What are Custom Resource Definitions (CRDs) in Kubernetes?

Custom Resource Definitions (CRDs) in Kubernetes

Custom Resource Definitions in Kubernetes are ways to add new types of resources to the Kubernetes API. This helps us manage complex applications and services. We can create and use custom resources that act like the regular Kubernetes resources. CRDs let us make Kubernetes work better for our needs. They help us customize how our applications run.

In this article, we will look at what Custom Resource Definitions are in Kubernetes. We will see how they work and what benefits they bring. We will give a simple step-by-step guide on how to create and manage CRDs using kubectl. We will also talk about common ways to use CRDs. We will cover how to validate them, how to use controllers for custom resources, and best tips for designing CRDs. Lastly, we will answer some common questions about CRDs.

  • What are Custom Resource Definitions in Kubernetes?
  • How Do Custom Resource Definitions Work?
  • What are the Benefits of Using CRDs?
  • How to Create a Custom Resource Definition?
  • How to Manage Custom Resources with kubectl?
  • What are Common Use Cases for CRDs?
  • How to Validate Custom Resource Definitions?
  • How to Implement Controllers for Custom Resources?
  • Best Practices for Designing Custom Resource Definitions
  • Frequently Asked Questions

If you want to learn more about Kubernetes, we can check out related topics. For example, What is Kubernetes and How Does it Simplify Container Management and What are Kubernetes Operators and How Do They Automate Tasks.

How Do Custom Resource Definitions Work?

Custom Resource Definitions (CRDs) in Kubernetes help us to add new types of resources to the Kubernetes API. With a CRD, we can create our own resource types that work like the built-in ones, such as Pods and Services.

Here is how CRDs work:

  1. Definition: We define a CRD using a YAML file. This file tells the system how the custom resource should look and act. It has details like the group, version, and kind of the resource.

    Example CRD definition:

    apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: myresources.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                foo:
                  type: string
  scope: Namespaced
  names:
    plural: myresources
    singular: myresource
    kind: MyResource
    shortNames:
      - mr

  2. Registration: After we apply a CRD definition, Kubernetes registers the new resource type. It becomes available through the Kubernetes API. Now, we can use our custom resource just like the built-in ones.

  3. Creating Custom Resources: Once we have a CRD defined and registered, we can create our own custom resources. This is similar to how we create a Pod or Service.

    Example of a custom resource instance:

    apiVersion: example.com/v1
kind: MyResource
metadata:
  name: my-custom-resource
spec:
  foo: "bar"

  4. Controller Interaction: Usually, we use CRDs with controllers. A controller watches for any changes in custom resources. It makes sure the system is in the state we want. When we create, update, or delete a custom resource, the controller acts based on what we set in the resource.

  5. Kubernetes API: We can access custom resources through the Kubernetes API. This allows us to use kubectl commands or REST API calls to manage our custom resources just like the built-in ones.

  6. Validation: CRDs can check custom resources using OpenAPI v3 schema. This way, only valid configurations will be accepted.

By using CRDs, Kubernetes gives us a strong way to add new features to fit our application needs. This helps us manage complex applications with our own resource types easily. For more details about managing Kubernetes resources, you can check what is kubectl and how do I use it to manage Kubernetes.

What are the Benefits of Using CRDs?

Custom Resource Definitions or CRDs in Kubernetes give us many benefits. They help us make Kubernetes clusters more flexible and easy to use. With CRDs, we can create our own resource types. These types fit right into the Kubernetes API. Here are some main benefits of using CRDs:

  1. Extensibility: CRDs let us add new features to Kubernetes. We can define custom resource types for our specific needs. This helps us manage settings or tasks for applications that the standard resources do not cover.

  2. Unified Management: We can manage custom resources just like we manage regular Kubernetes objects. We use the same tools and workflows. For example, we can use kubectl for creating, reading, updating, and deleting resources. This gives us a smooth management experience.

    # Example of getting custom resources
kubectl get <custom-resource-name>

# Example of creating a custom resource
kubectl apply -f custom-resource.yaml

  3. API Integration: CRDs work with the Kubernetes API. This means we have the same access control, versioning, and resource management as the built-in resources. This makes it easier to develop operators and controllers.

  4. Declarative Configuration: We can set the desired states for our applications using simple manifests. Kubernetes takes care of making sure the actual state matches what we want.

  5. Versioning and Schema Validation: CRDs allow us to version our resource definitions. This means we can change them over time without breaking what we already have. We can also define validation rules using OpenAPI. This makes sure only correct configurations get accepted.

    # Example of a CRD with validation schema
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: myresources.example.com
spec:
  group: example.com
  names:
    kind: MyResource
    plural: myresources
    singular: myresource
  validation:
    openAPIV3Schema:
      type: object
      properties:
        spec:
          type: object
          properties:
            field:
              type: string
              pattern: '^foo$'

  6. Rich Ecosystem and Community Support: By using CRDs, we can access a wide range of tools, libraries, and community-made operators. These resources add more features and make difficult tasks easier.

  7. Custom Controllers and Automation: CRDs let us create custom controllers. These controllers can do tasks automatically based on the state of our custom resources. This is important for things like reconciliation loops.

  8. Interoperability: CRDs work well with other Kubernetes features like RBAC, namespaces, and labels. This helps us keep our resources secure and organized.

By using these benefits, we can create better, customized, and efficient cloud-native applications with Kubernetes. For more details on Kubernetes, we can check articles like What are Kubernetes Operators and How Do They Automate Tasks?.

How to Create a Custom Resource Definition?

We can create a Custom Resource Definition (CRD) in Kubernetes. This helps us add our own resource types to the Kubernetes API. To make a CRD, we need to define its schema and then apply it to our Kubernetes cluster.

Step 1: Define the CRD

First, we create a YAML file. We can name it myresource-definition.yaml. This file will define our custom resource. Here is an example of a CRD for a resource called MyResource:

apiVersion:apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: myresources.example.com
spec:
  group: example.com
  names:
    kind: MyResource
    listKind: MyResourceList
    plural: myresources
    singular: myresource
  scope: Namespaced
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                field1:
                  type: string
                field2:
                  type: integer

Step 2: Apply the CRD

Next, we use kubectl to apply the CRD definition to our cluster. We run this command:

kubectl apply -f myresource-definition.yaml

Step 3: Verify the CRD

Now, we should check if the CRD is created successfully. We can do this by running:

kubectl get crd

We should see myresources.example.com in the list of CRDs.

Step 4: Create a Custom Resource

With the CRD ready, we can create an instance of our custom resource. We make another YAML file and name it myresource-instance.yaml:

apiVersion: example.com/v1
kind: MyResource
metadata:
  name: my-resource-instance
  namespace: default
spec:
  field1: "example value"
  field2: 42

Then, we apply the custom resource by running:

kubectl apply -f myresource-instance.yaml

Step 5: Check the Custom Resource

Finally, we verify that the custom resource is created. We run this command:

kubectl get myresources

We should see my-resource-instance in the list.

This way, we can create and manage Custom Resource Definitions in Kubernetes. It helps us to customize the API for our needs. If we want to learn more about Kubernetes resources, we can read about Kubernetes Operators and how they automate tasks.

How to Manage Custom Resources with kubectl?

We can manage Custom Resource Definitions (CRDs) in Kubernetes easily with the kubectl command-line tool. Here is how we can do important tasks on custom resources.

Listing Custom Resources

To see all instances of a custom resource, we can use this command:

kubectl get <custom-resource-type>

For example, if our custom resource is called MyResource, we can list all resources of that type like this:

kubectl get myresources

Creating Custom Resources

To create a custom resource, we need to define it in a YAML file. Then we can use kubectl to apply it. Here is a sample YAML definition:

apiVersion: example.com/v1
kind: MyResource
metadata:
  name: my-resource-instance
spec:
  field1: value1
  field2: value2

We can create the custom resource with:

kubectl apply -f myresource.yaml

Viewing Custom Resource Details

To see detailed information about a specific custom resource, we can use:

kubectl describe <custom-resource-type> <resource-name>

For example:

kubectl describe myresource my-resource-instance

Updating Custom Resources

We can update a custom resource with kubectl apply and the changed YAML file. Or, we can edit it directly:

kubectl edit <custom-resource-type> <resource-name>

For example:

kubectl edit myresource my-resource-instance

Deleting Custom Resources

To delete a specific custom resource, we use:

kubectl delete <custom-resource-type> <resource-name>

For example:

kubectl delete myresource my-resource-instance

Filtering and Selecting Custom Resources

We can filter custom resources by using labels. For example, to list resources with a certain label, we can do:

kubectl get <custom-resource-type> -l key=value

Watching Custom Resources

To watch changes in custom resources in real-time, we use:

kubectl get <custom-resource-type> --watch

Custom Resource Status

We can check the status of a custom resource with:

kubectl get <custom-resource-type> -o custom-columns=NAME:.metadata.name,STATUS:.status.phase

This command shows the name and status of our custom resources.

For more details on how to use kubectl, please check this guide on kubectl.

What are Common Use Cases for CRDs?

Custom Resource Definitions (CRDs) in Kubernetes help us extend the Kubernetes API. This lets us manage resources that are not built-in. Here are some common ways we use CRDs:

  1. Operators: We often use CRDs to create Kubernetes Operators. Operators help us automate the management of complex applications. They can watch the application’s state and change things when needed.

    Example:

    apiVersion: example.com/v1
kind: MyApp
metadata:
  name: myapp-instance
spec:
  replicas: 3
  image: myapp:latest

  2. Custom Controllers: We can use CRDs with custom controllers. These controllers help us manage the state of applications or parts of our infrastructure that Kubernetes does not handle by itself.

  3. Configuration Management: We can use CRDs to set up custom configurations for applications. This helps teams manage application settings like they are Kubernetes resources.

    Example:

    apiVersion: configuration.example.com/v1
kind: AppConfig
metadata:
  name: app-config
spec:
  settings:
    logLevel: INFO
    features:
      enableFeatureX: true

  4. Monitoring and Metrics: We can create CRDs for monitoring and alerting settings. This allows us to connect with our own monitoring systems.

  5. Batch Jobs: CRDs help us define custom job types for batch processing. This lets us manage and keep an eye on batch jobs in a way that fits with Kubernetes.

    Example:

    apiVersion: batch.example.com/v1
kind: MyBatchJob
metadata:
  name: my-batch-job
spec:
  jobType: dataProcessing
  inputData:
    source: s3://mybucket/data

  6. Service Mesh Implementations: In service mesh setups, CRDs can show routing rules, service settings, or policies. This helps us manage communication between microservices.

  7. Custom Resource Management: CRDs let teams create and manage resources that are special to their applications. This can be things like custom databases or queues.

  8. Multi-Tenancy: We can use CRDs to manage multi-tenant resources. This helps different teams or projects have their own settings and resources in the same cluster.

CRDs give us a lot of flexibility. They are a key tool in the Kubernetes world. They help us adjust our Kubernetes setup to meet our business needs. If you want to learn more about Kubernetes Operators, you can read this article on Kubernetes Operators.

How to Validate Custom Resource Definitions?

Validating Custom Resource Definitions (CRDs) in Kubernetes is very important. It helps us make sure that our custom resources match the schema we expect. Kubernetes uses JSON Schema to help with this. This way, we can define how our custom resources should look and what rules they must follow.

Steps to Validate CRDs:

  1. Define Schema: When we create a CRD, we can add a validation schema. We put this under the spec.versions.schema field. This schema is in JSON Schema format.

  2. Example CRD with Validation:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: myresources.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                name:
                  type: string
                  minLength: 1
                replicas:
                  type: integer
                  minimum: 1
  scope: Namespaced
  names:
    plural: myresources
    singular: myresource
    kind: MyResource
    shortNames:
      - mr

In this example, the spec field must have a name (which is a string) and replicas (which is an integer and must be at least 1).

  1. Validation during Creation/Update: When we create or update a custom resource, Kubernetes checks it against the schema we defined. If the resource does not fit the rules, the API server will not accept the request. It gives an error message.

  2. Error Messages: The error messages show us which field(s) did not pass the validation. This helps us understand and fix the problems.

  3. Testing Validation: We can test the CRD and its validation using kubectl commands:

kubectl apply -f myresource.yaml

If the resource does not pass validation, we will get a message that tells us what the errors are.

  1. Updating Validation: If we need to change the validation schema, we can update the CRD definition. We should be careful with versioning. Changes to the schema can impact resources we already have.

By defining and validating CRDs well, we help keep data safe and enforce the right settings in our Kubernetes environment. For more details on CRDs and how to manage them, check out the Kubernetes Operators article.

How to Implement Controllers for Custom Resources?

To implement controllers for Custom Resource Definitions (CRDs) in Kubernetes, we can follow these steps:

  1. Understand the Controller Pattern: A controller looks for changes in custom resources. It acts based on the state of these resources. Each controller has a main loop that works with the Kubernetes API.

  2. Set Up Your Environment: Make sure we have the Go programming language and Kubernetes client libraries installed. We can use the client-go library to work with the Kubernetes API.

  3. Create Your Custom Resource: Define your CRD if we haven’t done this yet. For example, a CRD for a Foo resource might look like this:

    apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: foos.example.com
spec:
  group: example.com
  versions:
  - name: v1
    served: true
    storage: true
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              foo: 
                type: string
  scope: Namespaced
  names:
    plural: foos
    singular: foo
    kind: Foo
    shortNames:
    - f

  4. Write the Controller Logic: We create a Go application with a main loop to watch for changes in the CRD resources. Here’s a simple example of what the controller might look like:

    package main

import (
    "context"
    "log"
    "time"

    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/tools/clientcmd"
    "k8s.io/client-go/util/retry"
)

func main() {
    config, err := clientcmd.BuildConfigFromFlags("", "<path-to-kubeconfig>")
    if err != nil {
        log.Fatal(err)
    }

    clientset, err := kubernetes.NewForConfig(config)
    if err != nil {
        log.Fatal(err)
    }

    for {
        // Watch for changes in custom resources
        foos, err := clientset.CustomResourceDefinition("foos.example.com").List(context.TODO(), metav1.ListOptions{})
        if err != nil {
            log.Fatalf("Error listing foos: %v", err)
        }

        for _, foo := range foos.Items {
            // We implement logic for each foo resource
            log.Printf("Found foo: %s", foo.Name)
        }

        time.Sleep(10 * time.Second)
    }
}

  5. Build and Deploy the Controller: Compile the code and create a Docker image. We can deploy the controller in our Kubernetes cluster using a Deployment resource.

  6. Use Informers: For better efficiency, we can use informers. Informers give a higher-level API for managing the lifecycle of resources. They also reduce the number of API calls to the Kubernetes API server.

  7. Handle Events: We need to implement event handlers for create, update, and delete operations on our CRD. This helps us manage the state of our resources better.

  8. Deploy the Controller: Make sure the controller runs in our Kubernetes cluster. It will manage the lifecycle of our custom resources.

For more information about building Kubernetes operators that automate tasks, we might find this article helpful: What are Kubernetes Operators and how do they automate tasks?.

Best Practices for Designing Custom Resource Definitions

When we design Custom Resource Definitions (CRDs) in Kubernetes, it is important to follow some best practices. This helps to make our resources reliable, easy to maintain, and user-friendly. Here are some key guidelines we can follow:

  1. Naming Conventions:
    • We should use plural names for CRDs. For example, we use applications instead of application.
    • We need to stick to a naming scheme that shows what the resource does.
  2. Group and Versioning:
    • We define a unique API group for our CRD. This helps to organize and keep track of different versions of our resources.
    • We can use semantic versioning like v1alpha1, v1beta1, or v1 to show how stable it is.
  3. Schema Validation:
    • We should use OpenAPI v3 schema validation. This helps to enforce how our custom resources should look and what data types they can have.
    • Here is an example of a CRD schema:
    spec:
  validation:
    openAPIV3Schema:
      type: object
      properties:
        spec:
          type: object
          properties:
            replicas:
              type: integer
              minimum: 1
  4. Use Annotations and Labels:
    • We can use annotations for metadata. Labels help us select specific resources.
    • It is important to use labels correctly. This makes it easier to search and manage our resources.
  5. Status Management:
    • We can use the status subresource to give feedback about the state of our custom resource.
    • We should update status fields to show the current state. This makes it better for users.
  6. Keep It Simple:
    • We need to avoid making things too complex. Let’s focus on the main features that users need.
    • Our CRD should be easy to understand and use for developers.
  7. Documentation:
    • We should provide clear documentation for our CRDs. This includes examples and how to use them.
    • It is important to explain what each field in the CRD schema does.
  8. Versioning Strategy:
    • We need to plan for future changes. A versioning strategy helps to keep old versions working.
    • We can use kubectl convert to help move resources between different versions.
  9. Testing:
    • We should do thorough testing for our CRDs. This means running unit tests and integration tests.
    • It is good to check how the CRD works in different Kubernetes setups.
  10. Monitoring and Observability:
    • We can add metrics and logging to see how our CRDs behave.
    • Tools like Prometheus and Grafana can help us see the metrics related to custom resources.

If we follow these best practices, we can make sure our Custom Resource Definitions in Kubernetes are strong, easy to use, and can be maintained well over time. For more information on Kubernetes components, we can check this article on what are the key components of a Kubernetes cluster.

Frequently Asked Questions

1. What are Custom Resource Definitions (CRDs) in Kubernetes?

Custom Resource Definitions, or CRDs, in Kubernetes help us add new types of resources. This means we can create resources that work like regular Kubernetes objects. It gives us more options to customize. If we want to know more about how CRDs work, we can read our article on what are Kubernetes operators and how do they automate tasks.

2. How do I create a Custom Resource Definition in Kubernetes?

To create a Custom Resource Definition in Kubernetes, we need to write a YAML file. This file shows the CRD’s structure. It includes its kind, version, and schema. After we have the file, we can run the command kubectl apply -f <filename>.yaml. This step is important to make Kubernetes work for our needs.

3. How can I validate Custom Resource Definitions?

To validate Custom Resource Definitions (CRDs), we use validation schemas with OpenAPI v3. We can write the validation rules right in our CRD YAML file under the spec.validation section. This way, any custom resources we create will follow the set rules. It helps us check for errors and makes our work more reliable.

4. What tools can I use to manage Custom Resources in Kubernetes?

We can manage Custom Resources in Kubernetes with the kubectl command-line tool. It lets us create, update, delete, and list our custom resources just like we do with regular Kubernetes objects. We can also use special commands for CRDs to manage them better. If we want to learn more about kubectl, we can check our article on what is kubectl and how do I use it to manage Kubernetes.

5. What are some common use cases for Custom Resource Definitions in Kubernetes?

Some common uses for Custom Resource Definitions (CRDs) in Kubernetes are managing complex applications, making new controllers, and automating tasks in infrastructure. By making CRDs, we can adjust Kubernetes to handle our specific jobs better and work with different tools and services. This helps us be more efficient. For more details on managing applications, we can read our article on how to deploy a simple web application on Kubernetes.