How to Use a Single Informer to Monitor Multiple CRD Changes
Monitoring multiple Custom Resource Definitions (CRDs) for changes using a single Kubernetes informer can significantly streamline your application's logic and improve efficiency. Instead of creating separate informers for each CRD, a single, cleverly configured informer can handle all the changes, simplifying your codebase and reducing resource consumption. This guide will detail how to achieve this, addressing common questions along the way.
What is a Kubernetes Informer?
Before diving into the solution, let's briefly recap what a Kubernetes informer is. An informer is a component within the Kubernetes client library that efficiently watches for changes (creation, update, deletion) to resources within the cluster. It provides a mechanism to receive events related to these changes, allowing your application to react accordingly. Using informers is generally preferred over direct polling of the API server as it's more efficient and less taxing on the cluster.
Using a Single Informer for Multiple CRDs: The SharedIndexInformer
The key to monitoring multiple CRDs with a single informer lies in leveraging the SharedIndexInformer. This informer type allows you to watch multiple resource types (defined by their GroupVersionResource) concurrently. This means you can specify all the CRDs you're interested in and receive a single stream of events representing changes across all of them.
Here's how you would typically implement this in Go:
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/apimachinery/pkg/watch"
"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 for your CRDs. Replace with your actual CRD information.
gvrs := []schema.GroupVersionResource{
{Group: "mygroup.example.com", Version: "v1", Resource: "mycrd"},
{Group: "anothergroup.example.com", Version: "v1beta1", Resource: "anothercrd"},
}
// Create a SharedInformerFactory. This is crucial for sharing informers efficiently.
factory := informers.NewSharedInformerFactory(clientset, 0) // 0 for resync period - adjust if needed
// Add the specific informers using the dynamic client (for CRDs outside core Kubernetes)
for _, gvr := range gvrs {
factory.DynamicSharedInformerFactory.ForResource(gvr).Informer()
}
stopCh := make(chan struct{})
defer close(stopCh)
factory.Start(stopCh)
//Example of processing the informers
for _, gvr := range gvrs {
informer := factory.DynamicSharedInformerFactory.ForResource(gvr).Informer()
informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
AddFunc: func(obj interface{}) {
fmt.Printf("Added: %+v\n", obj)
},
UpdateFunc: func(oldObj, newObj interface{}) {
fmt.Printf("Updated: %+v -> %+v\n", oldObj, newObj)
},
DeleteFunc: func(obj interface{}) {
fmt.Printf("Deleted: %+v\n", obj)
},
})
cache.WaitForCacheSync(stopCh, informer.HasSynced) // Wait until the cache is synced
}
<-stopCh
}
How to Handle Different CRD Types
The example above uses the dynamic client which is excellent for handling CRDs whose specific client-go packages might not be available. Within your AddFunc, UpdateFunc, and DeleteFunc handlers, you'll need to type assert the obj interface{} to the correct custom resource struct. This allows your code to work with the specific fields and data within each CRD. For example:
AddFunc: func(obj interface{}) {
myCRD, ok := obj.(*mygroupv1.MyCRD) // mygroupv1.MyCRD is your custom struct
if !ok {
log.Printf("Unexpected object type: %T", obj)
return
}
fmt.Printf("Added MyCRD: Name=%s, Spec=%+v\n", myCRD.Name, myCRD.Spec)
},
Remember to replace "mygroup.example.com", "v1", "mycrd", and mygroupv1.MyCRD with your actual CRD group, version, resource, and Go struct.
Choosing a Resync Period
The informers.NewSharedInformerFactory takes a resyncPeriod argument. This sets how often the informer resynchronizes its cache with the API server. A value of 0 (as shown) means it resynchronizes only when an event occurs – the most efficient option. If your application requires a periodic check even without changes, you'd set a non-zero duration (e.g., time.Second * 30).
Error Handling and Robustness
Production-ready code should include comprehensive error handling. Check for errors at each step (config creation, clientset creation, informer startup, etc.) and implement appropriate logging and recovery mechanisms.
This approach, using a SharedIndexInformer and the dynamic client, offers a clean and scalable way to monitor multiple CRD changes using a single informer, leading to a more maintainable and efficient Kubernetes application. Remember to always adapt the code snippets with your specific CRD details.
