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Kubewarden is a Kubernetes policy engine that uses policies written using WebAssembly.

The Kubewarden stack is made by the following components:

  • Kubewarden Custom Resources: these are Kubernetes Custom Resources that simplify the process of managing policies.

  • kubewarden-controller: this is a Kubernetes controller that reconciles Kubewarden's Custom Resources. This component creates parts of the Kubewarden stack and, most important of all, translates Kubewarden's concepts into native Kubernetes directives.

  • Kubewarden policies: these are WebAssembly modules that hold the validation or mutation logic. These are covered in depth inside of this chapter.

  • policy-server: this component receives the requests to be validated. It does that by executing Kubewarden's policies.

At the bottom of the stack, Kubewarden's integrates with Kubernetes using the concept of Dynamic Admission Control. In particular, Kubewarden operates as a Kubernetes Admission Webhook. policy-server is the actual Webhook endpoint that is reached by Kubernetes API server to validate relevant requests.

Kubernetes is made aware of the existence of Kubewarden's Webhook endpoints by kubewarden-controller. This is done by registering either a MutatingWebhookConfiguration or a ValidatingWebhookConfiguration object.

This diagram shows the full architecture overview of a cluster running the Kubewarden stack:

Full architecture

Journey of a Kubewarden policy

The architecture diagram from above can be intimidating at first, this section explains it step by step.

Default Policy Server

On a fresh new cluster, the Kubewarden components defined are its Custom Resource Definitions, the kubewarden-controller Deployment and a PolicyServer Custom Resource named default.

Defining the first ClusterAdmissionPolicy resource

kubewarden-controller notices the default PolicyServer resource and, as a result of that, it creates a Deployment of the policy-server component.

As stated above, Kubewarden works as a Kubernetes Admission Webhook. Kubernetes dictates that all the Webhook endpoints must be secured with TLS. kubewarden-controller takes care of setting up this secure communication by doing these steps:

  1. Generate a self-signed Certificate Authority
  2. Use this CA to generate a TLS certificate and a TLS key for the policy-server Service.

All these objects are stored into Kubernetes as Secret resources.

Finally, kubewarden-controller will create the policy-server Deployment and a Kubernetes ClusterIP Service to expose it inside of the cluster network.

Defining the first policy

This chart shows what happens when the first policy bounded to the default policy-server is defined inside of the cluster:

Defining the first ClusterAdmissionPolicy resource

kubewarden-controller notices the new ClusterAdmissionPolicy resource and, as a result of that, it finds the bounded PolicyServer and reconciles it.

Reconciliation of policy-server

When a ClusterAdmissionPolicy is created, modified or deleted a reconciliation loop for the PolicyServer that owns the policy is triggered inside the kubewarden-controller. In this reconciliation loop, a ConfigMap with all the polices bounded to the PolicyServer is created. Then the a Deployment rollout of the interested policy-server is started. As a result of that, the new policy-server instance will be started with the updated configuration.

At start time, policy-server reads its configuration and downloads all the Kubewarden policies. Policies can be downloaded from remote endpoints like HTTP(s) servers and container registries.

Policies' behaviour can be tuned by users via policy-specific configuration parameters. Once all the policies are downloaded, policy-server will ensure the policy settings provided by the user are valid.

policy-server performs the validation of policies's settings by invoking the validate_setting function exposed by each policy. This topic is covered more in depth inside of this section of the documentation.

policy-server will exit with an error if one or more policies received wrong configuration parameters from the end user.

If all the policies are properly configured, policy-server will spawn a pool of worker threads to evaluate incoming requests using the Kubewarden policies specified by the user.

Finally, policy-server will start a HTTPS server that listens to incoming validation requests. The web server is secured using the TLS key and certificate that have been previously created by kubewarden-controller.

Each policy is exposed by the web server via a dedicated path that follows this naming convention: /validate/<policy ID>.

This is how the cluster looks like once the initialization of policy-server is completed:

policy-server initialized

Making Kubernetes aware of the policy

The policy-server Pods have a Readiness Probe, kubewarden-controller relies on that to know when the policy-server Deployment is ready to evaluate admission reviews.

Once the policy-server Deployment is marked as Ready, kubewarden-controller will make the Kubernetes API server aware of the new policy by creating either a MutatingWebhookConfiguration or a ValidatingWebhookConfiguration object.

Each policy has its dedicated MutatingWebhookConfiguration/ValidatingWebhookConfiguration which points to the Webhook endpoint served by policy-server. The endpoint is reachable by the /validate/<policy ID> URL mentioned before.

Kubernetes Webhook endpoint configuration

Policy in action

Now that all the plumbing has been done, Kubernetes will start sending the relevant Admission Review requests to the right policy-server endpoint.

Policy in action

policy-server receives the Admission Request object and, based on the endpoint that received the request, uses the right policy to evaluate it.

Each policy is evaluated inside of its own dedicated WebAssembly sandbox. The communication between policy-server (the "host") and the WebAssembly policy (the "guest") is done using the waPC communication protocol. This is covered in depth inside of this section of the documentation.

How multiple policy servers and policies are handled

A cluster can have multiple policy servers and Kubewarden policies defined.

Benefits of having multiple policy servers:

  • Noisy Namespaces/Tenants generating lots of policy evaluations can be isolated from the rest of the cluster and do not affect other users.
  • Mission critical policies can be run inside of a Policy Server "pool", making your whole infrastructure more resilient.

Each policy-server is defined via its own PolicyServer resource and each policy is defined via its own ClusterAdmissionPolicy resource.

This leads back to the initial diagram:

Full architecture

A ClusterAdmissionPolicy is bounded to a PolicyServer. ClusterAdmissionPolicies that don't specify any PolicyServer will be bounded to the PolicyServer named default. If a ClusterAdmissionPolicy references a PolicyServer that doesn't exist, it will be in an unschedulable state.

Each policy-server defines multiple validation endpoints, one per policy defined inside of its configuration file. It's also possible to load the same policy multiple times, just with different configuration parameters.

The Kubernetes API server is made aware of these policy via the ValidatingWebhookConfiguration and MutatingWebhookConfiguration resources that are kept in sync by kubewarden-controller.

Finally, the incoming admission requests are then dispatched by the Kubernetes API server to the right validation endpoint exposed by policy-server.