Open Policy Agent (OPA)
A general-purpose policy engine, and Rego, its query language, that evaluate policy against JSON input — deployable as a library, sidecar, or standalone service.
aka: OPA, Rego
OPA decouples policy from any one application by design — the same engine and the same Rego rule can run embedded in a Go binary, as a sidecar next to a service, or centrally behind an API, and opa test gives policy its own unit-testing story independent of the application it protects. That portability is also why OPA policies compile to WebAssembly, letting the same logic run at the edge.