聊聊storm的direct grouping

序
本文主要研究一下storm的direct grouping
direct grouping
direct grouping是一种特殊的grouping，它是由上游的producer直接指定下游哪个task去接收它发射出来的tuple。direct grouping的使用有如下几个步骤：
1、上游在prepare方法保存下游bolt的taskId列表
public class SentenceDirectBolt extends BaseRichBolt {

private static final Logger LOGGER = LoggerFactory.getLogger(SentenceDirectBolt.class);

private OutputCollector collector;

private List<Integer> taskIds;

private int numCounterTasks;

public void prepare(Map config, TopologyContext context, OutputCollector collector) {
this.collector = collector;
//NOTE 1 这里要取到下游的bolt的taskId，用于emitDirect时指定taskId
this.taskIds = context.getComponentTasks(“count-bolt”);
this.numCounterTasks = taskIds.size();
}
//……
}
这里保存了下游的bolt的taskId列表，用于emitDirect时选择taskId
2、上游在declareOutputFields使用declareStream声明streamId
public class SentenceDirectBolt extends BaseRichBolt {
//……
public void declareOutputFields(OutputFieldsDeclarer declarer) {
declarer.declare(new Fields(“word”));
//NOTE 2 这里要通过declareStream声明direct stream，并指定streamId
declarer.declareStream(“directStreamDemo1”,true,new Fields(“word”));
declarer.declareStream(“directStreamDemo2”,true,new Fields(“word”));
}
}
这里声明了两个streamId，一个是directStreamDemo1，一个是directStreamDemo2
3、上游采用emitDirect指定下游taskId及streamId
public class SentenceDirectBolt extends BaseRichBolt {
//……
public void execute(Tuple tuple) {
String sentence = tuple.getStringByField(“sentence”);
String[] words = sentence.split(” “);
for(String word : words){
int targetTaskId = getWordCountTaskId(word);
LOGGER.info(“word:{} choose taskId:{}”,word,targetTaskId);
// NOTE 3 这里指定发送给下游bolt的哪个taskId，同时指定streamId
if(targetTaskId % 2 == 0){
this.collector.emitDirect(targetTaskId,”directStreamDemo1″,new Values(word));
}else{
this.collector.emitDirect(targetTaskId,”directStreamDemo2″,new Values(word));
}
}
this.collector.ack(tuple);
}
}
这里使用emitDirect(int taskId, String streamId, List<Object> tuple)方法指定了下游的taskId以及要发送到的streamId
4、下游使用directGrouping连接上游bolt及streamId
@Test
public void testDirectGrouping() throws InvalidTopologyException, AuthorizationException, AlreadyAliveException {
TopologyBuilder builder = new TopologyBuilder();

builder.setSpout(“sentence-spout”, new SentenceSpout());
// SentenceSpout –> SplitSentenceBolt
builder.setBolt(“split-bolt”, new SentenceDirectBolt()).shuffleGrouping(“sentence-spout”);
// SplitSentenceBolt –> WordCountBolt
//NOTE 4这里要指定上游的bolt以及要处理的streamId
builder.setBolt(“count-bolt”, new WordCountBolt(),5).directGrouping(“split-bolt”,”directStreamDemo1″);
// WordCountBolt –> ReportBolt
builder.setBolt(“report-bolt”, new ReportBolt()).globalGrouping(“count-bolt”);

submitRemote(builder);
}
这里count-bolt作为split-bolt的下游，使用了directGrouping，同时指定了要接收的streamId为directStreamDemo1
小结

direct grouping是一种特殊的grouping，它是由上游的producer直接指定下游哪个task去接收它发射出来的tuple。
下游使用directGrouping连接上游同时指定要消费的streamId，上游在prepare的时候保存下游的taskId列表，然后在declareOutputFields的时候使用declareStream来声明streamId，最后在execute方法里头使用emitDirect(int taskId, String streamId, List<Object> tuple)方法指定了下游的taskId以及要发送到的streamId

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Concepts
Common Topology Patterns
关于Storm Stream grouping