A policy is a set of rules that governs the behavior of a software service. That policy could describe rate-limits, names of trusted servers, the clusters an application should be deployed to, permitted network routes, or accounts a user can withdraw money from.

Authorization is a special kind of policy that often dictates which people or machines can run which actions on which resources. Authorization is sometimes confused with Authentication: how people or machines prove they are who they say they are. Authorization and more generally policy often utilize the results of authentication (the username, user attributes, groups, claims), but makes decisions based on far more information than just who the user is. Generalizing away from authorization back to policy makes the distinction even clearer because some policy decisions have nothing to do with users, e.g. policy simply describes invariants that must hold in a software system (e.g. all binaries must come from a trusted source).

Today policy is often a hard-coded feature of the software service it actually governs. Open Policy Agent lets you decouple policy from that software service so that the people responsible for policy can read, write, analyze, version, distribute, and in general manage policy separate from the service itself. OPA also gives you a unified toolset to decouple policy from any software service you like, and to write context-aware policies using any context that you like. In short, OPA helps you decouple any policy using any context from any software system.

What is Policy?

All organizations have policies. Policies are essential to the long-term success of organizations because they encode important knowledge about how to comply with legal requirements, work within technical constraints, avoid repeating mistakes, and so on.

In their simplest form, policies can be applied manually based on rules that are written down or conventions that are unspoken but permeate an organization’s culture. Policies may also be enforced with application logic or statically configured at deploy time.

What is Policy Decoupling?

Software services should allow policies to be specified declaratively, updated at any time without recompiling or redeploying, and enforced automatically (which is especially valuable when decisions need to be made faster than humanly possible).

Decoupling policy helps you build such software services at scale, makes them adaptable to changing business requirements, improves the ability to discover violations and conflicts, increases the consistency of policy compliance, and mitigates the risk of human error. The policies you write can adapt more easily to the external environment–to factors that the developer could never have imagined at the time the software service was designed.

For example, a cloud computing service could answer questions such as:

• Can I add compute capacity?
• In what regions can I add compute capacity?
• Which instances are currently running in the wrong region?

What is OPA?

OPA is a lightweight general-purpose policy engine that can be co-located with your service. You can integrate OPA as a sidecar, host-level daemon, or library.

Services offload policy decisions to OPA by executing queries. OPA evaluates policies and data to produce query results (which are sent back to the client). Policies are written in a high-level declarative language and can be loaded dynamically into OPA remotely via APIs or through the local filesystem.

Why use OPA?

OPA is a full-featured policy engine that offloads policy decisions from your service. You can think of it as a concierge for your service who can answer detailed questions on behalf of your users to meet their specific needs.