How to Use a Single Informer to Monitor Multiple CRD Changes
Monitoring multiple Custom Resource Definitions (CRDs) for changes can be efficiently handled using a single Kubernetes informer. This approach avoids the overhead of creating separate informers for each CRD, leading to improved resource utilization and simplified management. This guide details how to achieve this using the Kubernetes Go client library.
Understanding Kubernetes Informers
Kubernetes informers provide a mechanism to efficiently watch for changes in Kubernetes resources. They cache resource data, providing a consistent view of the cluster state and notifying users of updates via event channels. While you could create an informer for each CRD, using a single informer with a more sophisticated resource selection mechanism is far more efficient.
Utilizing ListWatch with FieldSelectors
The key to monitoring multiple CRDs with a single informer lies in using a ListWatch object with a well-defined fieldSelector. The fieldSelector allows you to specify criteria for filtering the resources the informer watches. This lets you select resources from multiple CRDs based on shared metadata labels or other fields.
Let's assume you have two CRDs: mycrd.example.com/v1 and anothercrd.example.com/v1. Both might share a label like app: my-application. You could create a ListWatch that selects all resources with that label, regardless of their specific CRD:
package main
import (
"context"
"fmt"
"log"
metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
"k8s.io/apimachinery/pkg/runtime"
"k8s.io/apimachinery/pkg/runtime/schema"
"k8s.io/client-go/dynamic"
"k8s.io/client-go/informers"
"k8s.io/client-go/kubernetes"
"k8s.io/client-go/rest"
"k8s.io/client-go/tools/cache"
)
func main() {
// creates the in-cluster config
config, err := rest.InClusterConfig()
if err != nil {
panic(err.Error())
}
// creates the clientset
clientset, err := kubernetes.NewForConfig(config)
if err != nil {
panic(err.Error())
}
dynamicClient, err := dynamic.NewForConfig(config)
if err != nil {
panic(err.Error())
}
// Define the GVRs you want to watch. This example assumes you know the GVRs in advance. For a more dynamic approach, see the section below.
gvrList := []schema.GroupVersionResource{
{Group: "mycrd.example.com", Version: "v1", Resource: "mycrds"},
{Group: "anothercrd.example.com", Version: "v1", Resource: "anothercrds"},
}
// Create a ListWatch to select resources based on label selector
lw := &cache.ListWatch{
ListFunc: func(options metav1.ListOptions) (runtime.Object, error) {
// This will iterate through each GVR, get a list and append them together. You'll need error handling.
var list runtime.Object
for _, gvr := range gvrList {
obj, err := dynamicClient.Resource(gvr).List(context.TODO(), metav1.ListOptions{LabelSelector: "app=my-application"})
if err != nil {
return nil, err
}
// Append to list, appropriate type assertion needed depending on list structure of each GVR
// ... handle merging of lists ...
}
return list, nil
},
}
_, controller := informers.NewSharedInformerFactoryWithOptions(clientset, 0, informers.WithTweakListOptionsFunc(func(options *metav1.ListOptions) {
options.LabelSelector = "app=my-application"
})).Core().V1().ConfigMaps().Informer() // You'll need to adapt this to your resource type
// This will only work if your GVRs are of the same type and use the same controller
// Otherwise, you have to create multiple informers.
controller.AddEventHandler(cache.ResourceEventHandlerFuncs{
AddFunc: func(obj interface{}) {
fmt.Println("Resource added:", obj)
},
UpdateFunc: func(oldObj, newObj interface{}) {
fmt.Println("Resource updated:", newObj)
},
DeleteFunc: func(obj interface{}) {
fmt.Println("Resource deleted:", obj)
},
})
stop := make(chan struct{})
defer close(stop)
controller.Run(stop)
}
Important Considerations:
- Error Handling: The provided code snippet lacks robust error handling. Production-ready code must include comprehensive error checks and logging.
- Resource Type Consistency: This approach works best when all your CRDs share a common base type or if you are comfortable with merging different resource lists. If the CRD structures are significantly different, a more complex approach or separate informers might be necessary.
- Dynamic GVR Discovery: If you don't know the GVRs beforehand, you will need to dynamically discover them using the Kubernetes API server. This typically involves querying the API server for available CRDs and constructing
ListWatchobjects accordingly.
Handling Different Resource Types
If your CRDs have different structures and you still want to use a single informer, you will likely need a more complex handler function to correctly type assert the incoming objects. For instance, you might have a switch statement within your event handler function that checks the object type and handles it accordingly.
This more sophisticated approach allows for efficient monitoring of multiple CRDs using a single informer, reducing the complexity and resource consumption associated with managing numerous individual informers. Remember to adapt the code to your specific CRDs and consider the complexity introduced by different resource types or dynamic GVR discovery. Always prioritize robust error handling in production environments.
