When it comes to enforcing policies in Kubernetes, two projects stand out as the clear leaders: OPA/Gatekeeper and Kyverno. Both are powerful CNCF (Cloud Native Computing Foundation) graduated projects designed to act as admission controllers, but they approach the problem from different philosophical standpoints.

Choosing between them is a critical architectural decision. Should you use the general-purpose power of OPA with its specialized Rego language, or the Kubernetes-native simplicity of Kyverno with its declarative YAML policies? This guide will break down the key differences to help you choose the right engine for your team.

What is OPA/Gatekeeper?

Open Policy Agent (OPA) is a general-purpose policy engine that can be used for authorization across your entire stack—microservices, CI/CD pipelines, APIs, and more. It uses a high-level declarative language called **Rego** to define policies.

Gatekeeper is a specialized Kubernetes-native project that uses OPA to enforce policies on your cluster. It provides a way to instantiate OPA as an admission controller and manage its policies through Custom Resource Definitions (CRDs).

  • Core Idea: Unify policy enforcement across many different technologies with one language (Rego).
  • Strength: Extremely powerful and flexible. If you can express logic, you can write a policy for it.
  • Challenge: Requires learning Rego, which has a steeper learning curve than simple YAML.

What is Kyverno?

Kyverno (Greek for "govern") is a policy engine designed *specifically for Kubernetes*. Instead of using a separate language like Rego, Kyverno policies are themselves Kubernetes resources written in familiar YAML. This makes it feel very natural to anyone already working with Kubernetes manifests.

  • Core Idea: Manage Kubernetes policies the same way you manage other Kubernetes resources.
  • Strength: Easy to learn for Kubernetes developers. Policies can validate, mutate (modify), and generate resources, which is incredibly powerful.
  • Challenge: It is purpose-built for Kubernetes and cannot be used for other parts of your stack.

Head-to-Head Comparison

Feature OPA/Gatekeeper Kyverno
Policy Language Rego (Custom, powerful, general-purpose) Declarative YAML (Kubernetes-native, easy to read)
Learning Curve Steeper, requires learning a new language (Rego) Gentle, leverages existing Kubernetes knowledge
Primary Functions Validate, Audit Validate, Mutate, Generate, Audit
Ecosystem General-purpose (APIs, SSH, etc.) Kubernetes-specific
Community & Adoption Larger, more established across many domains Growing rapidly, very strong within the K8s community

Policy in Action: A Simple Example

Let's compare how each engine enforces a common policy: **"all pods must have resource limits defined."**

Kyverno Policy

The policy is a single, easy-to-read `ClusterPolicy` resource.

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: require-resource-limits
spec:
  validationFailureAction: Enforce
  rules:
  - name: check-for-limits
    match:
      any:
      - resources:
          kinds:
          - Pod
    validate:
      message: "CPU and memory resource limits are required."
      pattern:
        spec:
          containers:
          - resources:
              limits:
                cpu: "?*"
                memory: "?*"

OPA/Gatekeeper Policy

This requires two resources: a `ConstraintTemplate` to define the policy logic in Rego, and a `Constraint` to apply it.

# 1. The ConstraintTemplate (Defines the "how")
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8sresourcelimits
spec:
  crd:
    spec:
      names:
        kind: K8sResourceLimits
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sresourcelimits

        violation[{"msg": msg}] {
          container := input.review.object.spec.containers[_]
          not container.resources.limits.cpu
          msg := sprintf("Container '%v' is missing CPU limits", [container.name])
        }

        violation[{"msg": msg}] {
          container := input.review.object.spec.containers[_]
          not container.resources.limits.memory
          msg := sprintf("Container '%v' is missing memory limits", [container.name])
        }
---
# 2. The Constraint (Applies the "what" and "where")
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sResourceLimits
metadata:
  name: pods-must-have-limits
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]

When to Choose Which?

🚀 Choose Kyverno if:

  • Your primary focus is exclusively on Kubernetes.
  • Your team is highly proficient with Kubernetes YAML and prefers a declarative, resource-centric approach.
  • You need powerful **mutate** and **generate** capabilities (e.g., automatically adding a sidecar container or a default network policy).
  • You want a lower barrier to entry for writing and understanding policies.

🔒 Choose OPA/Gatekeeper if:

  • You need a **single policy language (Rego)** to govern your entire stack, including APIs, microservices, and CI/CD pipelines, not just Kubernetes.
  • Your policies require complex, conditional logic that is easier to express in a general-purpose language.
  • Your organization is willing to invest in learning Rego for its long-term flexibility and power.

Conclusion: No Wrong Answer

Both OPA/Gatekeeper and Kyverno are fantastic, mature solutions for Kubernetes policy enforcement. The "right" choice depends entirely on your team's needs and philosophy. Kyverno offers a seamless, Kubernetes-native experience with powerful generation features, while OPA/Gatekeeper provides unparalleled flexibility and the ability to unify policy across your entire cloud-native ecosystem.