Rego

Rego is a declarative, domain-specific policy language designed to express complex security and compliance rules over structured JSON data.

Built specifically for the Open Policy Agent (OPA) engine, Rego decouples authorization logic from application code by treating policy as code. It extends Datalog to support nested JSON documents, allowing platform engineers to write clean assertions for Kubernetes admission control, Terraform security checks, and microservice API authorization. Because Rego is declarative, the engine handles query optimization automatically: you define what the system state must look like, and OPA evaluates the rules in memory for high-performance decision-making.

https://www.openpolicyagent.org/docs/latest/policy-language/