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Freedom251 › Blog › CI/CD

Open Policy Agent for Continuous Delivery: Ship Fast Without Unsafe Changes

By Freedom251 Team · · 10 min read

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Open Policy Agent (OPA) lets a delivery system ask a separate policy engine whether a proposed action is acceptable. The caller supplies structured data—such as a deployment manifest, infrastructure plan, or authorization request—OPA evaluates versioned Rego rules, and the caller enforces the returned decision. That separation gives teams a consistent place to check policy in CI, at Kubernetes admission, and in running services, but it does not make releases safe by itself.

Reliable continuous delivery with OPA comes from treating policy as code: define the decision and its inputs, test known-good and known-bad fixtures, enforce at the points that matter, distribute policy deliberately, and decide what happens when the policy service or its latest bundle is unavailable.

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What OPA changes in a continuous-delivery pipeline

OPA is a general-purpose, open-source policy decision engine. It separates decision-making from enforcement. A caller sends JSON or another structured document as input; OPA evaluates Rego policy; the caller receives a result and allows, rejects, or otherwise handles the operation. The same model can serve a build pipeline, an admission webhook, an API gateway, or an application.

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That boundary is important operationally. OPA does not deploy software, approve a change, mutate a cluster, or roll back a release. Your pipeline or service performs those actions after interpreting the decision. OPA documentation lists CI/CD pipelines among its uses, but it does not prescribe a universal pipeline design or promise a delivery-speed improvement.

A useful mental model is:

  • Input: a deliberately defined document, such as a Kubernetes object, Terraform plan, image metadata, requester identity, or environment name.
  • Policy: Rego rules that query that document and produce a decision or collection of violations.
  • Enforcement: the integrating system decides whether to continue, reject, warn, mutate, or request an exception.
  • Operations: people own policy reviews, distribution, availability, observability, and emergency procedures.

Start with a precise delivery decision

Before writing Rego, state the decision in terms a machine can evaluate. “Keep production secure” is not an input contract. “A production deployment must identify an image by digest, declare resource requests, and come from an approved registry” is testable.

Define the input contract

Document the fields the policy expects and their types. An example deployment check might accept:

  • input.environment: a string such as staging or production.
  • input.object: the proposed Kubernetes object.
  • input.image_metadata: registry, digest, signature status, and build provenance.
  • input.actor: the pipeline identity and change ticket.

Keep optional fields explicit. A missing field should result in a deliberate violation or a documented default, not an accidental pass caused by an empty value. Version the contract alongside the policy so a producer and consumer can change it intentionally.

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Return decisions that callers can use

Teams commonly expose a boolean allow decision plus structured violations. A violation should identify the object, field, rule, and remediation. Returning data rather than only a yes/no value lets a pull request check show every problem at once and lets an admission webhook produce a useful message.

Author and test Rego before delivery

Rego is declarative and designed to query structured documents, including API requests and infrastructure-as-code data. Keep policy files and test fixtures in version control. Review them like application code, with ownership, change history, and a required test run.

Example policy

The following is an illustrative pattern, not a claim that it has been executed here. It rejects a production workload that uses a mutable image tag or omits CPU and memory requests:

package delivery

default allow := false

allow if {
  input.environment != "production"
}

allow if {
  input.environment == "production"
  count(violations) == 0
}

violations contains {"rule": "image-must-use-digest", "message": msg} if {
  input.environment == "production"
  some container in input.object.spec.template.spec.containers
  not contains(container.image, "@sha256:")
  msg := sprintf("container %q is not pinned by digest", [container.name])
}

violations contains {"rule": "resources-required", "message": msg} if {
  input.environment == "production"
  some container in input.object.spec.template.spec.containers
  not container.resources.requests.cpu
  msg := sprintf("container %q has no CPU request", [container.name])
}

The exact shape of a policy is a design choice. Some teams return deny messages, others return an object containing allow, warnings, and violations. Choose one contract and make every integration consume it consistently.

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Run representative fixtures in CI

OPA’s command-line interface can evaluate a policy against input files. A simple pipeline step can evaluate a known-good fixture and a known-bad fixture, then fail when the result is not what the test expects:

opa eval --format pretty 
  --data policy/ 
  --input testdata/production-good.json 
  'data.delivery.allow'

opa eval --format pretty 
  --data policy/ 
  --input testdata/production-bad.json 
  'data.delivery.violations'

Use the CLI or an API integration to run these checks in the workflow. Add cases for missing fields, malformed objects, exemptions, namespace boundaries, and policy-version changes. The OPA documentation describes the language and CLI capabilities, but does not prescribe a particular CI vendor or complete test strategy; your team must define coverage and review requirements.

Place enforcement where it catches the right failure

There is no mandatory sequence. Multiple checkpoints are often appropriate because each sees different information and has a different failure boundary.

CI checks: early, cheap feedback

Evaluate manifests, infrastructure plans, image metadata, and change descriptors before merge or deployment. CI is the best place to give developers line-level remediation and to stop an obviously invalid artifact before it reaches a cluster. It cannot, however, see every runtime fact, and a later change or manual action may bypass it.

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Kubernetes admission: cluster-side guardrails

Kubernetes sends an AdmissionReview request to an admission webhook. The policy integration can allow, reject, or mutate the object before it is persisted. OPA’s Kubernetes documentation recommends Gatekeeper as the specialized integration. Gatekeeper supplies a Kubernetes-native constraint and template workflow, including an audit-oriented operating model. Plain OPA integrations, including approaches built around kube-mgmt, can offer more direct control but leave more distribution, schema, and lifecycle work to the platform team.

Admission policy should be deterministic and fast enough for the API server’s request path. Decide which rules are hard rejects, which produce warnings, and which objects or namespaces are exempt. Test updates and deletes as well as creates; a rule that only understands one operation can create an unintended gap.

Runtime authorization: decisions close to the request

An application or gateway can call OPA when a user requests an action. This is useful for rules that depend on current identity, resource ownership, or request context rather than deployment metadata. Runtime calls introduce a request-time availability and latency dependency, so define caching, timeouts, and failure behavior before enabling enforcement.

Choose an integration pattern deliberately

Pattern Where evaluation runs Strengths Trade-offs and ownership
Sidecar Alongside the workload Local network path, isolated policy process, and independent policy updates More containers and lifecycle work; monitor each sidecar and its policy revision
REST service A shared OPA endpoint Works across languages and centralizes serving Network latency and an availability boundary; capacity, authentication, and scaling are yours
Go API Embedded in a Go program No separate network hop and direct integration with Go code Couples policy runtime upgrades to the application and limits the choice of implementation language
WebAssembly Inside a compatible host Portable policy module for supported runtimes Host integration, input preparation, and observability remain application responsibilities
Kubernetes deployment Admission or cluster services Fits cluster control points and Kubernetes object workflows Webhook availability, certificates, upgrades, and failure policy require platform ownership

OPA documentation describes these integration choices but does not publish comparative benchmark results. Select based on decision latency, network fault tolerance, language fit, deployment complexity, and who will operate the policy runtime. Avoid choosing a remote service merely because it appears architecturally central; a local evaluation may be safer for a critical request path, while a shared service may simplify updates for many clients.

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Distribute policy with bundles and discovery

Local policy files are easy to understand for a small deployment. As instances multiply, manually copying files creates drift. OPA supports bundles for packaging policy and related data, and discovery mechanisms for configuring where instances obtain policy and configuration. Status reporting can show whether a bundle was downloaded and activated.

OPA’s management APIs provide mechanisms, not a ready-made management product. As the documentation states, “OPA does not provide a control plane service out-of-the-box.” Operate the bundle or configuration endpoint yourself, or adopt an external management approach that meets your security and availability requirements.

Track revision and activation

  • Assign every policy build a version or revision and include it in deployment metadata.
  • Record which revision each OPA instance has activated, not merely which revision was published.
  • Roll out a new revision to a controlled population, compare decisions, then expand adoption.
  • Keep the previous known-good bundle available for rollback.
  • Define whether an instance may continue using its last active bundle when an update endpoint is unavailable.

These are operational recommendations, not automated OPA behavior. A bundle mechanism will not decide whether your organization should fail closed, fail open, or pause a deployment.

Design failure behavior before enabling enforcement

Policy availability is part of the delivery design. A CI job may be able to stop safely when OPA is unreachable; a production authorization request may require a different response; a cluster admission webhook has its own Kubernetes failure-policy setting and operational consequences.

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For each integration, document:

  • Timeout and retry limits.
  • Whether the caller uses a cached decision or last active policy.
  • Whether an unavailable policy service blocks, allows, or quarantines the operation.
  • Who can grant a time-limited emergency exception and how it is recorded.
  • How policy and caller versions remain compatible during rollback.

Fail-open can preserve availability while allowing a prohibited action. Fail-closed can protect a control while preventing legitimate work during an outage. Neither is universally correct; the resource, threat model, and recovery process determine the choice.

Observe decisions without leaking sensitive input

OPA supports status reporting and decision logs. Logs can include request details and policy inputs, which may contain credentials, personal data, source code, or infrastructure secrets. Treat logging as a data-handling decision, not a default debugging switch.

Choose the destination and retention

Console output is useful during development and isolated troubleshooting. Remote decision logs support fleet-wide analysis, but require an endpoint, transport security, access controls, retention limits, and a plan for delivery failures. Verify the actual configuration rather than assuming a default destination.

Mask before upload

OPA supports masking or modifying data before remote upload. Identify sensitive paths in the input, remove values that are not needed for diagnosis, and test the redaction rules with realistic fixtures. Understand buffering and retry behavior: a log that is queued locally may be delivered later, duplicated after a retry, or lost when a process terminates. Document those limits so incident responders do not treat logs as a perfect audit record.

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Roll out policy changes safely

  1. Write the decision contract. Define inputs, output shape, owners, and the enforcement action for each result.
  2. Build fixtures. Include compliant, non-compliant, boundary, malformed, and exemption cases. Keep them versioned with the policy.
  3. Run in observation mode. Collect violations without blocking where the integration allows it; measure false positives and remediation effort rather than assuming every match is correct.
  4. Fix producers and documentation. Give application teams an actionable path to satisfy the rule, and document approved exceptions.
  5. Promote deliberately. Enable blocking for a limited environment, namespace, or service population before expanding it.
  6. Monitor activation and outcomes. Correlate policy revision, caller, decision, latency, errors, and override events.
  7. Keep rollback ready. Revert the policy bundle or enforcement setting independently of the application release when possible.

OPA evaluates the rules you provide; it does not canary a policy rollout for you. The controlled-adoption process is an engineering practice built around OPA’s evaluation and distribution features.

Common failure modes and fixes

“The rule passed, but the deployment was still unsafe”

Check whether the policy received the fields it relied on and whether a later path bypassed the check. Add the missing context to the input contract, enforce at the cluster or runtime boundary, and verify that every deployment route invokes the same decision.

“A harmless change is blocked”

Inspect the fixture and the rule’s assumptions about absent fields, defaults, and object kinds. Return structured remediation, add a narrowly scoped exception with an owner and expiry, and add a regression test before changing the rule globally.

“Different instances make different decisions”

Compare active bundle revisions, data files, configuration, and clock-sensitive inputs. Centralize distribution where appropriate, expose status, and alert when an instance falls behind the approved revision.

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“The policy service outage stops everything”

Apply the failure decision documented for that integration. If the behavior was never specified, pause rollout, restore the last known-good policy or local cache where safe, and resolve the ownership gap before adding more enforcement points.

A practical readiness checklist

  • Policy input and output schemas are documented and versioned.
  • Rego rules have known-good, known-bad, boundary, and malformed fixtures.
  • CI evaluates policies before merge or deployment.
  • Kubernetes controls use an explicitly chosen integration and tested webhook failure behavior.
  • Runtime calls have timeout, retry, cache, and unavailable-service decisions.
  • Bundle revisions, activation status, and rollback procedures are visible.
  • Decision logs are redacted, access-controlled, retained appropriately, and monitored for delivery failures.
  • Policy owners, approvers, exception authorities, and on-call responsibilities are named.
  • No claim of safety or speed depends on OPA alone; enforcement and recovery remain with the surrounding systems.

The operational boundary to remember

OPA makes policy decisions reusable across delivery and runtime systems. Rego makes those decisions reviewable and testable, while bundles, discovery, status, and decision logs help operate them at scale. The hard engineering work remains: selecting trustworthy inputs, writing rules that match real risk, integrating at every relevant boundary, protecting sensitive telemetry, and choosing an explicit response to outages and exceptions. With those responsibilities assigned, teams can move quickly without confusing a policy engine with a guarantee that nothing will break.

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CI/CDcontinuous deliveryKubernetesOPAOpen Policy Agentpolicy as codeRegoSRE

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