Community post by Kevin Conner, Chief Engineer, Getup Cloud and co-author of Kubernetes in Action 2nd Edition
As Kubernetes deployments grow in scale and complexity, policy enforcement becomes a critical aspect of maintaining secure and reliable operations. The Common Expression Language (CEL) has emerged as a powerful solution for simplifying Kubernetes policy management. CEL’s integration into Kubernetes enables the creation of sophisticated, efficient policies that are both human-readable and resource-friendly.
CEL is specifically designed to evaluate expressions quickly and predictably, making it ideal for environments where performance and resource efficiency are paramount. Unlike external tools that rely on network calls, CEL is embedded directly into the Kubernetes API server. This eliminates latency issues and enhances reliability while avoiding the unpredictability of Turing-complete languages. By eliminating constructs like loops and recursion, CEL ensures expressions execute in linear time, making it a robust choice for performance-critical applications.
The applications of CEL within Kubernetes are extensive. Validating admission policies, for instance, allow the enforcement of declarative rules that reject or warn about invalid resource requests directly within the API server. This integration removes the need for external webhooks, streamlining operations and improving policy enforcement reliability. The recent introduction of mutating admission policies in Kubernetes v1.32 extends CEL’s capabilities further by enabling resource mutations through server-side apply or JSON Patch configurations, offering a comprehensive toolkit for cluster compliance and operational consistency.
CEL Playground, an open-source tool designed for experimenting with CEL expressions, provides an invaluable resource for policy testing and validation. The browser-based application runs entirely locally, allowing users to test CEL expressions and integration points such as admission policies without risking cluster stability. Its intuitive interface enables users to input expressions, provide sample data, and view results in real time, including cost evaluations. This environment simplifies the adoption of CEL, making it accessible for both experienced professionals and those new to Kubernetes policy enforcement.
Practical use cases demonstrate CEL’s flexibility and utility. Policies can be crafted to prevent the deployment of privileged containers, enforce minimum replica counts in deployments, and manage Pod Disruption Budgets (PDBs) to avoid conflicts. Using CEL variables and operators, such as those that support optional types, simplifies expressions, reduces redundancy, and results in more maintainable policies. Parameterization further enhances flexibility, allowing policies to dynamically adjust values without requiring redeployment. This capability is particularly effective for tailoring rules to specific namespaces or environments.
Performance considerations are integral to CEL’s adoption in Kubernetes. The introduction of cost estimators in Kubernetes v1.32 ensures that CEL expressions adhere to predefined resource budgets, preventing overloads in the API server. This safeguard maintains cluster stability while ensuring the efficiency of CEL-based policies, making it a reliable framework for high-performance environments.
CEL transforms Kubernetes policy management by enabling declarative, embedded rules that are easier to create, manage, and enforce. With its robust syntax, extensibility, and resource efficiency, CEL provides a superior alternative to traditional webhook-based solutions. Tools like CEL Playground further enhance its utility, offering a safe and accessible platform for experimentation and development.
By adopting CEL, Kubernetes administrators and developers can enhance security, improve operational standards, and streamline cluster management. CEL’s integration into the Kubernetes API server represents a significant step forward in policy enforcement, offering a modern, efficient approach to managing complex cloud-native environments.
To explore its capabilities and start crafting effective Kubernetes policies, visit CEL Playground.
This article is based on a talk given at KubeCon + CloudNativeCon North America 2024 by Kevin Conner (Getup Cloud) and Anish Ramasekar (Microsoft). You can watch the full talk on the KubeCon NA 2024 video archive here.