OPA vs Cedar: 7 Proven Steps to Ship Policy-as-Code

Why this matters now

Modern apps need authorization that’s testable, reviewable, and observable. Two strong options are OPA (Open Policy Agent) with Rego and Cedar (policy language + embeddable engine). Below is a practical, code-heavy guide to help developers and engineering leaders choose wisely and ship policy-as-code with CI gates—without slowing delivery.

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1) OPA vs Cedar: When to choose which

Dimension	OPA (Rego)	Cedar
Model fit	General-purpose policies; great with K8s/Envoy/Cloud Native	Fine-grained authz with principals/actions/resources; strong ABAC/RBAC
Runtime shape	Sidecar (Envoy ext_authz), daemon, or library	Primarily in-process library (very low latency); can be wrapped as a local service
Latency/footprint	In-memory eval is fast (<1–2 ms typical); sidecar adds ~network hop	In-process eval typically sub-millisecond; tiny footprint
Tooling	Conftest, Bundles, partial eval, decision logs, Gatekeeper	Schemas, policies, templates; SDKs; policy validation and tests
Best for	Platform policy, K8s admission, API gateways, multi-language estates	App-level authz in services where consistency & perf are critical
Policy language	Rego (Datalog-style)	Cedar (resource-centric with schema)

Rule of thumb: If you already run Envoy/K8s and want a unified policy plane, OPA is ergonomic. If you need ultra-low latency, in-process checks with a clear principal-action-resource schema, Cedar shines.

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2) Deployment patterns that scale (sidecar vs library vs gateway)

A. Sidecar (OPA + Envoy ext_authz)

Good when you want language-agnostic enforcement at the edge or per-service.

Kubernetes Deployment (excerpt):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: orders
spec:
  template:
    spec:
      containers:
        - name: app
          image: ghcr.io/example/orders:latest
          ports: [{containerPort: 8080}]
          env:
            - name: AUTHZ_ADDR
              value: "http://127.0.0.1:8181"
        - name: envoy
          image: envoyproxy/envoy:v1.29.0
          args: ["-c", "/etc/envoy/envoy.yaml"]
          ports: [{containerPort: 8443}]
          volumeMounts:
            - { name: envoy-config, mountPath: /etc/envoy }
        - name: opa
          image: openpolicyagent/opa:latest-envoy
          args:
            - "run"
            - "--server"
            - "--addr=localhost:8181"
            - "--set=plugins.envoy_ext_authz_grpc.addr=:9191"
            - "--set=decision_logs.console=true"
          ports: [{containerPort: 8181}, {containerPort: 9191}]
      volumes:
        - name: envoy-config
          configMap: { name: envoy-config }

Envoy ext_authz filter (snippet):

http_filters:
- name: envoy.filters.http.ext_authz
  typed_config:
    "@type": type.googleapis.com/envoy.extensions.filters.http.ext_authz.v3.ExtAuthz
    transport_api_version: V3
    grpc_service:
      envoy_grpc: { cluster_name: opa_ext_authz }
    include_peer_certificate: false

B. Library (Cedar in-process)

Tight latency budgets and local context? Embed Cedar directly in your service (example in Rust):

use cedar_policy::*;
fn allow(request: Request, policy: &PolicySet, entities: &EntityStore) -> bool {
    let res = authorize(&request, policy, entities, None);
    res.decision() == Decision::Allow
}

C. Gateway (OPA at API gateway)

Centralize checks at ingress; useful for REST/GraphQL without touching every service. Combine with service-level checks for defense-in-depth.

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3) Authoring policies: ABAC/RBAC for REST, gRPC, GraphQL

OPA / Rego — hybrid RBAC + ABAC

Input (from Envoy): method, path, user claims, resource, action.

package httpapi.authz

default allow = false

# Basic RBAC
role_bindings := {
  "alice": ["admin"],
  "bob":   ["support"]
}

role_perms := {
  "admin":   [{"action": "write", "resource": "orders:*"},
              {"action": "read",  "resource": "orders:*"}],
  "support": [{"action": "read",  "resource": "orders:*"}]
}

# ABAC condition: region must match
region_match {
  input.user.region == input.resource.region
}

# Resource match helper
resource_matches(pattern, res) {
  startswith(res, trim(pattern, "*"))
}

allow {
  user := input.user.name
  some role
  role := role_bindings[user][_]
  some p
  p := role_perms[role][_]
  p.action == input.action
  resource_matches(p.resource, sprintf("%s:%s", [input.resource.type, input.resource.id]))
  region_match
}

Evaluate via OPA HTTP API:

curl -s localhost:8181/v1/data/httpapi/authz/allow -d @- <<'JSON'
{
  "input": {
    "action": "read",
    "user": {"name":"bob","region":"eu-west-1"},
    "resource": {"type":"orders","id":"123","region":"eu-west-1"}
  }
}
JSON

Cedar — schema + policies

Schema (entities + actions):

entity User {}
entity Order {
  attributes { region: String }
}
action ReadOrder appliesTo (principal, resource) when { principal is User, resource is Order };
action WriteOrder appliesTo (principal, resource) when { principal is User, resource is Order };

RBAC binding as entities:

{
  "uid": "User::\"bob\"",
  "attrs": { "region": "eu-west-1", "roles": ["support"] },
  "parents": []
}

Policy: allow read if role has read AND ABAC region rule

permit(
  principal, 
  action in [ReadOrder], 
  resource
)
when {
  "support" in principal.roles && principal.region == resource.region
};

Request evaluation (JSON-ish):

{
  "principal": "User::\"bob\"",
  "action": "ReadOrder",
  "resource": {
    "__entity": "Order::\"123\"",
    "region": "eu-west-1"
  }
}

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4) Wiring REST, gRPC, and GraphQL

REST (Node/Express) → OPA sidecar

import fetch from "node-fetch";
export async function authz(req, res, next) {
  const decision = await fetch("http://127.0.0.1:8181/v1/data/httpapi/authz/allow", {
    method: "POST",
    headers: {"Content-Type":"application/json"},
    body: JSON.stringify({
      input: {
        action: req.method === "GET" ? "read" : "write",
        user: req.user, // set by upstream authn
        resource: { type: "orders", id: req.params.id, region: req.user.region }
      }
    })
  }).then(r => r.json());
  if (decision.result === true) return next();
  res.status(403).json({error:"forbidden"});
}

gRPC (Go interceptor) → OPA sidecar

func AuthzUnaryInterceptor(next grpc.UnaryHandler) grpc.UnaryHandler {
  return func(ctx context.Context, req interface{}) (interface{}, error) {
    input := map[string]interface{}{
      "action": "write",
      "user": ctx.Value(UserKey{}),
      "resource": map[string]interface{}{"type":"orders","id":"123","region":"eu-west-1"},
    }
    allowed, err := queryOPA(ctx, input) // http POST -> /v1/data/httpapi/authz/allow
    if err != nil { return nil, status.Error(codes.Internal, "authz error") }
    if !allowed { return nil, status.Error(codes.PermissionDenied, "forbidden") }
    return next(ctx, req)
  }
}

GraphQL (Apollo) directive → Cedar in-process

const isAllowed = (principal, action, resource) =>
  cedar.authorize({ principal, action, resource }).decision === "Allow";

const resolvers = {
  Query: {
    order: async (_, { id }, ctx) => {
      const resource = { __entity: `Order::"${id}"`, region: await regionOf(id) };
      if (!isAllowed(`User::"${ctx.user.id}"`, "ReadOrder", resource)) throw new Error("Forbidden");
      return loadOrder(id);
    }
  }
}

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5) CI gates: block merges on failing authz checks

Rego tests with Conftest (GitHub Actions)

name: policy-checks
on: [pull_request]
jobs:
  rego-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run Rego unit tests
        run: opa test policies/*.rego
      - name: Validate manifests/contracts
        run: |
          conftest test k8s/ --policy policies/
          conftest test api/openapi.yaml --policy policies/

Cedar policy/unit tests (custom script)

#!/usr/bin/env bash
set -euo pipefail
for t in tests/*.json; do
  echo "Running $t"
  cedar-eval --policies cedar/policies/ --schema cedar/schema/ --request "$t" | \
    jq -e '.decision=="Allow"' > /dev/null
done

Gate merges if tests or cedar-eval checks fail. Add required checks in your repo rules.

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6) Observability: decision logs, metrics, and drift

OPA decision logs (ship to your logs/OTel)

{
  "labels": {"service":"orders","version":"2025.11.1"},
  "decision_id":"6f6f-...",
  "path":"/httpapi/authz/allow",
  "input":{"action":"read","user":{"name":"bob","region":"eu-west-1"}},
  "result":true,
  "metrics":{"timer_rego_query_ns":380000}
}

Quick wins

• Emit policy version and bundle SHA in every decision log.
• Export counters: authz_allow_total, authz_deny_total, authz_error_total, and latency histograms.
• Alert on drift (runtime policy SHA ≠ main branch SHA).

Cedar in-process

• Wrap authorize to log principal/action/resource, decision, policy set version, and timing.
• Expose /metrics in your service; scrape with Prometheus.

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7) Gradual rollout & rollback guardrails

1. Shadow mode: evaluate decisions but don’t enforce; compare with app logic.
2. 10%→50%→100% traffic: turn on enforcement behind a feature flag.
3. Fail-closed for Denies; fail-open for Errors only during early phases, then converge to fail-closed.
4. Version pins: OPA bundles via digest; Cedar policy set versioned in lockfiles.
5. Instant rollback: keep N-1 policy bundle and feature flag to revert in minutes.

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End-to-end “real-time” quickstart (you can copy-paste)

1. Model your world (users/resources/actions).
2. Pick runtime: OPA sidecar (Envoy) or Cedar library.
3. Write policies (Rego/Cedar) with unit tests.
4. Wire app (REST/gRPC/GraphQL) using snippets above.
5. Add CI gates (Rego tests, Cedar eval).
6. Turn on logs & metrics; alert on drift.
7. Roll out gradually with clear rollback.

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Performance notes & tips

• Keep inputs small; pass only what the policy needs.
• Prefer in-process where tail latency is critical (Cedar), or sidecar when you want a common control point (OPA).
• Use partial evaluation (OPA) or pre-compiled policies (both) for hot paths.
• Cache subject → entitlements for read-heavy APIs; invalidate on role/attr change.

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Security hardening checklist

• Schema everything (Cedar schema, input contracts for OPA).
• Least privilege defaults; explicit denies for sensitive actions.
• Request-scoped context only; never pull secrets at decision time.
• Log redaction for PII/PHI.
• Timeouts (e.g., 10–30 ms sidecar) with safe fallbacks during staged rollout.

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Related services & next steps

• At Cyber Rely, we implement and validate policy-as-code pipelines end-to-end—see Web Application Penetration Testing.
• For audit-ready artifacts and sprint-friendly fixes, see Risk Assessment and Remediation Services.
• Keep learning on the Cyber Rely Blog.

Recent Cyber Rely posts you can reference inside your team:

• Prevent Sensitive Data Exposure in React.js — 7 Best Ways
• Best 5 Ways for CSRF Prevention in React.js
• Prevent Directory Traversal Attack in React.js — 7 Best Ways
• npm supply chain attack 2025: ‘Shai-Hulud’ CI fixes
  (These are great companions to convert findings into CI checks.)

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