gRPC is a lightweight, open-source RPC framework. It was originally developed by Google, with version 1.0 released in August 2016. Since then, it's gained much attention and wide adoption.
Whereas JSON transmits as human-readable text, gRPC is binary. The binary format makes data transfer faster and more compact. In the benchmarks we've seen, gRPC has proved much faster than REST, gRPC's more traditional, JSON-based counterpart. The messages and services used for gRPC are described in .proto files, containing definitions for Protocol Buffers (protobuf).
k6 also supports unary gRPC requests to the k6/net/grpc built-in module. For further information, read our tutorial about performance testing gRPC services.