Yep, Kafka is probably the most popular one. I've heard that the team is looking forward to replace the remaining Scala code in the project with Java once pattern matching and co. land. Spark is another beast that was written in Scala. Many are reporting a high cost for compatibility. Nowadays, Scala community is all about Typelevel and ZIO, if you are not a category theory minded person, then you will have a hard time picking it up.