Fleet Autoscaler Specification

A full FleetAutoscaler specification is available below and in the example folder for reference :

apiVersion: "autoscaling.agones.dev/v1"
kind: FleetAutoscaler
# FleetAutoscaler Metadata
# https://v1-26.docs.kubernetes.io/docs/reference/generated/kubernetes-api/v1.26/#objectmeta-v1-meta
metadata:
  name: fleet-autoscaler-example
spec:
  # The name of the fleet to attach to and control. Must be an existing Fleet in the same namespace
  # as this FleetAutoscaler
  fleetName: fleet-example
  # The autoscaling policy
  policy:
    # type of the policy. for now, only Buffer is available
    type: Buffer
    # parameters of the buffer policy
    buffer:
      # Size of a buffer of "ready" game server instances
      # The FleetAutoscaler will scale the fleet up and down trying to maintain this buffer, 
      # as instances are being allocated or terminated
      # it can be specified either in absolute (i.e. 5) or percentage format (i.e. 5%)
      bufferSize: 5
      # minimum fleet size to be set by this FleetAutoscaler. 
      # if not specified, the actual minimum fleet size will be bufferSize
      minReplicas: 10
      # maximum fleet size that can be set by this FleetAutoscaler
      # required
      maxReplicas: 20
  # The autoscaling sync strategy
  sync:
    # type of the sync. for now, only FixedInterval is available
    type: FixedInterval
    # parameters of the fixedInterval sync
    fixedInterval:
      # the time in seconds between each auto scaling
      seconds: 30

Or for Webhook FleetAutoscaler below and in example folder :

apiVersion: "autoscaling.agones.dev/v1"
kind: FleetAutoscaler
metadata:
  name: webhook-fleet-autoscaler
spec:
  fleetName: simple-game-server
  policy:
    # type of the policy - this example is Webhook
    type: Webhook
    # parameters for the webhook policy - this is a WebhookClientConfig, as per other K8s webhooks
    webhook:
      # use a service, or URL
      service:
        name: autoscaler-webhook-service
        namespace: default
        path: scale
      # optional for URL defined webhooks
      # url: ""
      # caBundle:  optional, used for HTTPS webhook type
  # The autoscaling sync strategy
  sync:
    # type of the sync. for now, only FixedInterval is available
    type: FixedInterval
    # parameters of the fixedInterval sync
    fixedInterval:
      # the time in seconds between each auto scaling
      seconds: 30

Since Agones defines a new Custom Resources Definition (CRD) we can define a new resource using the kind FleetAutoscaler with the custom group autoscaling.agones.dev and API version v1

The spec field is the actual FleetAutoscaler specification and it is composed as follows:

• fleetName is name of the fleet to attach to and control. Must be an existing Fleet in the same namespace as this FleetAutoscaler.
• policy is the autoscaling policy
  • type is type of the policy. “Buffer” and “Webhook” are available
  • buffer parameters of the buffer policy type
    • bufferSize is the size of a buffer of “ready” and “reserved” game server instances. The FleetAutoscaler will scale the fleet up and down trying to maintain this buffer, as instances are being allocated or terminated. Note that “reserved” game servers could not be scaled down. It can be specified either in absolute (i.e. 5) or percentage format (i.e. 5%)
    • minReplicas is the minimum fleet size to be set by this FleetAutoscaler. if not specified, the minimum fleet size will be bufferSize if absolute value is used. When bufferSize in percentage format is used, minReplicas should be more than 0.
    • maxReplicas is the maximum fleet size that can be set by this FleetAutoscaler. Required.
  • webhook parameters of the webhook policy type
    • service is a reference to the service for this webhook. Either service or url must be specified. If the webhook is running within the cluster, then you should use service. Port 8000 will be used if it is open, otherwise it is an error.
      • name is the service name bound to Deployment of autoscaler webhook. Required (see example) The FleetAutoscaler will scale the fleet up and down based on the response from this webhook server
      • namespace is the kubernetes namespace where webhook is deployed. Optional If not specified, the “default” would be used
      • path is an optional URL path which will be sent in any request to this service. (i. e. /scale)
      • port is optional, it is the port for the service which is hosting the webhook. The default is 8000 for backward compatibility. If given, it should be a valid port number (1-65535, inclusive).
    • url gives the location of the webhook, in standard URL form ([scheme://]host:port/path). Exactly one of url or service must be specified. The host should not refer to a service running in the cluster; use the service field instead. (optional, instead of service)
    • caBundle is a PEM encoded certificate authority bundle which is used to issue and then validate the webhook’s server certificate. Base64 encoded PEM string. Required only for HTTPS. If not present HTTP client would be used.
  • Note: only one buffer or webhook could be defined for FleetAutoscaler which is based on the type field.
• sync is autoscaling sync strategy. It defines when to run the autoscaling
  • type is type of the sync. For now only “FixedInterval” is available
  • fixedInterval parameters of the fixedInterval sync
    • seconds is the time in seconds between each auto scaling

Webhook Endpoint Specification

Webhook endpoint is used to delegate the scaling logic to a separate pod or server.

FleetAutoscaler would send a request to the webhook endpoint every sync period (which is currently 30s) with a JSON body, and scale the target fleet based on the data that is returned. JSON payload with a FleetAutoscaleReview data structure would be sent to webhook endpoint and received from it with FleetAutoscaleResponse field populated. FleetAutoscaleResponse contains target Replica count which would trigger scaling of the fleet according to it.

In order to define the path to your Webhook you can use either URL or service. Note that caBundle parameter is required if you use HTTPS for webhook fleetautoscaler, caBundle should be omitted if you want to use HTTP webhook server.

The connection to this webhook endpoint should be defined in FleetAutoscaler using Webhook policy type.

// FleetAutoscaleReview is passed to the webhook with a populated Request value,
// and then returned with a populated Response.
type FleetAutoscaleReview struct {
	Request  *FleetAutoscaleRequest  `json:"request"`
	Response *FleetAutoscaleResponse `json:"response"`
}

type FleetAutoscaleRequest struct {
	// UID is an identifier for the individual request/response. It allows us to distinguish instances of requests which are
	// otherwise identical (parallel requests, requests when earlier requests did not modify etc)
	// The UID is meant to track the round trip (request/response) between the Autoscaler and the WebHook, not the user request.
	// It is suitable for correlating log entries between the webhook and apiserver, for either auditing or debugging.
	UID types.UID `json:"uid""`
	// Name is the name of the Fleet being scaled
	Name string `json:"name"`
	// Namespace is the namespace associated with the request (if any).
	Namespace string `json:"namespace"`
	// The Fleet's status values
	Status v1.FleetStatus `json:"status"`
}

type FleetAutoscaleResponse struct {
	// UID is an identifier for the individual request/response.
	// This should be copied over from the corresponding FleetAutoscaleRequest.
	UID types.UID `json:"uid"`
	// Set to false if no scaling should occur to the Fleet
	Scale bool `json:"scale"`
	// The targeted replica count
	Replicas int32 `json:"replicas"`
}

// FleetStatus is the status of a Fleet
type FleetStatus struct {
	// Replicas the total number of current GameServer replicas
	Replicas int32 `json:"replicas"`
	// ReadyReplicas are the number of Ready GameServer replicas
	ReadyReplicas int32 `json:"readyReplicas"`
	// ReservedReplicas are the total number of Reserved GameServer replicas in this fleet.
	// Reserved instances won't be deleted on scale down, but won't cause an autoscaler to scale up.
	ReservedReplicas int32 `json:"reservedReplicas"`
	// AllocatedReplicas are the number of Allocated GameServer replicas
	AllocatedReplicas int32 `json:"allocatedReplicas"`
}

For Webhook Fleetautoscaler Policy either HTTP or HTTPS could be used. Switching between them occurs depending on https presence in URL or by the presence of caBundle. The example of the webhook written in Go could be found here .

It implements the scaling logic based on the percentage of allocated gameservers in a fleet.