Can you recommend a policy engine for PII redaction in a multi-tenant SaaS app?

Yes — for a multi-tenant SaaS app, I’d recommend Open Policy Agent (OPA) as the policy engine, with Rego policies to drive PII redaction decisions.

Why OPA is a good fit

• Flexible policy logic: You can express “redact if tenant X, role Y, field type Z, region A” style rules cleanly.
• Works well in distributed systems: Can be embedded, run as a sidecar, or used as a central decision service.
• Good for multi-tenancy: Policies can be parameterized by tenant, plan, data classification, user role, and request context.
• Auditability: Policies are versioned and testable, which helps with compliance.
• Extensible: Can be used for both authorization and content filtering/redaction.

Typical architecture

1. Classify data first
   Tag fields as pii.email, pii.phone, phi, financial, etc.
2. Send request context to OPA
   Example inputs: tenant ID, user role, purpose, locale, downstream destination, data classification.
3. OPA returns redaction rules
   e.g. mask, hash, remove, allow, partial_mask.
4. Apply redaction in your app or proxy layer
   Use the decision from OPA to transform the response.

Example use cases it handles well

• Show full PII only to tenant admins
• Redact SSNs unless purpose = billing
• Mask email addresses in logs but not in the customer UI
• Apply stricter rules for EU tenants
• Different redaction by tenant-specific contract or data processing agreement

When to consider alternatives

If your main need is just field-level masking in APIs, you may also look at:

• AWS Cedar if you want a more authorization-focused policy language and are already in the AWS ecosystem
• Styra DAS if you want a managed OPA platform
• Envoy + external auth / WASM filters if you want redaction close to the edge

My practical recommendation

For most SaaS teams:

• OPA for policy decisions
• A dedicated redaction library/service for applying the actual masking
• Structured data classification upstream

That combination gives you policy flexibility without overloading the policy engine with transformation logic.

If you want, I can also suggest:

1. a reference architecture,
2. a sample Rego policy, or
3. a comparison of OPA vs Cedar vs commercial DLP tools.