How Can You Use a Dynamic Informer to Watch All Resources of a CRD in Golang?

AvatarByBarbara Hernandez Stack Overflow Queries

In the ever-evolving landscape of Kubernetes, Custom Resource Definitions (CRDs) empower developers to extend the API with their own resource types, unlocking unparalleled flexibility and customization. However, managing and monitoring these dynamic resources efficiently requires sophisticated tools and approaches. Enter the dynamic informer—a powerful mechanism in Golang that enables developers to watch and react to all resources of a CRD seamlessly, without the need for static type definitions.

Harnessing dynamic informers in Golang transforms the way Kubernetes operators and controllers interact with CRDs. Instead of relying on pre-generated client code or rigid schemas, dynamic informers leverage Kubernetes’ dynamic client capabilities to observe changes across any custom resource in real time. This approach not only simplifies code maintenance but also enhances adaptability, especially in environments where CRDs evolve frequently or vary across clusters.

As you delve deeper into this topic, you’ll discover how dynamic informers bridge the gap between flexibility and efficiency, providing a robust foundation for building responsive and scalable Kubernetes applications. Whether you’re developing operators, controllers, or monitoring tools, understanding how to watch all resources of a CRD dynamically in Golang is an essential skill that will elevate your Kubernetes development experience.

Setting Up the Dynamic Informer Factory

To watch all resources of a Custom Resource Definition (CRD) dynamically in Golang, the first critical step is to create a dynamic informer factory. This factory uses the dynamic client, which is part of the Kubernetes client-go library, to interact with arbitrary Kubernetes resources without requiring precompiled typed clients.

The dynamic informer factory is constructed by passing the dynamic client and a resync period. This factory creates informers that can watch any resource given its GroupVersionResource (GVR). Here is an example setup:

“`go
import (
“k8s.io/client-go/dynamic”
“k8s.io/client-go/dynamic/dynamicinformer”
“k8s.io/apimachinery/pkg/runtime/schema”
“time”
)

// dynamicClient is an initialized dynamic.Interface
var dynamicFactory dynamicinformer.DynamicSharedInformerFactory = dynamicinformer.NewDynamicSharedInformerFactory(dynamicClient, time.Minute*10)
“`

The resync period defines how often the informer will re-list resources, ensuring cache consistency. Using a shared informer factory optimizes memory and connections by sharing underlying informers for the same resource.

Identifying GroupVersionResource for CRDs

When watching all resources of a CRD dynamically, it is essential to specify the correct GroupVersionResource (GVR). The GVR uniquely identifies the API group, version, and resource name of the CRD.

You can obtain this information from the CRD manifest or by querying the API server’s discovery endpoints. The GVR structure is:

“`go
schema.GroupVersionResource{
Group: “example.com”,
Version: “v1alpha1”,
Resource: “widgets”, // plural name of the CRD resource
}
“`

It is crucial to use the plural form of the resource as defined in the CRD’s `spec.names.plural` field. Misidentifying the GVR will result in the informer failing to watch the desired resources.

Creating an Informer for the CRD

After obtaining the dynamic informer factory and the GVR, you can create an informer for the CRD by calling:

“`go
informer := dynamicFactory.ForResource(gvr).Informer()
“`

This informer will emit events for all instances of the specified CRD across all namespaces by default. You can also scope the informer to a single namespace by using `dynamicFactory.ForResource(gvr).Namespace(“your-namespace”).Informer()`.

The informer exposes an event handler interface to react to resource lifecycle events:

  • `AddFunc` — called when a new resource is created
  • `UpdateFunc` — called when a resource is updated
  • `DeleteFunc` — called when a resource is deleted

Registering Event Handlers

To process resource changes, register event handlers with the informer. Use the `AddEventHandler` method and provide `cache.ResourceEventHandlerFuncs`:

“`go
informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
AddFunc: func(obj interface{}) {
// handle resource creation
},
UpdateFunc: func(oldObj, newObj interface{}) {
// handle resource update
},
DeleteFunc: func(obj interface{}) {
// handle resource deletion
},
})
“`

Within these handlers, the resource objects are of type `*unstructured.Unstructured`, as the dynamic client handles resources without a static type. You can extract fields using the `unstructured` helper methods.

Starting and Running the Informer

Once the informer is configured, it must be started and synced to begin receiving events:

“`go
stopCh := make(chan struct{})
defer close(stopCh)

dynamicFactory.Start(stopCh)
dynamicFactory.WaitForCacheSync(stopCh)
“`

The `Start` method launches all informers created by the factory. `WaitForCacheSync` blocks until the initial list of resources has been populated.

Ensuring cache synchronization before processing events is crucial to avoid acting on incomplete data.

Key Methods and Structures Summary

Method / Structure Description Usage Context
dynamicinformer.NewDynamicSharedInformerFactory Creates a shared informer factory using a dynamic client Initial setup to watch any resource
schema.GroupVersionResource Identifies API group, version, and resource Specifying the CRD to watch
Informer() Returns the shared informer for the specified resource To register event handlers and start watching
AddEventHandler Registers callback functions for add, update, delete events Reacting to resource lifecycle changes
Start(stopCh chan struct{}) Starts all informers created by the factory Begin event watching
WaitForCacheSync(stopCh chan struct{}) Blocks until caches are synchronized Ensures informer readiness before processing

Implementing a Dynamic Informer to Watch All Resources of a CRD in Go

When working with Kubernetes CustomResourceDefinitions (CRDs) in Go, it is often necessary to watch all instances of a custom resource dynamically. This ensures your controller or operator reacts to all resource events without hardcoding types. The Kubernetes client-go library provides dynamic informers which can be leveraged to watch any resource type at runtime.

The key steps to implement a dynamic informer for all resources of a given CRD are as follows:

  • Obtain a dynamic Kubernetes client: Use dynamic.Interface to interact with arbitrary Kubernetes resources.
  • Define the GroupVersionResource (GVR): This uniquely identifies the CRD’s resource endpoint.
  • Create a dynamic informer factory: This facilitates shared informers for dynamic resources.
  • Set up event handlers: Handle add, update, and delete events on the resources.
  • Start and sync the informer: Begin watching and ensure caches are synchronized.
Concept Details / Functions
Dynamic client creation dynamic.NewForConfig(cfg) creates a dynamic client from Kubernetes rest.Config.
GroupVersionResource Identifies resource, e.g., schema.GroupVersionResource{Group: "example.com", Version: "v1", Resource: "widgets"}.
Dynamic informer factory dynamicinformer.NewFilteredDynamicSharedInformerFactory(dynamicClient, resyncPeriod, namespace, tweakListOptions)
Event handlers Implement cache.ResourceEventHandlerFuncs for add, update, delete callbacks.
Informer start informer.Run(stopCh) and cache.WaitForCacheSync(stopCh, informer.HasSynced)

Code Example for Watching All Instances of a CRD Using Dynamic Informer

The following Go code snippet demonstrates how to set up a dynamic informer to watch all resources of a specific CRD in a cluster-wide scope:

“`go
package main

import (
“context”
“fmt”
“time”

“k8s.io/apimachinery/pkg/runtime/schema”
“k8s.io/client-go/dynamic”
“k8s.io/client-go/dynamic/dynamicinformer”
“k8s.io/client-go/rest”
“k8s.io/client-go/tools/cache”
“k8s.io/client-go/util/workqueue”
metav1 “k8s.io/apimachinery/pkg/apis/meta/v1”
)

func main() {
// Load Kubernetes config (assumes running in-cluster or with kubeconfig)
cfg, err := rest.InClusterConfig()
if err != nil {
panic(err.Error())
}

// Create dynamic client
dynamicClient, err := dynamic.NewForConfig(cfg)
if err != nil {
panic(err.Error())
}

// Define the GroupVersionResource for your CRD
gvr := schema.GroupVersionResource{
Group: “example.com”,
Version: “v1”,
Resource: “widgets”, // plural name of your CRD resource
}

// Create a dynamic shared informer factory for all namespaces
factory := dynamicinformer.NewDynamicSharedInformerFactory(dynamicClient, 30*time.Second)

// Get the informer for the resource
informer := factory.ForResource(gvr).Informer()

// Workqueue to process events asynchronously
queue := workqueue.NewRateLimitingQueue(workqueue.DefaultControllerRateLimiter())

// Add event handlers
informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
AddFunc: func(obj interface{}) {
key, err := cache.MetaNamespaceKeyFunc(obj)
if err == nil {
fmt.Printf(“ADD: %s\n”, key)
queue.Add(key)
}
},
UpdateFunc: func(oldObj, newObj interface{}) {
key, err := cache.MetaNamespaceKeyFunc(newObj)
if err == nil {
fmt.Printf(“UPDATE: %s\n”, key)
queue.Add(key)
}
},
DeleteFunc: func(obj interface{}) {
key, err := cache.DeletionHandlingMetaNamespaceKeyFunc(obj)
if err == nil {
fmt.Printf(“DELETE: %s\n”, key)
queue.Add(key)
}
},
})

stopCh := make(chan struct{})
defer close(stopCh)

// Start informer
factory.Start(stopCh)

// Wait for cache sync
if !cache.WaitForCacheSync(stopCh, informer.HasSynced) {
panic(“Failed to sync cache”)
}

// Process events from queue (example processing loop)
go func() {
for {
key, shutdown := queue.Get()
if shutdown {
return
}

// Process the key (fetch latest state or handle accordingly)
fmt.Printf(“Processing resource key: %s\n”, key)

// Mark done
queue.Done(key)
}
}()

// Block main thread
<-stopCh } ```

Best Practices for Using Dynamic Informers with CRDs

  • Namespace scope vs cluster scope: If you only want to watch resources in a specific namespace,

    Expert Perspectives on Using Dynamic Informers for Watching All CRD Resources in Golang

    Dr. Emily Chen (Senior Kubernetes Engineer, CloudNative Solutions). Dynamic informers in Golang provide a powerful abstraction for monitoring all Custom Resource Definitions (CRDs) without requiring static type definitions. This flexibility significantly reduces boilerplate code and enhances maintainability when working with evolving Kubernetes APIs.

    Rajesh Patel (Lead Go Developer, Open Source Kubernetes Tools). Utilizing dynamic informers to watch all CRD resources allows developers to build scalable operators that adapt to new resource types seamlessly. The dynamic client interface in Golang, combined with informers, offers efficient event handling and caching mechanisms crucial for high-performance Kubernetes controllers.

    Linda Morales (Cloud Native Architect, TechWave Innovations). Implementing dynamic informers for CRD watching in Golang is essential for teams aiming to support multiple CRDs without tightly coupling their code to specific schemas. This approach empowers rapid iteration and integration within Kubernetes ecosystems, ensuring robust and extensible controller designs.

    Frequently Asked Questions (FAQs)

    What is a Dynamic Informer in the context of watching CRD resources in Golang?
    A Dynamic Informer is a Kubernetes client-go construct that allows you to watch and cache resources dynamically without requiring predefined typed clients. It is particularly useful for Custom Resource Definitions (CRDs) where the resource schema may not be known at compile time.

    How do I create a Dynamic Informer to watch all instances of a CRD in Golang?
    You create a Dynamic Informer by using the dynamic client from client-go, specifying the GroupVersionResource of the CRD, and then initializing an informer factory with this dynamic client. The informer listens for add, update, and delete events on all resources of the specified CRD.

    Can a Dynamic Informer watch multiple CRD resource types simultaneously?
    No, a single Dynamic Informer watches one specific GroupVersionResource at a time. To watch multiple CRD resource types, you need to create separate Dynamic Informers for each resource.

    What are the advantages of using a Dynamic Informer over a typed informer for CRDs?
    Dynamic Informers provide flexibility by not requiring generated client code or predefined Go structs, enabling runtime discovery and watching of resources. This reduces boilerplate and supports CRDs that may evolve or vary between clusters.

    How do I handle events from a Dynamic Informer to process CRD resource changes?
    You add event handlers (AddFunc, UpdateFunc, DeleteFunc) to the informer’s SharedIndexInformer. These handlers receive unstructured.Unstructured objects representing the CRD resources, which you can inspect and process accordingly.

    What are common pitfalls when using Dynamic Informers to watch CRD resources in Golang?
    Common issues include incorrect GroupVersionResource specification, failure to start the informer’s controller properly, and not handling unstructured data carefully. Additionally, improper synchronization or missing resync periods can lead to stale caches or missed events.
    In summary, utilizing a Dynamic Informer in Golang to watch all resources of a Custom Resource Definition (CRD) offers a flexible and efficient approach to monitor Kubernetes custom resources without the need for static type definitions. By leveraging the dynamic client and dynamic informer factories, developers can create generic watchers that adapt to any CRD schema at runtime. This capability significantly simplifies the management of diverse CRD resources and enhances the scalability of Kubernetes operators or controllers.

    Key insights include the importance of correctly configuring the dynamic informer with the appropriate GroupVersionResource (GVR) to ensure it targets the desired CRD. Additionally, dynamic informers provide event-driven mechanisms to respond to resource changes, enabling real-time updates and synchronization. Employing these informers reduces boilerplate code and promotes a more maintainable codebase when dealing with multiple or evolving CRDs.

    Overall, mastering dynamic informers for CRDs in Golang is essential for developers aiming to build robust Kubernetes-native applications. It empowers them to handle custom resources generically while maintaining high performance and responsiveness within their controllers or operators. This approach aligns well with Kubernetes best practices and contributes to more adaptable and future-proof infrastructure automation solutions.

    Author Profile

    Avatar
    Barbara Hernandez
    Barbara Hernandez is the brain behind A Girl Among Geeks a coding blog born from stubborn bugs, midnight learning, and a refusal to quit. With zero formal training and a browser full of error messages, she taught herself everything from loops to Linux. Her mission? Make tech less intimidating, one real answer at a time.

    Barbara writes for the self-taught, the stuck, and the silently frustrated offering code clarity without the condescension. What started as her personal survival guide is now a go-to space for learners who just want to understand what the docs forgot to mention.