Extending the Kubernetes API

How to extend the Kubernetes API using CustomResourceDefinitions and CustomResources. Also, how to write a custom controller to operate on these CustomResources.

Presented as a talk at Kubernetes Pune meetup.

Transcript

1. Extending the Kubernetes API
   Thanks to Ashley McNamara for the wonderful artwork!
   

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2. Who am I?
   ● Hi, I am Nikhita Raghunath!
   ● Google Summer of Code student for CNCF
   (Kubernetes)
   ● Undergrad student from VJTI, Mumbai
   You can find me on:
   ➔ Twitter: @TheNikhita
   ➔ Github: nikhita
   ➔ Website: https://www.nikhita.github.io
   @TheNikhita
   

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3. What comes to your mind when you
   think of Kubernetes?
   @TheNikhita
   

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4. What comes to your mind when you
   think of Kubernetes?
   Pods
   @TheNikhita
   

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5. What comes to your mind when you
   think of Kubernetes?
   Pods Deployments
   @TheNikhita
   

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6. What comes to your mind when you
   think of Kubernetes?
   Pods Deployments Services
   @TheNikhita
   

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7. What comes to your mind when you
   think of Kubernetes?
   Pods Deployments Services StatefulSets
   DaemonSet CronJob ConfigMap
   Secrets Node PersistentVolumeClaim
   @TheNikhita
   

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8. What comes to your mind when you
   think of Kubernetes?
   Pods Deployments Services StatefulSets
   DaemonSet CronJob ConfigMap
   Secrets Node PersistentVolumeClaim
   @TheNikhita
   

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9. We are going to build our own objects!
   @TheNikhita
   

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10. We are going to build our own objects!
    custom
    @TheNikhita
    

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11. We are going to build our own
    custom resources!
    @TheNikhita
    

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12. We are going to build our own
    custom resources!
    That’s cool. But...why?
    Operator pattern: encode domain knowledge for specific
    applications as an extension of Kubernetes API.
    @TheNikhita
    

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13. Step 1: Tell Kubernetes about our object
    CustomResourceDefinition
    Step 2: Create the custom object
    CustomResource
    @TheNikhita
    

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14. CustomResourceDefinition
    @TheNikhita
    

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15. CustomResource
    @TheNikhita
    

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16. DEMO TIME!
    @TheNikhita
    

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17. What now?
    Let’s create a
    CONTROLLER
    (yes, you’ll have to write some code)
    @TheNikhita
    

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18. CONTROLLER
    ● Active reconciliation process
    Current state Desired state
    Status Spec
    @TheNikhita
    

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19. What will our controller do?
    1. Try to make the current state same as the desired state.
    2. Do something extra on add, update, delete of a resource.
    Example:
    - Write a message in a CustomResource.
    - Create the CustomResource.
    - A Github comment is created with this message.
    Link: https://github.com/nikhita/kube-custom-controller
    (inspired from https://github.com/munnerz/k8s-api-pager-demo)
    @TheNikhita
    

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20. Kubernetes API structure
    Internal version External version(s)
    @TheNikhita
    

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21. Step 1: Create internal types
    Inside pkg/apis/github
    - doc.go
    - register.go
    - types.go
    Step 2: Create external types
    Inside pkg/apis/github/v1
    - doc.go
    - register.go
    - types.go
    Step 3: Install your API group
    Inside
    pkg/apis/github/install
    - install.go
    @TheNikhita
    

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22. Step 4: GENERATE CODE!
    Use https://github.com/kubernetes/code-generator
    1. client-gen
    2. conversion-gen
    3. deepcopy-gen
    4. defaulter-gen
    5. informer-gen
    6. lister-gen
    @TheNikhita
    

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23. Step 5: Create the controller
    1. Create clients:
    - kubernetes client => kubeconfig file
    - github client => github API token
    2. Create an informer:
    Use sharedInformers:
    - notify or “inform” about add/update/delete of a resource
    - use a common cache across all controllers
    @TheNikhita
    

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24. Step 5: Create the controller
    3. Add event handler to the informer:
    - to tell what happens on add/update/delete
    - here, we will put it into a queue
    4. Start the informer
    - wait for the informer cache sync across all resources
    @TheNikhita
    

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25. Step 5: Create the controller
    5. Take the object from the queue:
    - take the object from the queue
    - get it’s latest version
    - sync it!
    6. Sync it! (current state -> desired state):
    - do some action. Here, we send a Github comment.
    - update the status (the current state)
    - update the resource
    @TheNikhita
    

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26. That’s it!
    We learnt how to:
    - Create a CustomResourceDefinition
    - Create a CustomResource
    - Write your own controller to use the CustomResource!
    If you have even more specific needs, you should use Aggregated API
    Servers....but we won’t be discussing that today.
    @TheNikhita
    

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27. Thanks for coming!!!!
    

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