Fleet Autoscaler Specification

A FleetAutoscaler’s job is to automatically scale up and down a Fleet in response to demand.

A full FleetAutoscaler specification is available below and in the example folder for reference, but here are several examples that show different autoscaling policies.

Ready Buffer Autoscaling

Fleet autoscaling with a buffer can be used to maintain a configured number of game server instances ready to serve players based on number of allocated instances in a Fleet. The buffer size can be specified as an absolute number or a percentage of the desired number of Ready game server instances over the Allocated count.

apiVersion: "autoscaling.agones.dev/v1"
kind: FleetAutoscaler
# FleetAutoscaler Metadata
# https://v1-31.docs.kubernetes.io/docs/reference/generated/kubernetes-api/v1.31/#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

Counter and List Autoscaling

A Counter based autoscaler can be used to autoscale GameServers based on a Count and Capacity set on each of the GameServers in a Fleet to ensure there is always a buffer of available capacity available.

For example, if you have a game server that can support 10 rooms, and you want to ensure that there are always at least 5 rooms available, you could use a counter-based autoscaler with a buffer size of 5. The autoscaler would then scale the Fleet up or down based on the difference between the count of rooms across the Fleet and the capacity of rooms across the Fleet to ensure the buffer is maintained.

Counter-based FleetAutoscaler specification below and in the example folder :

apiVersion: autoscaling.agones.dev/v1
kind: FleetAutoscaler
metadata:
  name: fleet-autoscaler-counter
spec:
  fleetName: fleet-example
  policy:
    type: Counter  # Counter based autoscaling
    counter:
      # Key is the name of the Counter. Required field.
      key: rooms
      # BufferSize is the size of a buffer of counted items that are available in the Fleet (available capacity).
      # Value can be an absolute number (ex: 5) or a percentage of the Counter available capacity (ex: 5%).
      # An absolute number is calculated from percentage by rounding up. Must be bigger than 0. Required field.
      bufferSize: 5
      # MinCapacity is the minimum aggregate Counter total capacity across the fleet.
      # If BufferSize is specified as a percentage, MinCapacity is required and cannot be 0.
      # If non zero, MinCapacity must be smaller than MaxCapacity and must be greater than or equal to BufferSize.
      minCapacity: 10
      # MaxCapacity is the maximum aggregate Counter total capacity across the fleet.
      # MaxCapacity must be greater than or equal to both MinCapacity and BufferSize. Required field.
      maxCapacity: 100

A List based autoscaler can be used to autoscale GameServers based on the List length and Capacity set on each of the GameServers in a Fleet to ensure there is always a buffer of available capacity available.

For example, if you have a game server that can support 10 players, and you want to ensure that there are always room for at least 5 players across GameServers in a Fleet, you could use a list-based autoscaler with a buffer size of 5. The autoscaler would then scale the Fleet up or down based on the difference between the total length of the players and the total players capacity across the Fleet to ensure the buffer is maintained.

List-based FleetAutoscaler specification below and in the example folder :

apiVersion: autoscaling.agones.dev/v1
kind: FleetAutoscaler
metadata:
  name: fleet-autoscaler-list
spec:
  fleetName: fleet-example
  policy:
    type: List  # List based autoscaling.
    list:
      # Key is the name of the List. Required field.
      key: players
      # BufferSize is the size of a buffer based on the List capacity that is available over the current
      # aggregate List length in the Fleet (available capacity).
      # It can be specified either as an absolute value (i.e. 5) or percentage format (i.e. 5%).
      # Must be bigger than 0. Required field.
      bufferSize: 5
      # MinCapacity is the minimum aggregate List total capacity across the fleet.
      # If BufferSize is specified as a percentage, MinCapacity is required must be greater than 0.
      # If non-zero, MinCapacity must be smaller than MaxCapacity and must be greater than or equal to BufferSize.
      minCapacity: 10
      # MaxCapacity is the maximum aggregate List total capacity across the fleet.
      # MaxCapacity must be greater than or equal to both MinCapacity and BufferSize. Required field.
      maxCapacity: 100

Webhook Autoscaling

A webhook-based FleetAutoscaler can be used to delegate the scaling logic to a separate http based service. This can be useful if you want to use a custom scaling algorithm or if you want to integrate with other systems. For example, you could use a webhook-based FleetAutoscaler to scale your fleet based on data from a match-maker or player authentication system or a combination of systems.

Webhook based autoscalers have the added benefit of being able to scale a Fleet to 0 replicas, since they are able to scale up on demand based on an external signal before a GameServerAllocation is executed from a match-maker or similar system.

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.

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

See the Webhook Endpoint Specification for the specification of the incoming and outgoing JSON packet structure for the webhook endpoint.

Schedule and Chain Autoscaling

A schedule-based autoscaler can automatically scale GameServers at a specific time based on a FleetAutoscaler policy.

For example, if you have an in-game event happening at 12:00 AM PST on October 31, 2024 and you want to ensure that there are always at least 5 ready game servers at that specific time, you could use a schedule-based autoscaler that applies a buffer policy with a buffer size of 5.

Schedule-based FleetAutoscaler specification below and in the example folder :

apiVersion: autoscaling.agones.dev/v1
kind: FleetAutoscaler
metadata:
  name: schedule-fleet-autoscaler
spec:
  fleetName: fleet-example
  policy:
    # Schedule based policy for autoscaling.
    type: Schedule
    schedule:
      between:
        # The policy becomes eligible for application starting on October 31st, 2024 at 12:00 AM PST. If not set, the policy will immediately be eligible for application.
        start: "2024-10-31T00:00:00-07:00"
        # The policy is never ineligible for application. If not set, the policy will always be eligible for application (after the start time).
        end: ""
      activePeriod:
        # Use PST time for the startCron field. Defaults to UTC if not set.
        timezone: "America/Los_Angeles"
        # Start applying the policy everyday at 12:00 AM PST. If not set, the policy will always be applied in the .between window.
        # (Only eligible starting on October 31, 2024 at 12:00 AM PST).
        startCron: "0 0 * * *"
        # Apply this policy indefinitely. If not set, the duration will be defaulted to always/indefinite.
        duration: ""
      # Policy to be applied during the activePeriod. Required.
      policy:
        type: Buffer
        buffer:
          bufferSize: 5
          minReplicas: 10
          maxReplicas: 20
  # The autoscaling sync strategy, this will determine how frequent the schedule is evaluated.
  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

Note

While it’s possible to use multiple FleetAutoscalers to perform the same actions on a fleet, this approach can result in unpredictable scaling behavior and overwhelm the controller. Multiple FleetAutoscalers pointing to the same fleet can lead to conflicting schedules and complex management. To avoid these issues, it’s recommended to use the Chain Policy for defining scheduled scaling. This simplifies management and prevents unexpected scaling fluctuations.

A Chain-based autoscaler can be used to autoscale GameServers based on a list of policies. The index of each policy within the list determines its priority, with the first item in the chain having the highest priority and being evaluated first, followed by the second item, and so on. The order in the list is crucial as it directly affects the sequence in which policies are evaluated and applied.

For example, if you have an in-game event happening between 12:00 AM and 10:00 PM on October 31, 2024 EST, and you want to ensure there are always at least 5 ready game servers during that time, you chain add a schedule policy that applies a buffer policy with a buffer size of 5. Now, let’s say you want to adjust the number of game servers after the event based on external factors after the in-game event, you could chain a webhook policy to dynamically. Finally as a fallback, for if the webhook invocation fails, you can chain another buffer policy to default to.

Chain-based FleetAutoscaler specification below and in the example folder :

apiVersion: autoscaling.agones.dev/v1
kind: FleetAutoscaler
metadata:
  name: chain-fleet-autoscaler
spec:
  fleetName: fleet-example
  policy:
    # Chain based policy for autoscaling.
    type: Chain
    chain:
      # Id of chain entry. If not set, the Id will be defaulted to the index of the entry within the chain.
      - id: "in-game-event"
        type: Schedule
        schedule:
          between:
            # The policy becomes eligible for application starting on October 31st, 2024 at 12:00 AM PST. If not set, the policy will immediately be eligible for application.
            start: "2024-10-31T00:00:00-07:00"
            # The policy is no longer eligible for application starting on  October 31st, 2024 at 10:00 PM PST. If not set, the policy will always be eligible for application (after the start time).
            end: "2024-10-31T22:00:00-07:00"
          activePeriod:
            # Use PST time for the startCron field. Defaults to UTC if not set.
            timezone: "America/Los_Angeles"
            # Start applying the policy everyday at 1:00 AM PST. If not set, the policy will always be applied in the .between window.
            # (Only eligible starting on October 31, 2024 at 12:00 AM PST).
            startCron: "0 0 * * *"
            # Apply this policy indefinitely. If not set, the duration will be defaulted to always/indefinite.
            duration: ""
          # Policy to be applied during the activePeriod. Required.
          policy:
            type: Buffer
            buffer:
              bufferSize: 5
              minReplicas: 10
              maxReplicas: 20
      # Id of chain entry. If not set, the Id will be defaulted to the index of the entry within the chain list.
      - id: "webhook"
         type: Webhook
          webhook:
            service:
              name: autoscaler-webhook-service
              namespace: default
              path: scale
      # Id of chain entry. If not set, the Id will be defaulted to the index of the entry within the chain list.
      - id: "default"
        # Policy will always be applied when no other policy is applicable. Required.
        type: Buffer
        buffer:
          bufferSize: 2
          minReplicas: 5
          maxReplicas: 10
  # The autoscaling sync strategy, this will determine how frequent the chain is evaluated.
  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

Spec Field Reference

The spec field of the FleetAutoscaler 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.
- counter parameters of the counter policy type
  - counter contains the settings for counter-based autoscaling:
    - key is the name of the counter to use for scaling decisions.
    - bufferSize is the size of a buffer of counted items that are available in the Fleet (available capacity). Value can be an absolute number or a percentage of desired game server instances. An absolute number is calculated from percentage by rounding up. Must be bigger than 0.
    - minCapacity is the minimum aggregate Counter total capacity across the fleet. If zero, MinCapacity is ignored. If non zero, MinCapacity must be smaller than MaxCapacity and bigger than BufferSize.
    - maxCapacity is the maximum aggregate Counter total capacity across the fleet. It must be bigger than both MinCapacity and BufferSize.
- list parameters of the list policy type
  - list contains the settings for list-based autoscaling:
    - key is the name of the list to use for scaling decisions.
    - bufferSize is the size of a buffer based on the List capacity that is available over the current aggregate List length in the Fleet (available capacity). It can be specified either as an absolute value or percentage format.
    - minCapacity is the minimum aggregate List total capacity across the fleet. If zero, it is ignored. If non zero, it must be smaller than MaxCapacity and bigger than BufferSize.
    - maxCapacity is the maximum aggregate List total capacity across the fleet. It must be bigger than both MinCapacity and BufferSize. Required 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 autoscaling

Webhook Endpoint Specification

A webhook based FleetAutoscaler sends an HTTP POST request to the webhook endpoint every sync period (default is 30s) with a JSON body, and scale the target fleet based on the data that is returned.

The JSON payload that is sent is a FleetAutoscaleReview data structure and a FleetAutoscaleResponse data structure is expected to be returned.

The FleetAutoscaleResponse’s Replica field is used to set the target Fleet count with each sync interval, thereby providing the autoscaling functionality.

// 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"`
	// Standard object metadata; More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata.
	MetaData *metav1.ObjectMeta `json:"metadata,omitempty"`
}

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.

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Last modified April 15, 2025: Fix CRD API docs generation script (#4152) (a8a5950)