使用Kafka Streams在输出中设置时间戳无法进行转换

问题描述

假设我们有一个变压器(用Scala编写)

Suppose we have a transformer (written in Scala)

new Transformer[String, V, (String, V)]() {
  var context: ProcessorContext = _

  override def init(context: ProcessorContext): Unit = {
    this.context = context
  }

  override def transform(key: String, value: V): (String, V) = {
    val timestamp = toTimestamp(value)
    context.forward(key, value, To.all().withTimestamp(timestamp))
    key -> value
  }

  override def close(): Unit = ()
}

其中，toTimestamp只是一个函数，该函数返回从记录值获取的时间戳.一旦执行，就会有一个NPE:

where toTimestamp is just a function which returns an a timestamp fetched from the record value. Once it gets executed, there's an NPE:

Exception in thread "...-6f3693b9-4e8d-4e65-9af6-928884320351-StreamThread-5" java.lang.NullPointerException
    at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:110)
    at CustomTransformer.transform()
    at CustomTransformer.transform()
    at org.apache.kafka.streams.scala.kstream.KStream$$anon$1$$anon$2.transform(KStream.scala:302)
    at org.apache.kafka.streams.scala.kstream.KStream$$anon$1$$anon$2.transform(KStream.scala:300)
    at

实际上发生的是ProcessorContextImpl失败:

public <K, V> void forward(final K key, final V value, final To to) {
    toInternal.update(to);
    if (toInternal.hasTimestamp()) {
        recordContext.setTimestamp(toInternal.timestamp());
    }
    final ProcessorNode previousNode = currentNode();

因为recordContext尚未初始化(只能由KafkaStreams在内部完成).

because the recordContext was not initialized (an it could only be done internally by KafkaStreams).

这是一个后续问题在Kafka Streams 1中设置输出的时间戳

推荐答案

如果使用transformer，则需要确保在调用TransformerSupplier#get()时创建新的Transformer对象. (请参见 https://docs.confluent.io/current/streams/faq.html#why-do-i-get-an-illegalstateexception-when-accessing-record-metadata )

If you work with transformer, you need to make sure that a new Transformer object is create when TransformerSupplier#get() is called. (cf. https://docs.confluent.io/current/streams/faq.html#why-do-i-get-an-illegalstateexception-when-accessing-record-metadata)

在最初的问题中，我认为这与导致NPE的context变量有关，但现在我意识到这与Kafka Streams内部有关.

In the original question, I thought it's about your context variable that results in NPE, but now I realized it's about the Kafka Streams internals.

Scala API在2.0.0中有一个bug，可能会导致相同的Transformer实例被重用( https://issues.apache.org/jira/browse/KAFKA-7250 ).我认为您正在遇到此错误.稍微重写一下代码就可以解决问题.请注意，Kafka 2.0.1和Kafka 2.1.0包含修复程序.

The Scala API has a bug in 2.0.0 that may result in the case that the same Transformer instance is reused (https://issues.apache.org/jira/browse/KAFKA-7250). I think that you are hitting this bug. Rewriting your code a little bit should fix the issues. Note, that Kafka 2.0.1 and Kafka 2.1.0 contain a fix.

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