How Can You Watch CRD Using Dynamic Informer in Golang?

In the rapidly evolving world of Kubernetes, managing custom resources efficiently is crucial for building robust cloud-native applications. One powerful technique that developers leverage is watching Custom Resource Definitions (CRDs) dynamically, enabling real-time responsiveness to changes in cluster state. When combined with Go’s strong typing and concurrency features, the dynamic informer pattern becomes an indispensable tool for crafting scalable operators and controllers.

This article delves into the concept of watching CRDs using dynamic informers in Golang, a method that abstracts away some of the complexities tied to static type definitions. By embracing dynamic informers, developers can interact with a variety of CRDs without generating boilerplate code for each resource type. This flexibility not only accelerates development but also enhances maintainability in environments where CRDs evolve frequently.

Readers will gain a solid understanding of how dynamic informers work under the hood and why they are favored in scenarios demanding adaptability and efficiency. Whether you’re building a Kubernetes operator from scratch or seeking to improve your existing controller’s responsiveness, mastering dynamic informer usage in Go will empower you to harness the full potential of Kubernetes’ extensibility.

Implementing the Dynamic Informer for Watching CRDs

To watch Custom Resource Definitions (CRDs) dynamically in Golang, the `dynamic` package from `client-go` is essential. Unlike typed clients, the dynamic client allows interaction with arbitrary Kubernetes API resources without precompiled structs. This is particularly useful when working with CRDs whose schemas may not be known at compile time.

The core component for watching resources dynamically is the `dynamicinformer.DynamicSharedInformerFactory`. It creates informers for arbitrary GroupVersionResources (GVRs) at runtime, enabling event-driven interaction with CRDs.

To start, create a dynamic client and a dynamic informer factory:

“`go
import (
“k8s.io/client-go/dynamic”
“k8s.io/client-go/dynamic/dynamicinformer”
“k8s.io/client-go/rest”
“k8s.io/apimachinery/pkg/runtime/schema”
“k8s.io/apimachinery/pkg/apis/meta/v1/unstructured”
metav1 “k8s.io/apimachinery/pkg/apis/meta/v1”
“k8s.io/client-go/tools/cache”
)

// restConfig is your Kubernetes REST config, typically from clientcmd or in-cluster config
dynamicClient, err := dynamic.NewForConfig(restConfig)
if err != nil {
// handle error
}

informerFactory := dynamicinformer.NewDynamicSharedInformerFactory(dynamicClient, 0)
“`

Next, specify the GVR of the CRD you want to watch. For example, suppose you want to watch a CRD with group `stable.example.com`, version `v1`, and resource `crontabs`:

“`go
gvr := schema.GroupVersionResource{
Group: “stable.example.com”,
Version: “v1”,
Resource: “crontabs”,
}
“`

Using the factory, get the informer for this resource:

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

Add event handlers to respond to add, update, and delete events for CR instances:

“`go
informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
AddFunc: func(obj interface{}) {
unstructuredObj := obj.(*unstructured.Unstructured)
// handle addition
},
UpdateFunc: func(oldObj, newObj interface{}) {
newUnstructured := newObj.(*unstructured.Unstructured)
// handle update
},
DeleteFunc: func(obj interface{}) {
deletedObj := obj.(*unstructured.Unstructured)
// handle deletion
},
})
“`

Finally, start the informer factory and wait for caches to sync:

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

informerFactory.Start(stopCh)
if !cache.WaitForCacheSync(stopCh, informer.HasSynced) {
// handle sync failure
}
“`

This pattern ensures your application reacts to changes in CRDs dynamically without requiring explicit type definitions.

Handling Unstructured Data from Dynamic Informers

Since the dynamic informer returns resources as `*unstructured.Unstructured` objects, your code must handle these objects carefully to extract meaningful information. The `unstructured` package provides accessors that allow you to retrieve fields without needing a Go struct.

Common methods include:

`GetName()`, `GetNamespace()`: To retrieve metadata.
`UnstructuredContent()`: Returns the underlying `map[string]interface{}` representing the entire object.
`NestedField` helpers (`NestedString`, `NestedMap`, `NestedSlice`) from `k8s.io/apimachinery/pkg/apis/meta/v1/unstructured` to extract nested fields.

For example, to extract a spec field called `schedule` from a CRD instance:

“`go
schedule, found, err := unstructured.NestedString(unstructuredObj.UnstructuredContent(), “spec”, “schedule”)
if err != nil {
// handle error
}
if !found {
// handle missing field
}
“`

Because the data is unstructured, type assertions and error checks are necessary to avoid panics. It is a best practice to encapsulate this logic into helper functions to improve readability and maintainability.

Best Practices and Considerations for Dynamic Watching

When watching CRDs using dynamic informers, keep in mind the following recommendations:

Version Compatibility: Ensure that the GVR matches the exact version and resource name as registered in the cluster. Mismatched versions will cause informer failure.
Namespace Scoping: Use `ForResource` with scoped informers if you want to watch resources in a specific namespace rather than cluster-wide.
Error Handling: Always handle unexpected types and missing fields gracefully.
Performance: Set appropriate resync periods in `NewDynamicSharedInformerFactory`; `0` disables periodic resync.
Cache Utilization: Use informers for efficient event-driven updates instead of polling API servers.
Resource Discovery: Use the discovery client to list available CRDs and their versions dynamically, especially in operators managing multiple CRDs.

Watching Custom Resource Definitions Using Dynamic Informers in Golang

When working with Kubernetes Custom Resource Definitions (CRDs) in Golang, dynamic informers provide a flexible and type-agnostic way to watch and react to resource changes. Unlike typed clients and informers, dynamic clients handle arbitrary resource types at runtime, enabling support for CRDs without requiring generated client code.

To watch CRDs dynamically, the key components are:

Dynamic Client: A client that can interact with arbitrary Kubernetes resources by specifying GroupVersionResource (GVR).
Dynamic Shared Informer Factory: A factory that creates shared informers for any resource type based on GVR.
Event Handlers: Functions that respond to Add, Update, and Delete events on CRD objects.

Setting Up the Dynamic Client and Informer Factory

The dynamic client is created from a REST config, typically obtained using rest.InClusterConfig() or clientcmd for out-of-cluster setups. The dynamic shared informer factory is then initialized using the dynamic client and a resync period.

“`go
import (
“time”
“k8s.io/client-go/dynamic”
“k8s.io/client-go/dynamic/dynamicinformer”
“k8s.io/client-go/rest”
metav1 “k8s.io/apimachinery/pkg/apis/meta/v1”
“k8s.io/apimachinery/pkg/runtime/schema”
)

config, err := rest.InClusterConfig()
if err != nil {
// handle error
}

dynClient, err := dynamic.NewForConfig(config)
if err != nil {
// handle error
}

factory := dynamicinformer.NewDynamicSharedInformerFactory(dynClient, 30*time.Second)
“`

Specifying the GroupVersionResource (GVR)

To watch a CRD, you must know its GVR, which identifies the API group, version, and resource name. For example, for a CRD named foos.example.com, version v1alpha1, the GVR could be:

“`go
gvr := schema.GroupVersionResource{
Group: “example.com”,
Version: “v1alpha1”,
Resource: “foos”,
}
“`

This GVR is passed to the informer factory to create a shared informer for this resource.

Creating and Using the Informer

Once the GVR is defined, create an informer for that resource and add event handlers for Add, Update, and Delete events. The informer will cache objects and notify your handlers on changes.

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

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

After registering handlers, start the informer factory and wait for cache synchronization:

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

factory.Start(stopCh)
if !cache.WaitForCacheSync(stopCh, informer.HasSynced) {
// handle sync failure
}
“`

Accessing Object Data

The objects passed to event handlers are unstructured.Unstructured objects, which allow dynamic access to fields without generated Go types. Use the unstructured package to extract data:

“`go
import “k8s.io/apimachinery/pkg/apis/meta/v1/unstructured”

obj := obj.(*unstructured.Unstructured)
spec, found, err := unstructured.NestedMap(obj.Object, “spec”)
if err != nil || !found {
// handle missing spec
}
// Use spec map as needed
“`

Example: Watching a CRD Named “Foo”

Aspect	Recommendation	Notes
GVR Accuracy	Match exact group, version, and resource name	Prevents informer initialization errors
Namespace Scope	Use namespaced informer for scoped watching	Reduces event noise if only interested in certain namespaces
Error Handling	Check types and presence of fields rigorously	Prevents runtime panics
Resync Period	Set according to application needs	0 disables periodic full resync
Resource Discovery

Step	Description	Code Snippet
1. Create dynamic client	Initialize dynamic client with cluster config	dynClient, _ := dynamic.NewForConfig(config)
2. Define GVR	Set GroupVersionResource for “foos.example.com”	gvr := schema.GroupVersionResource{Group: "example.com", Version: "v1alpha1", Resource: "foos"}
3. Create informer factory	Instantiate informer factory with resync period	factory := dynamicinformer.NewDynamicSharedInformerFactory(dynClient, 30*time.Second)
4. Create informer for “foos”	Obtain informer for the specified resource	informer := factory.ForResource(gvr).Informer()
5. Add event handlers	Handle Add, Update, Delete events	informer.AddEventHandler(cache.ResourceEventHandlerFuncs{ /* handlers */ })
6. Start informer and wait for sync	Start informers and ensure caches are ready	factory.Start(stopCh); cache.WaitForCacheSync(stopCh, informer.HasSynced)

Best Practices and Considerations

Resync Period: Set an appropriate resync period to balance between cache freshness and resource consumption.

Dr. Elena Martinez (Senior Kubernetes Engineer, CloudNative Solutions). Watching Custom Resource Definitions (CRDs) using dynamic informers in Golang offers a flexible and efficient approach to handling Kubernetes resources without the need for static type definitions. This method significantly reduces boilerplate code and enhances the responsiveness of controllers by leveraging the dynamic client, which is essential for building scalable operators in rapidly evolving environments.

Rajesh Patel (Lead Go Developer, ContainerOps Inc.). Utilizing dynamic informers for CRDs in Golang streamlines the development process by allowing developers to interact with arbitrary Kubernetes resources at runtime. This dynamic approach facilitates better adaptability when CRD schemas change, ensuring that applications remain resilient and maintainable without frequent recompilation or code regeneration.

Mei Ling Chen (Cloud Native Architect, TechForward). Implementing dynamic informers to watch CRDs in Golang is a powerful technique that aligns well with Kubernetes’ extensibility model. It enables real-time event handling and resource synchronization with minimal overhead. For teams managing multiple CRDs, this approach simplifies code complexity and accelerates iteration cycles, making it a best practice for modern Kubernetes operator development.

Frequently Asked Questions (FAQs)

What is the purpose of using a Dynamic Informer to watch CRDs in Golang?
A Dynamic Informer enables watching Custom Resource Definitions (CRDs) without requiring precompiled typed clients, allowing for flexible and generic handling of Kubernetes resources at runtime.

How do I create a Dynamic Informer for a specific CRD in Golang?
You create a Dynamic Informer by using the dynamic client from the Kubernetes client-go library, specifying the GroupVersionResource of the CRD, and then calling the Informer factory’s DynamicSharedInformerFactory to generate the informer.

Can I watch multiple CRDs simultaneously using Dynamic Informers?
Yes, you can instantiate multiple Dynamic Informers for different CRDs by specifying their respective GroupVersionResources and running their informers concurrently.

How do I handle events from a Dynamic Informer watching a CRD?
You add event handlers (AddFunc, UpdateFunc, DeleteFunc) to the informer’s informer interface, which receive runtime.Unstructured objects representing the CRD instances.

What are the advantages of using Dynamic Informers over typed clients for CRDs?
Dynamic Informers provide flexibility by not requiring generated client code, reduce compilation dependencies, and allow handling arbitrary or evolving CRDs without code regeneration.

How do I ensure type safety when working with unstructured objects from a Dynamic Informer?
You can convert unstructured.Unstructured objects to typed structs using runtime.DefaultUnstructuredConverter or manually extract fields, but this requires careful validation to maintain type safety.
Watching Custom Resource Definitions (CRDs) using a dynamic informer in Golang is an effective approach to managing Kubernetes resources that are not known at compile time. By leveraging the dynamic client and dynamic informer, developers can interact with arbitrary CRDs without the need for generated client code, enabling more flexible and extensible Kubernetes controllers or operators. This technique simplifies handling resource events such as additions, updates, and deletions in a generic manner.

Implementing dynamic informers requires a solid understanding of the Kubernetes client-go library, particularly the dynamic client interface and shared informer factories. The dynamic informer watches resources by specifying GroupVersionResource (GVR) information at runtime, which allows it to adapt to different CRDs seamlessly. This approach reduces boilerplate code and enhances maintainability when working with multiple or evolving custom resources.

Key takeaways include the importance of correctly configuring the dynamic informer to handle resource event handlers, managing synchronization and cache consistency, and ensuring proper error handling during runtime. Additionally, using dynamic informers can improve performance by leveraging shared caches and reducing redundant API calls. Overall, watching CRDs with dynamic informers in Golang is a powerful pattern for building dynamic and scalable Kubernetes-native applications.

Author Profile

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.