Load-based Auto Scaling

Overview

Responding to fluctuating service demand is a common challenge for maintaining stable and responsive services. This policy introduces a mechanism to dynamically scale service resources based on observed load, optimizing resource allocation and ensuring that the service remains responsive even under high load.

This policy employs two key strategies: service protection and auto-scaling.

1. Service Protection: Based on the trend of observed latency, the service gets protected from sudden traffic spikes using a Load Scheduler component. Load on the service is throttled when the observed latency exceeds the long-term trend by a certain percentage threshold. This ensures the service stays responsive even under high load.
2. Auto-Scaling: The auto-scaling strategy is based on the throttling behavior of the service protection policy. An Auto Scaler component is used to dynamically adjust the number of service instances in response to changes in load. This load-based auto-scaling is enacted by a scale-out Controller that reads Load Scheduler signals. The service replicas are scaled out when the load is being throttled, effectively scaling resources to match the demand. During periods of low load, the policy attempts to scale in after periodic intervals to reduce excess replicas.

By combining service protection with auto-scaling, this policy ensures that the number of service replicas is adjusted to match persistent changes in demand, maintaining service stability and responsiveness.

Configuration

This policy is based on the Service Protection with Average Latency Feedback blueprint. It provides protection against overloads at the search-service.prod.svc.cluster.local service. Auto-scaling is applied to the Deployment search-service with a minimum of 1 and a maximum of 10 replicas.

To prevent frequent fluctuation in replicas, scale-in and scale-out cooldown periods are set to 40 and 30 seconds, respectively. A periodic scale-in interval of 60 seconds is also set to reduce excess replicas during periods of low load.

The below values.yaml file can be generated by following the steps in the Installation section.

aperturectl values.yaml

# yaml-language-server: $schema=../../../../../../blueprints/policies/load-scheduling/average-latency/gen/definitions.json
# Generated values file for load-scheduling/average-latency blueprint
# Documentation/Reference for objects and parameters can be found at:
# https://docs.fluxninja.com/reference/blueprints/load-scheduling/average-latency

policy:
  # Name of the policy.
  # Type: string
  # Required: True
  policy_name: load-based-auto-scale
  service_protection_core:
    adaptive_load_scheduler:
      load_scheduler:
        # The selectors determine the flows that are protected by this policy.
        # Type: []aperture.spec.v1.Selector
        # Required: True
        selectors:
        - control_point: ingress
          service: search-service.prod.svc.cluster.local
  latency_baseliner:
    # Tolerance factor beyond which the service is considered to be in overloaded state. E.g. if EMA of latency is 50ms and if Tolerance is 1.1, then service is considered to be in overloaded state if current latency is more than 55ms.
    # Type: float64
    latency_tolerance_multiplier: 1.1
    # Flux Meter defines the scope of latency measurements.
    # Type: aperture.spec.v1.FluxMeter
    # Required: True
    flux_meter:
      selectors:
      - control_point: ingress
        service: search-service.prod.svc.cluster.local
  components:
  - auto_scale:
      auto_scaler:
        scaling_backend:
          # Kubernetes replicas scaling backend.
          # Type: aperture.spec.v1.AutoScalerScalingBackendKubernetesReplicas
          # Required: True
          kubernetes_replicas:
            # Kubernetes object selector.
            # Type: aperture.spec.v1.KubernetesObjectSelector
            # Required: True
            kubernetes_object_selector:
              agent_group: default
              api_version: apps/v1
              kind: Deployment
              name: search-service
              namespace: prod
            # Minimum number of replicas.
            # Type: string
            # Required: True
            min_replicas: "1"
            # Maximum number of replicas.
            # Type: string
            # Required: True
            max_replicas: "10"
        # Dry run mode ensures that no scaling is invoked by this auto scaler.
        # Type: bool
        dry_run: false
        scale_in_controllers:
        - alerter:
            alert_name: Auto-scaler is scaling in
          controller:
            periodic:
              period: "60s"
              scale_in_percentage: 10
        scale_out_controllers:
        - alerter:
            alert_name: Auto-scaler is scaling out
          controller:
            gradient:
              in_ports:
                setpoint:
                  constant_signal:
                    value: 1
                signal:
                  signal_name: DESIRED_LOAD_MULTIPLIER
              parameters:
                slope: -1
        # Parameters that define the scaling behavior.
        # Type: aperture.spec.v1.AutoScalerScalingParameters
        # Required: True
        scaling_parameters:
          scale_in_alerter:
            alert_name: "Auto-scaler is scaling in"
          scale_in_cooldown: "40s"
          scale_out_alerter:
            alert_name: "Auto-scaler is scaling out"
          scale_out_cooldown: "30s"

Generated Policy

apiVersion: fluxninja.com/v1alpha1
kind: Policy
metadata:
  labels:
    fluxninja.com/validate: "true"
  name: load-based-auto-scale
spec:
  circuit:
    components:
    - flow_control:
        adaptive_load_scheduler:
          dry_run: false
          dry_run_config_key: dry_run
          in_ports:
            overload_confirmation:
              constant_signal:
                value: 1
            setpoint:
              signal_name: SETPOINT
            signal:
              signal_name: SIGNAL
          out_ports:
            desired_load_multiplier:
              signal_name: DESIRED_LOAD_MULTIPLIER
            observed_load_multiplier:
              signal_name: OBSERVED_LOAD_MULTIPLIER
          parameters:
            alerter:
              alert_name: Load Throttling Event
            gradient:
              max_gradient: 1
              min_gradient: 0.1
              slope: -1
            load_multiplier_linear_increment: 0.025
            load_scheduler:
              selectors:
              - control_point: ingress
                service: search-service.prod.svc.cluster.local
            max_load_multiplier: 2
    - auto_scale:
        auto_scaler:
          dry_run: false
          scale_in_controllers:
          - alerter:
              alert_name: Auto-scaler is scaling in
            controller:
              periodic:
                period: 60s
                scale_in_percentage: 10
          scale_out_controllers:
          - alerter:
              alert_name: Auto-scaler is scaling out
            controller:
              gradient:
                in_ports:
                  setpoint:
                    constant_signal:
                      value: 1
                  signal:
                    signal_name: DESIRED_LOAD_MULTIPLIER
                parameters:
                  slope: -1
          scaling_backend:
            kubernetes_replicas:
              kubernetes_object_selector:
                agent_group: default
                api_version: apps/v1
                kind: Deployment
                name: search-service
                namespace: prod
              max_replicas: "10"
              min_replicas: "1"
          scaling_parameters:
            scale_in_alerter:
              alert_name: Auto-scaler is scaling in
            scale_in_cooldown: 40s
            scale_out_alerter:
              alert_name: Auto-scaler is scaling out
            scale_out_cooldown: 30s
    - query:
        promql:
          evaluation_interval: 10s
          out_ports:
            output:
              signal_name: SIGNAL
          query_string: sum(increase(flux_meter_sum{flow_status="OK", flux_meter_name="load-based-auto-scale",
            policy_name="load-based-auto-scale"}[30s]))/sum(increase(flux_meter_count{flow_status="OK",
            flux_meter_name="load-based-auto-scale", policy_name="load-based-auto-scale"}[30s]))
    - query:
        promql:
          evaluation_interval: 30s
          out_ports:
            output:
              signal_name: SIGNAL_LONG_TERM
          query_string: sum(increase(flux_meter_sum{flow_status="OK", flux_meter_name="load-based-auto-scale",
            policy_name="load-based-auto-scale"}[1800s]))/sum(increase(flux_meter_count{flow_status="OK",
            flux_meter_name="load-based-auto-scale", policy_name="load-based-auto-scale"}[1800s]))
    - arithmetic_combinator:
        in_ports:
          lhs:
            signal_name: SIGNAL_LONG_TERM
          rhs:
            constant_signal:
              value: 1.1
        operator: mul
        out_ports:
          output:
            signal_name: SETPOINT
    evaluation_interval: 10s
  resources:
    flow_control:
      classifiers: []
      flux_meters:
        load-based-auto-scale:
          selectors:
          - control_point: ingress
            service: search-service.prod.svc.cluster.local
    infra_meters: {}

info

Circuit Diagram for this policy.

Installation

Generate a values file specific to the policy. This can be achieved using the command provided below.

aperturectl blueprints values --name=auto-scaling/pod-auto-scaler --version=v2.8.0 --output-file=values.yaml

Adjust the values to match the application requirements. Use the following command to generate the policy.

aperturectl blueprints generate --name=auto-scaling/pod-auto-scaler --values-file=values.yaml --output-dir=policy-gen --version=v2.8.0

Apply the policy using the aperturectl CLI or kubectl.

aperturectl

Pass the --kube flag with aperturectl to directly apply the generated policies on a Kubernetes cluster in the namespace where the Aperture Controller is installed.

aperturectl apply policy --file=policy-gen/configuration/auto-scaling.yaml --kube

kubectl

Apply the policy YAML generated (Kubernetes Custom Resource) using the above example with kubectl.

kubectl apply -f policy-gen/configuration/auto-scaling-cr.yaml -n aperture-controller

Policy in Action

During transient load spikes, the response latency on the service increases. The service protection policy queues a proportion of the incoming requests. The Auto Scaler makes a scale-out decision as the OBSERVED_LOAD_MULTIPLIER falls below 1. This triggers the auto-scale policy, which scales up the deployment. With the additional replicas in the deployment, the service is now better equipped to handle the increased load. The OBSERVED_LOAD_MULTIPLIER rises above 1, enabling the service to meet the heightened demand. As a result, the response latency returns to a normal range, and the Load Scheduler ceases throttling.

After the scale-out cooldown period, the periodic scale-in function is triggered, which reduces the number of replicas in response to decreased load.