Tip: ClassCastException in Kafka Streams reducer

If you use the groupByKey function on a KStream without specifying a Serdes, the (one configured in the StreamsConfig will be used by default e.g. in the below snippet, it’s Serdes.String(). As a result, you will face a ClassCastExcpetion in case

  • you execute an operation (e.g. mapValues)  which changes the data type of the key or value, and
  • do not specify the corresponding Serdes for the data type while execuing subsequent operations (e.g. groupByKey)

Quick Tip

The solution is to use the appropriate Serdes (in the below example, Serdes.Double() is explicitly specified)



About Abhishek

Loves Java EE, distributed KV stores and messaging systems. Frequently blogs at abhirockzz.wordpress.com as well as simplydistributed.wordpress.com. Oh, I have also authored a few (mini) books, articles, Refcards etc. :-)
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