Azure Policy definitions effect basics

Each policy definition in Azure Policy has a single effect in its policyRule. That effect determines what happens when the policy rule is evaluated to match. The effects behave differently if they are for a new resource, an updated resource, or an existing resource.

The following are the supported Azure Policy definition effects:

Interchanging effects

Sometimes multiple effects can be valid for a given policy definition. Parameters are often used to specify allowed effect values (allowedValues) so that a single definition can be more versatile during assignment. However, it's important to note that not all effects are interchangeable. Resource properties and logic in the policy rule can determine whether a certain effect is considered valid to the policy definition. For example, policy definitions with effect auditIfNotExists require other details in the policy rule that aren't required for policies with effect audit. The effects also behave differently. audit policies assess a resource's compliance based on its own properties, while auditIfNotExists policies assess a resource's compliance based on a child or extension resource's properties.

The following list is some general guidance around interchangeable effects:

  • audit, deny, and either modify or append are often interchangeable.
  • auditIfNotExists and deployIfNotExists are often interchangeable.
  • manual isn't interchangeable.
  • disabled is interchangeable with any effect.

Order of evaluation

Azure Policy's first evaluation is for requests to create or update a resource. Azure Policy creates a list of all assignments that apply to the resource and then evaluates the resource against each definition. For a Resource Manager mode, Azure Policy processes several of the effects before handing the request to the appropriate Resource Provider. This order prevents unnecessary processing by a Resource Provider when a resource doesn't meet the designed governance controls of Azure Policy. With a Resource Provider mode, the Resource Provider manages the evaluation and outcome and reports the results back to Azure Policy.

  • disabled is checked first to determine whether the policy rule should be evaluated.
  • append and modify are then evaluated. Since either could alter the request, a change made might prevent an audit or deny effect from triggering. These effects are only available with a Resource Manager mode.
  • deny is then evaluated. By evaluating deny before audit, double logging of an undesired resource is prevented.
  • audit is evaluated.
  • manual is evaluated.
  • auditIfNotExists is evaluated.
  • denyAction is evaluated last.

After the Resource Provider returns a success code on a Resource Manager mode request, auditIfNotExists and deployIfNotExists evaluate to determine whether more compliance logging or action is required.

PATCH requests that only modify tags related fields restricts policy evaluation to policies containing conditions that inspect tags related fields.

Layering policy definitions

Several assignments can affect a resource. These assignments might be at the same scope or at different scopes. Each of these assignments is also likely to have a different effect defined. The condition and effect for each policy is independently evaluated. For example:

  • Policy 1
    • Restricts resource location to westus
    • Assigned to subscription A
    • Deny effect
  • Policy 2
    • Restricts resource location to eastus
    • Assigned to resource group B in subscription A
    • Audit effect

This setup would result in the following outcome:

  • Any resource already in resource group B in eastus is compliant to policy 2 and non-compliant to policy 1
  • Any resource already in resource group B not in eastus is non-compliant to policy 2 and non-compliant to policy 1 if not in westus
  • Policy 1 denies any new resource in subscription A not in westus
  • Any new resource in subscription A and resource group B in westus is created and non-compliant on policy 2

If both policy 1 and policy 2 had effect of deny, the situation changes to:

  • Any resource already in resource group B not in eastus is non-compliant to policy 2
  • Any resource already in resource group B not in westus is non-compliant to policy 1
  • Policy 1 denies any new resource in subscription A not in westus
  • Any new resource in resource group B of subscription A is denied

Each assignment is individually evaluated. As such, there isn't an opportunity for a resource to slip through a gap from differences in scope. The net result of layering policy definitions is considered to be cumulative most restrictive. As an example, if both policy 1 and 2 had a deny effect, a resource would be blocked by the overlapping and conflicting policy definitions. If you still need the resource to be created in the target scope, review the exclusions on each assignment to validate the right policy assignments are affecting the right scopes.

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