Observing Changes to CRDs with client-go

Hey! It’s great to see you’re diving into Kubernetes and working with Custom Resource Definitions (CRDs). Observing changes to CRDs can be really powerful for automation. I’ve dealt with this before, so here’s how you can set up a watch on CRDs using client-go.

Sample Code to Watch CRDs

package main

import (
    "context"
    "fmt"
    "log"
    "time"

    v1 "k8s.io/api/core/v1"
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/apimachinery/pkg/watch"
    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/tools/clientcmd"
)

func main() {
    // Load kubeconfig
    kubeconfig := "/path/to/your/kubeconfig"
    config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
    if err != nil {
        log.Fatalf("Failed to build kubeconfig: %v", err)
    }

    // Create a clientset
    clientset, err := kubernetes.NewForConfig(config)
    if err != nil {
        log.Fatalf("Failed to create clientset: %v", err)
    }

    // Start watching CRD changes
    watchCRDs(clientset)
}

func watchCRDs(clientset *kubernetes.Clientset) {
    watchInterface, err := clientset.CoreV1().Pods("default").Watch(context.TODO(), metav1.ListOptions{})
    if err != nil {
        log.Fatalf("Error watching CRDs: %v", err)
    }
    
    defer watchInterface.Stop()
    
    fmt.Println("Watching for changes to CRDs...")
    for event := range watchInterface.ResultChan() {
        switch event.Type {
        case watch.Added:
            fmt.Printf("Added: %v\n", event.Object)
        case watch.Modified:
            fmt.Printf("Modified: %v\n", event.Object)
        case watch.Deleted:
            fmt.Printf("Deleted: %v\n", event.Object)
        }
    }
}

Best Practices

• Handle Resource Version: Ensure that you handle the resource version properly to avoid stale data.
• Graceful Shutdown: Make sure to implement a way to gracefully handle shutdowns and stop your watches cleanly.
• Implement Exponential Backoff: In case of errors, consider implementing an exponential backoff strategy to avoid overwhelming the API server.

Common Pitfalls

• Not Checking for Errors: Always check for errors returned from the watch calls.
• Ignoring API Rate Limits: Be mindful of the API server’s rate limits; making too many requests can lead to throttling.

Feel free to ask if you have more questions or need further assistance. Happy coding with Kubernetes!

To observe changes to your Custom Resource Definitions (CRDs) in a Kubernetes cluster using client-go, you will primarily use the `Watch` method that is part of the client-go API. First, ensure you have a properly configured Kubernetes client set up. You can use the `dynamic` client interface to manage CRDs easily. Here’s a snippet of code that initializes a watch on your CRD:

import (
    "context"
    "fmt"
    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/tools/clientcmd"
    "k8s.io/apimachinery/pkg/runtime/schema"
    "k8s.io/apimachinery/pkg/labels"
    "k8s.io/apimachinery/pkg/watch"
)

func main() {
    config, err := clientcmd.BuildConfigFromFlags("", "/path/to/kubeconfig")
    if err != nil {
        panic(err)
    }
    clientset, err := kubernetes.NewForConfig(config)
    if err != nil {
        panic(err)
    }

    gvr := schema.GroupVersionResource{Group: "mygroup.example.com", Version: "v1", Resource: "myresources"}
    watcher, err := clientset.Resource(gvr).Watch(context.Background(), metav1.ListOptions{
        LabelSelector: labels.SelectorFromSet(labels.Set{"key": "value"}).String(),
    })
    if err != nil {
        panic(err)
    }
    for event := range watcher.ResultChan() {
        fmt.Printf("Event Type: %s, Object: %v\n", event.Type, event.Object)
    }
}

In this example, replace `”mygroup.example.com”`, `”v1″`, and `”myresources”` with your specific CRD’s group, version, and resource name. A few best practices to keep in mind include managing the lifecycle of your watchers properly—make sure to stop them when they are no longer needed to avoid memory leaks—and handle reconnects gracefully in case the watch is interrupted. Additionally, be cautious about the load on your cluster; watching too many resources without proper filters can create an unnecessary burden on the API server. Happy coding!