gRPC is a lightweight open-source RPC framework. It was originally developed by Google, with 1.0 being released in August 2016. Since then, it's gained a lot of attention as well as a wide adoption.
In comparison to JSON, which is transmitted as human-readable text, gRPC is binary, making it both faster to transmit and more compact. In the benchmarks we've seen between gRPC and JSON-based REST, gRPC has proved to be a lot faster than it's more traditional counterpart. The messages and services used for gRPC are described in .proto files, containing Protocol Buffers (protobuf) definitions.
Starting with k6 v0.29.0, we support unary gRPC requests via the k6/net/grpc built-in module.