apricote.de/content/posts/ccm-from-scratch/part-3.md

author	title	date	description	summary	tags	ShowToc	ShowReadingTime	ShowBreadCrumbs
Julian Tölle	Part 3: Implementing Controller: Node	2023-09-06	Goal: Implement the Node controller with all features, deploy & verify.	Implement the Node controller with all features, deploy & verify.	kubernetes	true	true	true

This is part 3 out of 5 of the series [Building a Kubernetes cloud-controller-manager]({{< ref "." >}}).

Now that our side-quest is finished, we can finally go back to the part why you are all here: Implementing the controllers of our cloud-controller-manager.

This part focuses on the Node controller. To [recap]({{< ref ".#what-is-a-cloud-controller-manager" >}}) its functionality:

• Node

  • Match Kubernetes Node to Cloud API Server (/VM/Instance/Droplet/...) and set spec.providerID
  • Label Kubernetes Nodes with the Instance Type and Topology info (Region, Zone)
  • Figure out the Network addresses of the Node and set them in status.addresses.
  • Mark Node as initialized by removing the node.cloudprovider.kubernetes.io/uninitialized taint.

• Node Lifecycle

  The kubelet is usually responsible for creating the Node object in Kubernetes on its first startup. This does not work well for the removal of the node, as the kubelet might not get a chance to do this. Instead, the Node Lifecycle controller regularly checks if the Node was deleted in the Cloud API, and if it was it also deletes the Node object in Kubernetes.

Luck has is, k8s.io/cloud-provider already implements most of the logic, so we can focus on the integration with our Cloud Provider (Hetzner Cloud in my case). Similar to the general cloudprovider.Interface from [Part 1]({{< ref "part-1.md#cloudproviderinterface" >}}), this post focuses on the cloudprovider.InstancesV2 interface.

Lets start by stubbing out the interface in a new file:

package ccm
// ccm/instances_v2.go

import (
  "context"
  
  cloudprovider "k8s.io/cloud-provider"
	corev1 "k8s.io/api/core/v1"
)

type InstancesV2 struct {}

// Let's try to assign our struct to a var of the interface we try to implement.
// This way we get nice feedback from the compiler (and the IDE) if we break the contract.
var _ cloudprovider.InstancesV2 = InstancesV2{}

func (i *InstancesV2) InstanceExists(ctx context.Context, node *corev1.Node) (bool, error) { 
	// We return true for now, to avoid CCM removing any Nodes from the cluster.
	return true, nil
}

func (i *InstancesV2) InstanceShutdown(ctx context.Context, node *corev1.Node) (bool, error) {
	return false, nil
}

func (i *InstancesV2) InstanceMetadata(ctx context.Context, node *corev1.Node) (*cloudprovider.InstanceMetadata, error) {
	return nil, nil
}

Creating our Cloud Provider Client

Before we can make any requests to our Cloud Provider API, we need to read some credentials and create the client. As mentioned in Part 1, we are going to use hcloud-go here, but you should use a client for your API. Because we also need the client in other controllers, we will create it in ccm.NewCloudProvider() and save it to the CloudProvider struct.

package ccm
// ccm/cloud.go

import (
	"os"
	
	"github.com/hetznercloud/hcloud-go/v2/hcloud"
)

type CloudProvider struct {
	client *hcloud.Client
}

func NewCloudProvider(_ *config.CompletedConfig) cloudprovider.Interface {
	// In Part 2 we set up a Kubernetes Secret with our Hetzner Cloud API token.
	// We mounted it as the environment variable HCLOUD_TOKEN into our Pod.
	token := os.Getenv("HCLOUD_TOKEN")
	
	client := hcloud.NewClient(
    hcloud.WithToken(token),
	  // Setting a custom user agent
	  hcloud.WithApplication("ccm-from-scratch", ""),
  )
	
	
  return CloudProvider{client: client}
}

We can now initialize the InstancesV2 struct and pass the client to it:

package ccm

// ccm/instances_v2.go
type InstancesV2 struct {
	client *hcloud.Client
}

// ccm/cloud.go
func (c CloudProvider) InstancesV2() (cloudprovider.InstancesV2, bool) {
  return InstancesV2{client: c.client}, true
}

When we start our program now, it should detect that the InstancesV2 interface is supported and start the Node & NodeLifecycle controllers.

Matching Up Nodes

For all methods in this interface, we need to ask the Cloud API about details of our server. For this, we need to somehow match the Kubernetes Node with its Cloud API server equivalent.

On the first run, we need to use some heuristic to make this match. All methods in the interface receive the Node as a parameter, so we could use any of its fields, like the Name, IP, Labels or Annotations. To keep this simple, we are going to use the Node name and expect that the server in the Hetzner Cloud API has the same name.

In InstanceMetadata we return a ProviderID, which is written to Node.spec.providerID by k/cloud-provider. After the node was initialized by us, we can therefore use the ID we previously saved, and do not need to keep using the name heuristic. We are going to use the format ccm-from-scratch://$ID for our ProviderID. By using a unique prefix, we can avoid issues where users accidentally install multiple cloud-providers into their cluster.

InstanceMetadata

The InstanceMetadata method is called by the Node controller when the Node is initialized for the first time.

To get started, we need to get the Server from the API, using the method described in the previous section. Matching up by name is pretty easy:

package ccm

// ccm/instances_v2.go

import (
	"fmt"
)

const (
	// A constant for the ProviderID. If you want, you can also update `CloudProvider.ProviderName` to use it.
	providerName = "ccm-from-scratch"
)

func (i *InstancesV2) InstanceMetadata(ctx context.Context, node *corev1.Node) (*cloudprovider.InstanceMetadata, error) {
	var server *hcloud.Server
	var err error

	if node.Spec.ProviderID == "" {
	  // If no ProviderID was set yet, we use the name to get the right server
	  // Luckily this is pretty easy with hcloud-go:
	  server, _, err = i.client.Server.GetByName(ctx, node.Name)
	  if err != nil {
		  return nil, err
	  }
  } else {
	  // If the Node was previously initialized, it should have a ProviderID set.
	  // We can parse it to get the ID and then use that to get the server from the API.
	  providerID, found := strings.CutPrefix(node.Spec.ProviderID, fmt.Sprintf("%s://", providerName))
	  if !found {
		  return nil, fmt.Errorf("ProviderID does not follow expected format: %s", node.Spec.ProviderID)
	  }

	  id, err := strconv.ParseInt(providerID, 10, 64)
	  if err != nil {
		  return nil, fmt.Errorf("unable to parse ProviderID to integer: %w", err)
	  }
	  
	  server, _, err = i.client.Server.GetByID(ctx, id)

	  // TODO error handling

  }
  
  // TODO metadata return, possibly in another code block
  
  return nil, nil
}

InstanceExists

InstanceExists is being called by the NodeLifecycle controller. Based on the return value the Node will be deleted. It is probably the easiest out of the three in this interface, as we only need to return true if a server matching the Node exists in the API, and false otherwise. So let's implement it first.

Refactoring

InstanceShutdown

Testing (in prod!)