&gt;The rewrite was from Ruby to Scala<p>Sorry, you&#x27;re exactly correct. It was Ruby -&gt; Scala and Java.<p>To be precise, according to Tony Pritezis from Twitter, Scala is the main language, Java is next, Ruby was still in use in 2014 but migrating away, C&#x2F;C++ and Python for some specific tasks. He did also talk about some of the problems of using Scala, and I seem to recall someone saying that Scala may be less favoured at Twitter in the future.