如何转换/分叉Kafka流并将其发送到特定主题? [英] How do I transform/fork a Kafka stream and send it over to a specific topic?

问题描述

我正在尝试使用功能"mapValues"将在原始流"textlines"中获得的字符串值转换为JSONObject Messages到newStream中.然后将在newStream中获得的所有内容流式传输到一个名为"testoutput"的主题.但是，每当一条消息实际经过转换块时，我都会得到一个NullPointerException，错误仅指向kafka流库.不知道怎么回事:((

I am Trying to transform the string value obtained in my original stream "textlines" into JSONObject Messages using the function "mapValues" into newStream. Then stream whatever I get in newStream onto a topic called "testoutput". But everytime a message actually goes through the transformation block I get a NullPointerException with errors pointing only into kafka stream libraries. Have no idea what is going on :((

P.S.当我从原始流派生/创建新的kafka流时，新流是否属于原始构建器?因为我需要构建器来创建KafkaStreams对象并开始流式传输，所以我不确定是否需要对新流进行其他操作，而不仅仅是指定其.to("topic")

P.S. When I fork/create a new kafka stream from the original stream, does the new stream belong to the original builder? Since I need builder to create the KafkaStreams Object and start streaming I am not sure if I need to do something else with the new stream other than just specifying where its going .to("topic")

//Testing a Kafka Stream Application
public class testStream {

public static void main(String[] args) throws Exception {
    //Configurations
    Properties props = new Properties();
    props.put(StreamsConfig.APPLICATION_ID_CONFIG, "streams-teststream");
    props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "xxxxxxxxxxxx:xxxx");
    props.put(StreamsConfig.KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
    props.put(StreamsConfig.VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
    props.put(StreamsConfig.TIMESTAMP_EXTRACTOR_CLASS_CONFIG, WallclockTimestampExtractor.class);
    props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest");

    //Building Stream
    KStreamBuilder builder = new KStreamBuilder();
    KStream<String, String> textlines = builder.stream("mytest2"); 

    //Printout The Inputs just for testing purposes
    textlines.foreach(new ForeachAction<String, String>(){
        public void apply(String key, String value){
            for(int y=0; y<value.length(); y++){
                System.out.print(value.charAt(y));
            }
            System.out.print("\n");
        }
    });

    //Transform String Records to JSON Objects
    KStream<String, JSONObject> newStream = textlines.mapValues(new ValueMapper<String,JSONObject>(){
        @Override
        public JSONObject apply(String value) {

            JSONObject jsnobj = new JSONObject();

            //If the first 4 letters of the message is "xxxx" then parse it to a 
            //JSON Object, otherwise create a dummy
            if(value.substring(0, 4).equals("xxxx")){               
                jsnobj.put("Header_Title", value.substring(0, 4));
                jsnobj.put("Data_Part", value.substring(4));
            }else{
                jsnobj.put("Header_Title", "Not xxxx");
                jsnobj.put("Data_Part", "None");
            }
            return jsnobj;
        }
    });

    //Specify target
    newStream.to("testoutput");

    //Off you go
    KafkaStreams streams=new KafkaStreams(builder, props);
    streams.start();

  }
}

推荐答案

据我所知，您的问题是此行:

From what I can tell your problem is this line:

newStream.to("testoutput");

newStream的类型为KStream<String, JSONObject>.

但是，默认情况下，您的应用程序配置为使用String Serde来序列化/反序列化记录键和记录值:

However, your application is configured to use, by default, a String serde to serialize/deserialize record keys and record values:

props.put(StreamsConfig.KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
props.put(StreamsConfig.VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());

这意味着，当您未在to()调用中提供显式Serdes时，Kafka Streams将尝试将您的newStream作为KStream<String, String>(而不是KStream<String, JSONObject>)写回到Kafka.

This means that, when you do not provide explicit serdes in the to() call, Kafka Streams will attempt to write your newStream as KStream<String, String> (rather than KStream<String, JSONObject>) back to Kafka.

您需要做的是在to()调用中提供显式的serdes:

What you need to do is to provide explicit serdes in the to() call:

// Sth like this
newStream.to(Serdes.String(), myJsonSerde, "testoutput");

不幸的是，Kafka尚未包括现成的JSON Serde(已计划).幸运的是，您可以查看(并复制)Kafka自己的演示应用程序中包含的针对Kafka Streams API的示例JSON serde:

Unfortunately, Kafka doesn't include an out-of-the-box JSON serde yet (it's planned). Fortunately, you can look at (and copy) the example JSON serde included in Kafka's own demo applications for the Kafka Streams API: https://github.com/apache/kafka/tree/trunk/streams/examples/src/main/java/org/apache/kafka/streams/examples/pageview

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