How can I dynamically fetch the NetworkPolicy JSON Schema from a live Kubernetes cluster at runtime?

You can dynamically fetch the JSON Schema for the NetworkPolicy resource by querying the Kubernetes API server’s OpenAPI endpoint. The API server exposes an OpenAPI (formerly Swagger) specification that includes the JSON Schema definitions for all registered resources, including NetworkPolicy. This endpoint is typically available at:

/openapi/v2

or

/openapi

depending on the Kubernetes version.

How to Retrieve the Schema

1. Query the OpenAPI Endpoint:
   Make an HTTP GET request to the /openapi/v2 (or /openapi) path on your API server. For example, using curl:

   BASHCopied
   curl -k https://<api-server>:<port>/openapi/v2 --header "Authorization: Bearer <your-token>"

2. Filter for NetworkPolicy:
   The returned OpenAPI spec is a large JSON document. You need to locate the schema definitions for the NetworkPolicy resource. Look for the definition keyed by something like the fully qualified name of NetworkPolicy, for example: io.k8s.api.networking.v1.NetworkPolicy. The exact key might vary slightly based on the Kubernetes version.

3. Extract the Schema:
   Parse the JSON and extract the portion corresponding to the NetworkPolicy schema. For example, in a script or application, you can use a JSON parsing library to extract the definition you need.

Using Client Libraries

If you’re using a client library such as client-go in Go, the discovery client provides methods to retrieve the OpenAPI spec. For instance:

GOCopied
import (
    "context"
    "fmt"
    "log"

    "k8s.io/client-go/discovery"
    "k8s.io/client-go/rest"
)

func main() {
    config, err := rest.InClusterConfig()
    if err != nil {
        log.Fatal(err)
    }

    // Create a discovery client.
    dc, err := discovery.NewDiscoveryClientForConfig(config)
    if err != nil {
        log.Fatal(err)
    }

    // Fetch the OpenAPI schema.
    openapi, err := dc.OpenAPISchema()
    if err != nil {
        log.Fatal(err)
    }

    // Now, parse openapi to locate the "io.k8s.api.networking.v1.NetworkPolicy" definition.
    // You can use openapi.Raw (a json.RawMessage) and unmarshal it.
    fmt.Println(string(openapi.Raw))
}

This approach allows your application to dynamically retrieve the schema from whichever cluster it runs on, ensuring that you always have the most up-to-date definitions as provided by the cluster’s API server.

Notes

• The exact endpoint and structure of the returned OpenAPI JSON might vary with different Kubernetes versions.
• Ensure that your application has permissions (RBAC) to access the /openapi endpoint on the cluster.
• Once you have the full schema, you only need to extract the portion for NetworkPolicy, which is typically nested under a key corresponding to its API group, version, and kind.

Using this method, you can programmatically retrieve and work with the live JSON Schema for the NetworkPolicy resource on multiple clusters with potentially different versions.