在 Storm 中，什么是 Topology？它的作用是什么？

import backtype.storm.Config;
import backtype.storm.LocalCluster;
import backtype.storm.topology.TopologyBuilder;
import backtype.storm.tuple.Fields;

public class SimpleTopology {
    public static void main(String[] args) throws Exception {
        // 创建一个 TopologyBuilder 实例
        TopologyBuilder builder = new TopologyBuilder();

        // 定义一个 Spout
        builder.setSpout("spout", new MySpout(), 1);

        // 定义第一个 Bolt
        builder.setBolt("bolt1", new MyBolt1(), 2)
               .shuffleGrouping("spout");

        // 定义第二个 Bolt
        builder.setBolt("bolt2", new MyBolt2(), 2)
               .fieldsGrouping("bolt1", new Fields("field1"));

        // 配置 Topology
        Config config = new Config();
        config.setDebug(true);

        // 提交 Topology 到本地集群
        LocalCluster cluster = new LocalCluster();
        cluster.submitTopology("simple-topology", config, builder.createTopology());

        // 等待一段时间后关闭 Topology
        Thread.sleep(10000);
        cluster.shutdown();
    }
}