[feat][kubectl-plugin] add scale command

to scale a RayCluster's worker group.

closes #110

## Example Usage

```console
$ kubectl ray scale cluster -h                                                                                                                (base)
Scale a Ray cluster's worker group.

Usage:
  ray scale cluster (WORKERGROUP) (-c/--ray-cluster RAYCLUSTER) (-r/--replicas N) [flags]

Examples:
  # Scale a Ray cluster's worker group to 3 replicas
  kubectl ray scale cluster my-workergroup --ray-cluster my-raycluster --replicas 3

$ kubectl ray scale default-group --ray-cluster NONEXISTENT --replicas 0
Error: failed to scale worker group default-group in Ray cluster NONEXISTENT in namespace default: rayclusters.ray.io "NONEXISTENT" not found

$ kubectl ray scale DEADBEEF --ray-cluster dxia-test --replicas 1
Error: worker group DEADBEEF not found in Ray cluster dxia-test in namespace default. Available worker groups: default-group, another-group, yet-another-group

$ kubectl ray scale default-group --ray-cluster dxia-test --replicas 3
Scaled worker group default-group in Ray cluster dxia-test in namespace default from 0 to 3 replicas

$ kubectl ray scale default-group --ray-cluster dxia-test --replicas 1
Scaled worker group default-group in Ray cluster dxia-test in namespace default from 3 to 1 replicas

$ kubectl ray scale default-group --ray-cluster dxia-test --replicas -1
Error: must specify -r/--replicas with a non-negative integer
```

Signed-off-by: David Xia <david@davidxia.com>

kubectl-plugin/pkg/cmd/ray.go

@@ -10,6 +10,7 @@ import (
 	"github.com/ray-project/kuberay/kubectl-plugin/pkg/cmd/get"
 	"github.com/ray-project/kuberay/kubectl-plugin/pkg/cmd/job"
 	"github.com/ray-project/kuberay/kubectl-plugin/pkg/cmd/log"
+	"github.com/ray-project/kuberay/kubectl-plugin/pkg/cmd/scale"
 	"github.com/ray-project/kuberay/kubectl-plugin/pkg/cmd/session"
 	"github.com/ray-project/kuberay/kubectl-plugin/pkg/cmd/version"
 )
@@ -35,6 +36,7 @@ func NewRayCommand(streams genericiooptions.IOStreams) *cobra.Command {
 	cmd.AddCommand(version.NewVersionCommand(streams))
 	cmd.AddCommand(create.NewCreateCommand(streams))
 	cmd.AddCommand(kubectlraydelete.NewDeleteCommand(streams))
+	cmd.AddCommand(scale.NewScaleCommand(streams))
 
 	return cmd
 }

kubectl-plugin/pkg/cmd/scale/scale.go

@@ -0,0 +1,26 @@
+package scale
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/spf13/cobra"
+	"k8s.io/cli-runtime/pkg/genericclioptions"
+)
+
+func NewScaleCommand(streams genericclioptions.IOStreams) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:          "scale",
+		Short:        "Scale a Ray resource",
+		SilenceUsage: true,
+		Run: func(cmd *cobra.Command, args []string) {
+			if len(args) > 0 {
+				fmt.Println(fmt.Errorf("unknown command(s) %q", strings.Join(args, " ")))
+			}
+			cmd.HelpFunc()(cmd, args)
+		},
+	}
+
+	cmd.AddCommand(NewScaleClusterCommand(streams))
+	return cmd
+}

kubectl-plugin/pkg/cmd/scale/scale_cluster.go

@@ -0,0 +1,144 @@
+package scale
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/ray-project/kuberay/kubectl-plugin/pkg/util"
+	"github.com/ray-project/kuberay/kubectl-plugin/pkg/util/client"
+	"github.com/spf13/cobra"
+	"k8s.io/cli-runtime/pkg/genericclioptions"
+	"k8s.io/kubectl/pkg/util/templates"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	cmdutil "k8s.io/kubectl/pkg/cmd/util"
+)
+
+type ScaleClusterOptions struct {
+	configFlags *genericclioptions.ConfigFlags
+	ioStreams   *genericclioptions.IOStreams
+	replicas    *int32
+	workerGroup string
+	cluster     string
+}
+
+var (
+	scaleLong = templates.LongDesc(`
+		Scale a Ray cluster's worker group.
+	`)
+
+	scaleExample = templates.Examples(`
+		# Scale a Ray cluster's worker group to 3 replicas
+		kubectl ray scale cluster my-workergroup --ray-cluster my-raycluster --replicas 3
+	`)
+)
+
+func NewScaleClusterOptions(streams genericclioptions.IOStreams) *ScaleClusterOptions {
+	return &ScaleClusterOptions{
+		configFlags: genericclioptions.NewConfigFlags(true),
+		ioStreams:   &streams,
+		replicas:    new(int32),
+	}
+}
+
+func NewScaleClusterCommand(streams genericclioptions.IOStreams) *cobra.Command {
+	options := NewScaleClusterOptions(streams)
+	cmdFactory := cmdutil.NewFactory(options.configFlags)
+
+	cmd := &cobra.Command{
+		Use:          "cluster (WORKERGROUP) (-c/--ray-cluster RAYCLUSTER) (-r/--replicas N)",
+		Short:        "Scale a Ray cluster's worker group",
+		Long:         scaleLong,
+		Example:      scaleExample,
+		SilenceUsage: true,
+		Args:         cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if err := options.Complete(args); err != nil {
+				return err
+			}
+			if err := options.Validate(); err != nil {
+				return err
+			}
+			k8sClient, err := client.NewClient(cmdFactory)
+			if err != nil {
+				return fmt.Errorf("failed to create client: %w", err)
+			}
+			return options.Run(cmd.Context(), k8sClient, os.Stdout)
+		},
+	}
+
+	cmd.Flags().StringVarP(&options.cluster, "ray-cluster", "c", "", "Ray cluster of the worker group")
+	cobra.CheckErr(cmd.MarkFlagRequired("ray-cluster"))
+	cmd.Flags().Int32VarP(options.replicas, "replicas", "r", -1, "Desired number of replicas in worker group")
+	options.configFlags.AddFlags(cmd.Flags())
+	return cmd
+}
+
+func (options *ScaleClusterOptions) Complete(args []string) error {
+	if *options.configFlags.Namespace == "" {
+		*options.configFlags.Namespace = "default"
+	}
+
+	options.workerGroup = args[0]
+
+	return nil
+}
+
+func (options *ScaleClusterOptions) Validate() error {
+	config, err := options.configFlags.ToRawKubeConfigLoader().RawConfig()
+	if err != nil {
+		return fmt.Errorf("error retrieving raw config: %w", err)
+	}
+
+	if !util.HasKubectlContext(config, options.configFlags) {
+		return fmt.Errorf("no context is currently set, use %q or %q to select a new one", "--context", "kubectl config use-context <context>")
+	}
+
+	if options.cluster == "" {
+		return fmt.Errorf("must specify -c/--ray-cluster")
+	}
+
+	if options.replicas == nil || *options.replicas < 0 {
+		return fmt.Errorf("must specify -r/--replicas with a non-negative integer")
+	}
+
+	return nil
+}
+
+func (options *ScaleClusterOptions) Run(ctx context.Context, k8sClient client.Client, writer io.Writer) error {
+	cluster, err := k8sClient.RayClient().RayV1().RayClusters(*options.configFlags.Namespace).Get(ctx, options.cluster, metav1.GetOptions{})
+	if err != nil {
+		return fmt.Errorf("failed to scale worker group %s in Ray cluster %s in namespace %s: %w", options.workerGroup, options.cluster, *options.configFlags.Namespace, err)
+	}
+
+	// find the index of the worker group
+	var workerGroups []string
+	workerGroupIndex := -1
+	for i, workerGroup := range cluster.Spec.WorkerGroupSpecs {
+		workerGroups = append(workerGroups, workerGroup.GroupName)
+		if workerGroup.GroupName == options.workerGroup {
+			workerGroupIndex = i
+		}
+	}
+	if workerGroupIndex == -1 {
+		return fmt.Errorf("worker group %s not found in Ray cluster %s in namespace %s. Available worker groups: %s", options.workerGroup, options.cluster, *options.configFlags.Namespace, strings.Join(workerGroups, ", "))
+	}
+
+	previousReplicas := *cluster.Spec.WorkerGroupSpecs[workerGroupIndex].Replicas
+	if previousReplicas == *options.replicas {
+		fmt.Fprintf(writer, "worker group %s in Ray cluster %s in namespace %s already has %d replicas. Skipping\n", options.workerGroup, options.cluster, *options.configFlags.Namespace, previousReplicas)
+		return nil
+	}
+
+	cluster.Spec.WorkerGroupSpecs[workerGroupIndex].Replicas = options.replicas
+	_, err = k8sClient.RayClient().RayV1().RayClusters(*options.configFlags.Namespace).Update(ctx, cluster, metav1.UpdateOptions{})
+	if err != nil {
+		return fmt.Errorf("failed to scale worker group %s in Ray cluster %s in namespace %s: %w", options.workerGroup, options.cluster, *options.configFlags.Namespace, err)
+	}
+
+	fmt.Fprintf(writer, "Scaled worker group %s in Ray cluster %s in namespace %s from %d to %d replicas\n", options.workerGroup, options.cluster, *options.configFlags.Namespace, previousReplicas, *options.replicas)
+	return nil
+}