gRPC
A fast, contract-first RPC framework using Protocol Buffers over HTTP/2 — common for service-to-service calls.
gRPC is a high-performance remote-procedure-call framework: you define a service and its messages in a Protocol Buffers schema, and it generates typed client and server code that talk over HTTP/2 with compact binary payloads. It is popular for internal service-to-service communication.
Worked example: two backend microservices call each other through generated gRPC stubs — a strongly-typed contract, binary encoding, and streaming — far leaner than JSON over HTTP/1.1 for high-volume internal traffic. Gotcha: the binary format and HTTP/2 requirement make gRPC awkward from browsers (needs a proxy like gRPC-Web) and harder to debug by hand than REST/JSON, so it shines inside the datacenter but is rarely the choice for a public, browser-facing API.