Kafka流处理过程中处理数据的异常

Question

我正在使用以下代码处理 Kafka 流。如果"UserID":"1"，我检查来自JSON obj 的过滤条件。请参考下面的代码

builder.<String,String>stream(Serdes.String(), Serdes.String(), topic)
                   .filter(new Predicate <String, String>() {

               String userIDCheck = null;

               @Override
            public boolean test(String key, String value) {

                   try {
                       JSONObject jsonObj = new JSONObject(value);

                       userIDCheck = jsonObj.get("UserID").toString();
                       System.out.println("userIDCheck: " + userIDCheck);                          
                   } catch (JSONException e) {
                       // TODO Auto-generated catch block
                       e.printStackTrace();
                   }

                   return userIDCheck.equals("1");
               }
            })
           .to(streamouttopic);

值：{"UserID":"1","Address":"XXX","AccountNo":"989","UserName":"Stella","AccountType":"YYY"}

我收到以下错误：

    Exception in thread "SampleStreamProducer-4eecc3ab-858c-44a4-9b8c-5ece2b4ab21a-StreamThread-1" org.apache.kafka.streams.errors.StreamsException: Exception caught in process. taskId=0_0, processor=KSTREAM-SOURCE-0000000000, topic=testtopic1, partition=0, offset=270
    at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:203)
    at org.apache.kafka.streams.processor.internals.StreamThread.processAndPunctuate(StreamThread.java:679)
    at org.apache.kafka.streams.processor.internals.StreamThread.runLoop(StreamThread.java:557)
    at org.apache.kafka.streams.processor.internals.StreamThread.run(StreamThread.java:527)
Caused by: org.apache.kafka.streams.errors.StreamsException: A serializer (key: org.apache.kafka.common.serialization.ByteArraySerializer / value: org.apache.kafka.common.serialization.ByteArraySerializer) is not compatible to the actual key or value type (key type: unknown because key is null / value type: java.lang.String). Change the default Serdes in StreamConfig or provide correct Serdes via method parameters.
    at org.apache.kafka.streams.processor.internals.SinkNode.process(SinkNode.java:91)
    at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:82)
    at org.apache.kafka.streams.kstream.internals.KStreamFilter$KStreamFilterProcessor.process(KStreamFilter.java:43)
    at org.apache.kafka.streams.processor.internals.ProcessorNode$1.run(ProcessorNode.java:47)
    at org.apache.kafka.streams.processor.internals.StreamsMetricsImpl.measureLatencyNs(StreamsMetricsImpl.java:187)
    at org.apache.kafka.streams.processor.internals.ProcessorNode.process(ProcessorNode.java:133)
    at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:82)
    at org.apache.kafka.streams.processor.internals.SourceNode.process(SourceNode.java:80)
    at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:189)
    ... 3 more
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to [B
    at org.apache.kafka.common.serialization.ByteArraySerializer.serialize(ByteArraySerializer.java:21)
    at org.apache.kafka.streams.processor.internals.RecordCollectorImpl.send(RecordCollectorImpl.java:89)
    at org.apache.kafka.streams.processor.internals.RecordCollectorImpl.send(RecordCollectorImpl.java:76)
    at org.apache.kafka.streams.processor.internals.SinkNode.process(SinkNode.java:87)

上面的流代码中的值和条件都很好，我不明白为什么在执行流代码时会给出这个异常。

Answer 1

报告的问题应仅适用于 Kafka 2.0 及更早版本。从 2.1.0 版本开始，Kafka Streams 支持“serde push down”，to() 操作符应该从上游继承正确的 serdes（参见 https://issues.apache.org/jira/browse/KAFKA-7456）。

对于 Kafka 2.0 及更早版本，您必须为 to() 操作明确指定正确的 Serdes。否则，它使用来自 StreamsConfig 的默认 Serdes，即 ByteArraySerde（因为语义或 serde 覆盖是每个运算符“插入覆盖”）——并且 String 不能转换为 byte[]。

你需要做的：

.to(streamoutputtopic, Produced.with(Serdes.String(), Serdes.String()));

对于尚未使用Produced 参数的旧版本（1.0 之前），代码为：

.to(Serdes.String(), Serdes.String(), streamoutputtopic);