Rumors of Scala's dominance at Twitter are slightly exaggerated. While it's true that the non-revenue-related backend is almost all Scala, on the ads eng side, it's almost 100% Java. Which language a new project is written in has more to do with who's writing it than anything else.

Actually I just wondered cause of Finangle. I thought that their projects uses the RPC service heavily.

Ah, Finagle has a Java API too.

I think over the years they decided to use Scala. I'm not even sure if most of their codebase is still in Scala today.

Because Scala adds overhead.

Where? In operational cost? In development time? In performance?

Yes.

Depending on use-case, of course, there's lot of places where scala is preferable to Java. A byte-shuttling service where performance is important, mutability reigns and expressiveness is irrelevant.. not one of those places.

Yes.

Joke aside, at least in hiring people it should.