HashiCorp Sentinel

HashiCorp Sentinel manages enforcing fine-grained, logic-based policies across various infrastructure management tools, ensuring compliance with business and regulatory requirements. Here’s a technical overview of its architecture and operational considerations:

Sentinel is an embeddable policy-as-code framework integrated into HashiCorp's Enterprise products, including Terraform, Vault, Consul, and Nomad. It allows policy decisions to be made using real programming constructs, extending beyond the limitations of traditional ACL systems. Policies are defined using the Sentinel language, which can source external information to make holistic decisions, such as preventing Terraform executions during failing Consul health checks.

The technical architecture of Sentinel involves embedding the policy engine directly into the data path of these tools, enabling active rejection of violating behavior rather than passive detection. This is achieved through multiple enforcement levels: advisory, soft-mandatory, and hard-mandatory. Advisory mode logs warnings without preventing actions, soft-mandatory requires operator override, and hard-mandatory does not allow any override, ensuring strict compliance.

Operationally, Sentinel policies are configured at the organization level and can be applied to all workspaces within that organization. Policies are entered via the API or web UI and can soon support VCS integration. The enforcement mode determines the action taken upon policy failure, providing flexibility based on the severity of the policy.

Key operational considerations include the need for careful policy design to avoid overly restrictive or overly permissive rules. For example, policies can be written to ensure AWS instances have tags, restrict security group settings, or limit resource provisioning to specific regions or business hours. Additionally, Sentinel's integration with external systems allows for dynamic policy decisions, but this also introduces dependencies that must be managed.

From a technical detail perspective, Sentinel policies are written in a declarative language that supports conditional logic and external data sourcing. For instance, a policy might check if a Consul key-value update is in the proper format or if it is being updated during business hours. This fine-grained control ensures that infrastructure changes align with organizational policies, reducing the risk of non-compliant configurations.