Access Agones via the Kubernetes API

It’s likely that we will want to programmatically interact with Agones. Everything that can be done via the kubectl and yaml configurations can also be done via the Kubernetes API.

Installing Agones creates several Custom Resource Definitions (CRD), which can be accessed and manipulated through the Kubernetes API.

The detailed list of Agones CRDs with their parameters could be found here - Agones CRD API Reference.

Kubernetes has multiple client libraries, however, at time of writing, only the Go and Python clients are documented to support accessing CRDs.

This can be found in the Accessing a custom resource section of the Kubernetes documentation.

At this time, we recommend interacting with Agones through the Go client that has been generated in this repository, but other methods may also work as well.

Go Client

Kubernetes Go Client tooling generates a Client for Agones that we can use to interact with the Agones installation on our Kubernetes cluster.

Authentication

This client uses the same authentication mechanisms as the Kubernetes API.

If you plan to run your code in the same cluster as the Agones install, have a look at the in cluster configuration example from the Kubernetes Client.

If you plan to run your code outside the Kubernetes cluster as your Agones install, look at the out of cluster configuration example from the Kubernetes client.

Example

The following is an example of a in-cluster configuration, that creates a Clientset for Agones and then creates a GameServer.

A full example code is available in the example folder .

package main

import (
	"fmt"
	"context"

	agonesv1 "agones.dev/agones/pkg/apis/agones/v1"
	"agones.dev/agones/pkg/client/clientset/versioned"
	"agones.dev/agones/pkg/util/runtime"
	corev1 "k8s.io/api/core/v1"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/rest"
)

func main() {
	config, err := rest.InClusterConfig()
	logger := runtime.NewLoggerWithSource("main")
	if err != nil {
		logger.WithError(err).Fatal("Could not create in cluster config")
	}

	// Access to standard Kubernetes resources through the Kubernetes Clientset
	// We don't actually need this for this example, but it's just here for
	// illustrative purposes
	kubeClient, err := kubernetes.NewForConfig(config)
	if err != nil {
		logger.WithError(err).Fatal("Could not create the kubernetes clientset")
	}

	// Access to the Agones resources through the Agones Clientset
	// Note that we reuse the same config as we used for the Kubernetes Clientset
	agonesClient, err := versioned.NewForConfig(config)
	if err != nil {
		logger.WithError(err).Fatal("Could not create the agones api clientset")
	}

	// Create a GameServer
	gs := &agonesv1.GameServer{ObjectMeta: metav1.ObjectMeta{GenerateName: "simple-game-server", Namespace: "default"},
		Spec: agonesv1.GameServerSpec{
			Container: "simple-game-server",
			Ports: []agonesv1.GameServerPort{{
				ContainerPort: 7654,
				HostPort:      7654,
				Name:          "gameport",
				PortPolicy:    agonesv1.Static,
				Protocol:      corev1.ProtocolUDP,
			}},
			Template: corev1.PodTemplateSpec{
				Spec: corev1.PodSpec{
					Containers: []corev1.Container{{Name: "simple-game-server", Image: "gcr.io/agones-images/simple-game-server:0.13"}},
				},
			},
		},
	}
	newGS, err := agonesClient.AgonesV1().GameServers("default").Create(context.TODO(), gs, metav1.CreateOptions{})
	if err != nil {
		panic(err)
	}

	fmt.Printf("New game servers' name is: %s", newGS.ObjectMeta.Name)
}

In order to create GS using provided example, you can run it as a Kubernetes Job:

kubectl create -f https://raw.githubusercontent.com/googleforgames/agones/main/examples/crd-client/create-gs.yaml --namespace agones-system
kubectl get pods --namespace agones-system
NAME                                 READY   STATUS      RESTARTS   AGE
create-gs-6wz86-7qsm5                0/1     Completed   0          6s
kubectl logs create-gs-6wz86-7qsm5  --namespace agones-system
{"message":"\u0026{0xc0001dde00 default}","severity":"info","source":"main","time":"2020-04-21T11:14:00.477576428Z"}
{"message":"New GameServer name is: helm-test-server-fxfgg","severity":"info","time":"2020-04-21T11:14:00.516024697Z"}

You have just created a GameServer using Kubernetes Go Client.

Best Practice: Using Informers and Listers

Almost all Kubernetes' controllers and custom controllers utilise Informers and Listers to reduce the load on the Kubernetes’s control plane.

Repetitive, direct access of the Kubernetes control plane API can significantly reduce the performance of the cluster – and Informers and Listers help resolving that issue.

Informers and Listers reduce the load on the Kubernetes control plane by creating, using and maintaining an eventually consistent an in-memory cache. This can be watched and also queried with zero cost, since it will only read against its in-memory model of the Kubernetes resources.

Informer’s role and Lister’s role are different.

An Informer is the mechanism for watching a Kubernetes object’s event, such that when a Kubernetes object changes(e.g. CREATE,UPDATE,DELETE), the Informer is informed, and can execute a callback with the relevant object as an argument.

This can be very useful for building event based systems against the Kubernetes API.

A Lister is the mechanism for querying Kubernetes object’s against the client side in-memory cache. Since the Lister stores objects in an in-memory cache, queries against a come at practically no cost.

Of course, Agones itself also uses Informers and Listers in its codebase.

Example

The following is an example of Informers and Listers, that show the GameServer’s name & status & IPs in the Kubernetes cluster.

package main

import (
	"context"
	"time"

	"agones.dev/agones/pkg/client/clientset/versioned"
	"agones.dev/agones/pkg/client/informers/externalversions"
	"agones.dev/agones/pkg/util/runtime"
	"k8s.io/apimachinery/pkg/labels"
	"k8s.io/client-go/informers"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/cache"
)

func main() {
	config, err := rest.InClusterConfig()
	logger := runtime.NewLoggerWithSource("main")
	if err != nil {
		logger.WithError(err).Fatal("Could not create in cluster config")
	}
	kubeClient, err := kubernetes.NewForConfig(config)
	agonesClient, err := versioned.NewForConfig(config)
	if err != nil {
		logger.WithError(err).Fatal("Could not create the agones api clientset")
	}

	// Create InformerFactory which create the informer
	informerFactory := informers.NewSharedInformerFactory(kubeClient, time.Second*30)
	agonesInformerFactory := externalversions.NewSharedInformerFactory(agonesClient, time.Second*30)

	// Create Pod informer by informerFactory
	podInformer := informerFactory.Core().V1().Pods()

	// Create GameServer informer by informerFactory
	gameServers := agonesInformerFactory.Agones().V1().GameServers()
	gsInformer := gameServers.Informer()

	// Add EventHandler to informer
	// When the object's event happens, the function will be called
	// For example, when the pod is added, 'AddFunc' will be called and put out the "Pod Added"
	podInformer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
		AddFunc:    func(new interface{}) { logger.Infof("Pod Added") },
		UpdateFunc: func(old, new interface{}) { logger.Infof("Pod Updated") },
		DeleteFunc: func(old interface{}) { logger.Infof("Pod Deleted") },
	})
	gsInformer.AddEventHandler(cache.ResourceEventHandlerFuncs{
		AddFunc:    func(new interface{}) { logger.Infof("GameServer Added") },
		UpdateFunc: func(old, new interface{}) { logger.Infof("GameServer Updated") },
		DeleteFunc: func(old interface{}) { logger.Infof("GameServer Deleted") },
	})

	ctx := context.Background()

	// Start Go routines for informer
	informerFactory.Start(ctx.Done())
	agonesInformerFactory.Start(ctx.Done())
	// Wait until finish caching with List API
	informerFactory.WaitForCacheSync(ctx.Done())
	agonesInformerFactory.WaitForCacheSync(ctx.Done())

	// Create Lister which can list objects from the in-memory-cache
	podLister := podInformer.Lister()
	gsLister := gameServers.Lister()

	for {
		// Get List objects of Pods from Pod Lister
		p := podLister.Pods("default")
		// Get List objects of GameServers from GameServer Lister
		gs, err := gsLister.List(labels.Everything())
		if err != nil {
			panic(err)
		}
		// Show GameServer's name & status & IPs
		for _, g := range gs {
			a, err := p.Get(g.GetName())
			if err != nil {
				panic(err)
			}
			logger.Infof("------------------------------")
			logger.Infof("Name: %s", g.GetName())
			logger.Infof("Status: %s", g.Status.State)
			logger.Infof("External IP: %s", g.Status.Address)
			logger.Infof("Internal IP: %s", a.Status.PodIP)
		}
		time.Sleep(time.Second * 25)
	}
}

You can list GameServer’s name and status and IPs using Kubernetes Informers and Listers.

Direct Access to the REST API via Kubectl

If there isn’t a client written in your preferred language, it is always possible to communicate directly with Kubernetes API to interact with Agones.

The Kubernetes API can be authenticated and exposed locally through the kubectl proxy

For example:

kubectl proxy &
Starting to serve on 127.0.0.1:8001

List all Agones endpoints

curl http://localhost:8001/apis | grep agones -A 5 -B 5
{
    "name": "agones.dev",
    "versions": [
    {
        "groupVersion": "agones.dev/v1",
        "version": "v1"
    }
    ],
    "preferredVersion": {
    "groupVersion": "agones.dev/v1",
    "version": "v1"
    },
    "serverAddressByClientCIDRs": null
}

List Agones resources

curl http://localhost:8001/apis/agones.dev/v1
{
  "kind": "APIResourceList",
  "apiVersion": "v1",
  "groupVersion": "agones.dev/v1",
  "resources": [
    {
      "name": "gameservers",
      "singularName": "gameserver",
      "namespaced": true,
      "kind": "GameServer",
      "verbs": [
        "delete",
        "deletecollection",
        "get",
        "list",
        "patch",
        "create",
        "update",
        "watch"
      ],
      "shortNames": [
        "gs"
      ]
    }
  ]
}

List all gameservers in the default namespace

curl http://localhost:8001/apis/agones.dev/v1/namespaces/default/gameservers
{
    "apiVersion": "agones.dev/v1",
    "items": [
        {
            "apiVersion": "agones.dev/v1",
            "kind": "GameServer",
            "metadata": {
                "annotations": {
                    "kubectl.kubernetes.io/last-applied-configuration": "{\"apiVersion\":\"agones.dev/v1\",\"kind\":\"GameServer\",\"metadata\":{\"annotations\":{},\"name\":\"simple-game-server\",\"namespace\":\"default\"},\"spec\":{\"containerPort\":7654,\"hostPort\":7777,\"portPolicy\":\"static\",\"template\":{\"spec\":{\"containers\":[{\"image\":\"gcr.io/agones-images/simple-game-server:0.13\",\"name\":\"simple-game-server\"}]}}}}\n"
                },
                "clusterName": "",
                "creationTimestamp": "2018-03-02T21:41:05Z",
                "finalizers": [
                    "agones.dev"
                ],
                "generation": 0,
                "name": "simple-game-server",
                "namespace": "default",
                "resourceVersion": "760",
                "selfLink": "/apis/agones.dev/v1/namespaces/default/gameservers/simple-game-server",
                "uid": "692beea6-1e62-11e8-beb2-080027637781"
            },
            "spec": {
                "PortPolicy": "Static",
                "container": "simple-game-server",
                "containerPort": 7654,
                "health": {
                    "failureThreshold": 3,
                    "initialDelaySeconds": 5,
                    "periodSeconds": 5
                },
                "hostPort": 7777,
                "protocol": "UDP",
                "template": {
                    "metadata": {
                        "creationTimestamp": null
                    },
                    "spec": {
                        "containers": [
                            {
                                "image": "gcr.io/agones-images/simple-game-server:0.13",
                                "name": "simple-game-server",
                                "resources": {}
                            }
                        ]
                    }
                }
            },
            "status": {
                "address": "192.168.99.100",
                "nodeName": "agones",
                "port": 7777,
                "state": "Ready"
            }
        }
    ],
    "kind": "GameServerList",
    "metadata": {
        "continue": "",
        "resourceVersion": "1062",
        "selfLink": "/apis/agones.dev/v1/namespaces/default/gameservers"
    }
}

Allocate a gameserver from a fleet named ‘simple-game-server’, with GameServerAllocation

curl -d '{"apiVersion":"allocation.agones.dev/v1","kind":"GameServerAllocation","spec":{"required":{"matchLabels":{"agones.dev/fleet":"simple-game-server"}}}}' -H "Content-Type: application/json" -X POST http://localhost:8001/apis/allocation.agones.dev/v1/namespaces/default/gameserverallocations
{
    "kind": "GameServerAllocation",
    "apiVersion": "allocation.agones.dev/v1",
    "metadata": {
        "name": "simple-game-server-v6jwb-cmdcv",
        "namespace": "default",
        "creationTimestamp": "2019-07-03T17:19:47Z"
    },
    "spec": {
        "multiClusterSetting": {
            "policySelector": {}
        },
        "required": {
            "matchLabels": {
                "agones.dev/fleet": "simple-game-server"
            }
        },
        "scheduling": "Packed",
        "metadata": {}
    },
    "status": {
        "state": "Allocated",
        "gameServerName": "simple-game-server-v6jwb-cmdcv",
        "ports": [
            {
                "name": "default",
                "port": 7445
            }
        ],
        "address": "34.94.118.237",
        "nodeName": "gke-test-cluster-default-f11755a7-5km3"
    }
}

You may wish to review the Agones Kubernetes API for the full data structure reference.

The Kubernetes API Concepts section may also provide the more details on the API conventions that are used in the Kubernetes API.

Next Steps


Last modified June 23, 2022: Delete Katacoda link because Katacoda is closed (#2640) (079dced)