OPA vs Cedar vs Zanzibar: 2025 Policy Engine Guide

In this analysis, we'll examine three leading policy engine approaches: Open Policy Agent (OPA), AWS Cedar, and Google Zanzibar. Each tool features a different authorization approach, spanning general-purpose policy enforcement to application-specific design to relationship-based modeling. We'll also explore the benefits of Authorization as a Service (AaaS) platforms (like Oso) that shift your permissions logic into its own layer, creating a single source of truth for access control.

Summary of What We'll Cover

Here's what we'll examine to help you evaluate authorization solutions:

Open Policy Agent (OPA): We’ll explore OPA’s general-purpose policy engine capabilities, Rego language benefits, and operational considerations
AWS Cedar: We’ll discuss AWS Cedar’s application-focused design, readability improvements, and ecosystem maturity
Google Zanzibar: We’ll discuss Google Zanzibar’s relationship-based modeling, graph architecture trade-offs, and implementation complexity
Authorization as a Service: We’ll explore how platforms like Oso Cloud eliminate infrastructure overhead while providing enterprise-grade performance

Let's dive into how these approaches handle real-world authorization challenges.

Open Policy Agent (OPA): General-Purpose Policy Engine

Open Policy Agent is an open-source policy engine for policy enforcement across your stack. With OPA, you use the declarative Rego language to implement policy-as-code across microservices, Kubernetes, and other infrastructure components.

OPA's Core Strengths

Policy-as-Code Implementation: OPA provides unified policy enforcement across the stack , enabling policies to be versioned, tested, and reviewed using standard development practices. This approach brings software engineering discipline to authorization logic.

Interoperability. OPA can process data from external systems in the data’s native format. Rego (OPA’s rules engine) can transform and inspect data as needed during evaluation to make authorization decisions. This design champions interoperability with third-party systems and makes OPA well-suited for machine-to-machine systems.

Open Source: For companies that prefer open source software, OPA is a strong fit with its Apache 2.0 license. OPA has fostered a strong open source community, with ample custom plugins available.

OPA's Implementation Challenges

Rego Learning Curve: The Rego language operates more like logical programming languages (Datalog, Prolog) than familiar imperative languages. This creates a significant learning curve for most development teams, requiring dedicated time investment to achieve proficiency.

A simple OPA policy looks like the following:

package authz

import rego.v1

allow if {
	input.method == "GET"
	input.path[0] == "users"
	input.user.role == "admin"
}

allow if {
	input.method == "GET"
	input.path[0] == "users"
	input.path[1] == input.user.id
}

Operational Complexity: OPA deployments require careful consideration of data distribution, policy updates, and synchronization across instances. The challenge of keeping policy agents up-to-date is especially hard in applications where each user interaction may affect access-control decisions [3].

Real-Time Data Integration: Especially for products with dynamic authorization data, developers need additional infrastructure to handle real-time updates, adding operational overhead.

Performance. OPA can struggle with scaling, especially for applications with heavy clustering. To address this, OPA has documented work-arounds, but all of these add operational overhead.

Commercial backing: In August 2025, Apple hired the maintainers of OPA with plans to sunset the enterprise offerings. This has significantly raised doubts in OPA’s future, as OPA’s maintainers might not be able to pursue OPA’s roadmap without an underpinning financial incentive.

When OPA Makes Sense

OPA works well for teams that:

Need policy enforcement across multiple infrastructure components (Kubernetes, service mesh, etc.)
Have existing expertise in logical programming or are willing to invest in Rego training
Can dedicate resources to managing policy distribution and synchronization
Require highly customized policy logic that benefits from Rego's expressiveness

AWS Cedar: Application-Level Authorization Focus

AWS Cedar represents a newer approach to policy-as-code, specifically designed for application-level authorization with emphasis on readability and safety-oriented design.

Cedar's Design Philosophy

Human-Readable Policies: Cedar was designed to allow defining fine-grained, role-based access control (RBAC) and attribute-based access control (ABAC) policies in a safe, fast, and auditable way [4]. The syntax aims to be more intuitive than Rego for application developers.

Here's a Cedar policy example:

permit (
    principal == PhotoApp::User::"meghan",
    action == PhotoApp::Action::"viewPhoto",
    resource == PhotoApp::Photo::"vacationPhoto.jpg"
);

permit (
    principal == PhotoApp::User::"stephie",
    action == PhotoApp::Action::"viewPhoto",
    resource
)
when { resource in PhotoApp::Account::"stephie" };

Application-Specific Focus: Unlike OPA's general-purpose design, Cedar targets application authorization scenarios directly, providing relevant primitives and patterns for common use cases.

Cedar's Current Limitations

AWS Ecosystem Bias: AWS maintains Cedar as open source software. While open source, it’s designed to work within an AWS architecture.

Ecosystem Maturity: Cedar has limited tooling and smaller community support compared to more established alternatives. This means fewer integrations, tutorials, and community resources compared to alternatives like OPA.

Operational Requirements: Despite being application-focused, Cedar still requires teams to handle deployment, scaling, and synchronization concerns. You're responsible for the infrastructure that runs your authorization decisions.

When Cedar Is Appropriate

Cedar fits well for teams that:

Operate primarily in AWS environments or are comfortable with AWS-aligned tooling
Have straightforward application authorization needs without complex relationship modeling
Can invest in building operational processes around a newer technology

Google Zanzibar: Graph-Based Relationship Modeling

Google's Zanzibar paper introduced a relationship-based authorization model that excels at managing complex hierarchies and permissions through graph structures. However, while hypothetically versatile, implementing a Zanzibar-like system is difficult for the following reasons:

Zanzibar's Relationship-First Approach

Hierarchical Permission Management: Zanzibar-inspired systems provide fine-grained authorization through relationship-based access control (ReBAC), designed for high-scale, low-latency access control [4]. By using a graph-based data structure, nested relationships could be captured alongside their inherited permissions.

The Zanzibar model represents permissions as relationship tuples such as the following:

document:budget.xlsx#viewer@user:10 such as the following, where Alice is user ID 10: 
folder:reports#owner@user:bob
folder:reports#viewer@group:managers#member

These tuples denote the asset, asset type, the relationship, the user, and the user’s groups.

Google-Scale Validation: Zanzibar powers authorization across hundreds of Google services, including Calendar, Drive, Maps, and YouTube, demonstrating its capability at massive scale. Zanzibar requires centralized enforcement, where a single policy engine handles authorization for the entire ecosystem.

Zanzibar's Implementation Complexity

Centralization Requirements: Zanzibar introduces system complexity by requiring centralization of all authorization data . This architectural decision works for Google's scale but creates significant infrastructure overhead for most organizations.

Performance Trade-offs: Because Zanzibar’s data is centralized, there are inherent performance overheads of keeping authorization strongly consistent across all nodes.

Implementation Expertise: Unlike straightforward policy approaches, Zanzibar requires understanding object hierarchies, relationship tuples, and graph traversal algorithms [5]. Teams need specialized knowledge in graph databases and distributed systems.

When Zanzibar Makes Sense

Zanzibar-inspired approaches work best for:

Teams with dedicated infrastructure engineering resources who can manage the complexity of synchronization of data and can invest in building and maintaining distributed graph infrastructure
Monolithic applications with complex hierarchical relationships
Use cases where relationship inheritance and graph traversal are core requirements

Authorization as a Service: The Modern Alternative

While traditional policy engines require significant infrastructure investment, authorization as a service platforms represent a different approach to solving authorization complexity. Platforms like Oso provide authorization as a service, making sophisticated access control accessible through simple API calls [6].

The Service-Based Advantage

Infrastructure Elimination: Authorization as a service provides idiomatic SDKs, inline policy tests, logging, regression testing, and debugging built-in, removing the operational burden of managing policy engines, synchronization, and scaling within your code.

Performance Without Complexity: Platforms like Oso deliver sub-10ms latency through geo-replicated environments without requiring teams to become experts in distributed systems optimization.

Unified Access Models: Modern platforms unify RBAC, ReBAC, and ABAC into a single framework, allowing teams to start simple and add complexity as needed .

Here's how authorization as a service simplifies implementation:

# Traditional approach - custom authorization logic
def can_edit_document(user, document):
    if user.role == "admin":
        return True
    elif user.id == document.owner_id:
        return True
    elif user.id in document.collaborators and document.allow_edit:
        return True
    # Handle team permissions, time-based access, etc.
    return False

# Authorization as a service approach
result = oso.authorize(user, "edit", document)
return result.allowed

Real-World Adoption

Engineering leaders are adopting the as-a-service model for authorization, similar to how they've embraced services for feature flags and authentication [6]. Companies like Oyster, Webflow, and Duolingo use an Authorization as a Service model (via Oso).

Teams report implementing custom authorization solutions in minutes rather than weeks, choosing service-based approaches for developer experience, performance, and flexibility .

Summary: Choosing Your Authorization Path

Each authorization approach offers distinct trade-offs:

Open Policy Agent provides maximum flexibility and works well for teams with policy engine expertise who need infrastructure-wide policy enforcement. The investment in learning Rego and managing OPA infrastructure pays off for complex, multi-service architectures.

AWS Cedar offers improved readability over OPA while maintaining policy-as-code benefits. It's suitable for teams comfortable with AWS ecosystem alignment and willing to invest in a newer technology.

Google Zanzibar excels at relationship-based authorization for large-scale applications but requires significant infrastructure investment and specialized expertise in graph-based systems.

Authorization as a Service platforms eliminate infrastructure complexity while providing enterprise-grade performance and flexibility. This approach is particularly well-suited for teams implementing application authorization models without the overhead of managing policy engines.

‍

Feature	OPA	AWS Cedar	Zanzibar	Authorization as a Service (Oso)
Strengths	OSS and customizable	Readable syntax	Ideal for complex hierarchies	Works with any access model and easy to implement
Challenges	Steep learning curve	AWS-centric	Complex implementation	Requires external vendor

‍

Access control is fundamental to application security and user experience. Investing time in choosing the right approach pays dividends throughout your application's lifecycle .

The key is matching the solution complexity to your actual requirements while considering your team's expertise and operational capacity. For many development teams, the operational simplicity and comprehensive feature set of authorization as a service platforms like Oso Cloud provide the best balance of capability and practicality.

Ready to explore how authorization as a service can simplify your implementation while providing enterprise-grade capabilities? Learn more about Oso Cloud and see how modern authorization can accelerate your development without the infrastructure overhead.