Scalatra
Tiny Scala high-performance, async web framework
...It supports asynchronous request handling, routing, filters, content negotiation, and easy integration with templating, JSON libraries, and other web middleware. Being unopinionated, it lets developers pick their persistence, dependency injection, or templating layers, rather than enforcing heavy conventions. Scalatra is well-suited for services, APIs, or small–to–medium web applications where you want fine control over latency and structure, and it benefits from Scala’s type system and ecosystem.