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We currently build Kafka for 3 Scala versions: 2.10, 2.11 and the recently released 2.12 (not yet enabled in the "All" targets as it requires Java 8, but we have a Jenkins build for this version during PR validation). Since we have to compile and run the tests on each supported version, there is a non-trivial cost from a development and testing perspective

Scala 2.10 was released in March 2013 (support ended in March 2015, almost 2 years ago), 2.11 was released in April 2014 and 2.12 was released in November 2016. A reasonable policy is to support the 2 most recently released versions so that we can strike a good balance between supporting older versions, maintainability and taking advantage of language and library improvements. Furthermore, since the recommended clients (producer and consumer) are now pure Java, this should mostly affect the broker and tools where support for older Scala versions matters less.

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