Your go-to microservice framework for any situation, from the creator of Netty et al. You can build any type of microservice leveraging your favorite technologies, including gRPC, Thrift, Kotlin, Retrofit, Reactive Streams, Spring Boot and Dropwizard.
Smithy is an IDL from Amazon Web Services: https://smithy.io/ https://github.com/smithy-lang/smithy
Not as popular as gRPC or Thrift, but it has its own merits and niche where others don't fill exactly.