent to them on the direct stream
        //
        // You can, of course, keep this map as an instance variable of the bolt if you want to 
        // coordinate across multiple input tuples. This is how CoordinatedBolt works
        Map<Integer, Integer> outCounts = new HashMap<Integer, Integer>();
        
        
        for(int i=0; i < out; i++) {
            // collector.emit returns the task ids that the tuple was sent to
            List<Integer> tasks = collector.emit(1, new Values(i));
            
            for(int task: tasks) {
                Integer curr = outCounts.get(task);
                if(curr==null) curr = 0;
                outCounts.put(task, curr + 1);
            }
        }
        
        for(int task: outCounts.keySet()) {
            // tuples are emitted to a direct stream using the emitDirect method
            // the first argument to emitDirect is the task id
            collector.emitDirect(task, 2, new Values(outCounts.get(task)));
        }
    }

    @Override
    public void cleanup() {
    }

    @Override
    public void declareOutputFields(OutputFieldsDeclarer declarer) {
        declarer.declareStream(1, new Fields("output"));
        
        // Streams need to be explicitly declared as direct by saying "true" 
        // in the second parameter here
        declarer.declareStream(2, true, new Fields("count"));
    }    
}