This article is based on reporting originally published by TechTarget (techtarget.com) in September 2017, covering the launch of HashiCorp Sentinel.
DevOps Meets Policy: The Rise of DevSecOps
In heavily regulated enterprise environments, DevOps is increasingly evolving into DevSecOps — a practice that embeds security and compliance requirements directly into the software delivery pipeline rather than treating them as an afterthought. As automation spreads across infrastructure, the risk of codifying bad security decisions at scale grows alongside it. The answer, according to HashiCorp and a growing number of practitioners, is policy as code.
HashiCorp responded to this challenge by launching Sentinel, a policy-as-code framework integrated into the enterprise editions of its core DevOps tools. Sentinel allows teams to write, test, version-control, and enforce IT policies using a dedicated policy language — treating compliance rules the same way developers treat application code.
What Is HashiCorp Sentinel?
Sentinel is embedded directly into the enterprise versions of HashiCorp’s flagship tools:
- Terraform — Infrastructure as code, managing cloud and on-premises resources
- Vault — Secrets management and data encryption
- Consul — Service discovery and service mesh networking
- Nomad — Workload scheduling and container orchestration
Because Sentinel ships as a feature of existing tools rather than as a standalone product, it requires no separate installation or configuration overhead. Practitioners such as John Mitchell, infrastructure architect at SAP Ariba, highlighted this as a key advantage: “It doesn’t have an extra cost in terms of time to set up. It’s a natural part of other tools, not a bolt-on exterior piece.”
Policy as Code: What It Means in Practice
Traditional IT policy management — covering regulatory compliance, access controls, and security baselines — has historically been managed through documentation, manual approvals, and periodic audits. Policy as code changes this model by expressing those rules in a machine-readable language that can be:
- Tested — using automated test suites before deployment
- Version-controlled — tracked in Git alongside application and infrastructure code
- Reviewed — subjected to code review and pull request workflows
- Integrated into CI/CD pipelines — enforced automatically at every deployment stage
SAP Ariba, which had already adopted Terraform, Vault, Consul, Nomad, Packer, and Vagrant across its infrastructure, planned to layer Sentinel on top of all those tools. Mitchell described the philosophy as “policy everywhere”: “If policy isn’t everywhere — and that means enforceable everywhere, reproducible everywhere, and testable everywhere — then it’s not real.”
Active Enforcement vs. Passive Enforcement
One of Sentinel’s distinguishing design decisions is its active enforcement model. Rather than logging or alerting on policy violations after the fact, Sentinel sits in the data path and blocks non-compliant actions from completing. This contrasts with passive enforcement, which allows violations to occur but notifies administrators afterward.
For comparison, Chef InSpec — which also operates in the policy-as-code space — combines both approaches: it can actively enforce policies inside CI/CD pipelines and passively monitor deployed environments for drift. The two tools also target different infrastructure layers: Sentinel (via Terraform) addresses low-level server and container resources, while Chef InSpec focuses on the operating system and applications running on each server, making them complementary rather than mutually exclusive.
Trade-offs of Active Enforcement
Active enforcement introduces its own operational considerations:
- Sentinel itself must be highly available — because it sits in the data path, an outage could block legitimate infrastructure operations.
- It may add latency to continuous delivery workflows if not carefully tuned.
- Misconfiguration is a risk — overly restrictive policies could block valid deployments.
HashiCorp addressed these concerns through several design choices, according to Armon Dadgar, HashiCorp’s CTO at the time:
- Sentinel is deliberately constrained — it cannot run for extended time periods or consume large amounts of memory, limiting its attack surface.
- It inherits the high-availability features built into the underlying HashiCorp tools.
- A Sentinel Simulator tool allows teams to test and validate policies before deploying them, reducing misconfiguration risk.
Proponents of active enforcement argue that for companies subject to strict regulatory requirements, the ability to demonstrate that non-compliant states are impossible — not merely detected and remediated — is a meaningful compliance and audit advantage.
The Sentinel Plugin SDK: Extending Policy Beyond HashiCorp Tools
Alongside Sentinel itself, HashiCorp released a Sentinel Plugin SDK, enabling third-party tool vendors and internal platform teams to integrate Sentinel’s policy enforcement capabilities into non-HashiCorp tooling. This allows organizations to extend a consistent policy framework across their entire toolchain rather than managing separate policy systems for different products.
SAP Ariba planned to use the SDK to connect Sentinel to its internally developed tools and intended to advocate for its adoption among its external software vendors. As Mitchell put it: “HashiCorp has released the backbone, and now it’s up to us to help create the ecosystem and fill in the blanks.”
Why Policy as Code Matters for DevSecOps Teams
The policy-as-code market, while growing, was widely considered underdeveloped at the time of Sentinel’s launch. Zubin Irani, CEO of Agile consulting firm cPrime, framed the problem succinctly: “With automation, you can codify the same mistakes that people make in manual processes — you need a tool that can make sure the automation you build is doing the right things.”
For DevOps practitioners who are already comfortable with infrastructure as code and automated testing, the conceptual leap to policy as code is relatively small — but the operational impact is significant. Sentinel gives security and compliance teams a way to participate in the same code review and CI/CD workflows that development and operations teams already use, rather than operating as a separate gate that slows delivery.
Key Takeaways
- HashiCorp Sentinel embeds policy-as-code enforcement directly into the enterprise editions of Terraform, Vault, Consul, and Nomad.
- Sentinel policies can be tested, version-controlled, and integrated into CI/CD pipelines like any other code.
- Its active enforcement model blocks non-compliant actions at the point of execution, rather than alerting after the fact.
- The Sentinel Plugin SDK allows integration with non-HashiCorp tools, enabling organisation-wide policy coverage.
- Design constraints — limited execution time and memory usage — reduce Sentinel’s own attack surface.
- Early enterprise adopters included Barclays and SAP Ariba, both of which participated in the beta program.