Defining a scaling policy

A target-tracking scaling policy configuration is represented by a JSON block that the metrics and target values are defined in. You can save a scaling policy configuration as a JSON block in a text file. You use that text file when invoking the Amazon CLI or the Application Auto Scaling API. For more information about policy configuration syntax, see TargetTrackingScalingPolicyConfiguration in the Application Auto Scaling API Reference.

The following options are available for defining a target-tracking scaling policy configuration:

Using a predefined metric

By using predefined metrics, you can quickly define a target-tracking scaling policy for an ElastiCache for Redis cluster that works with target tracking in ElastiCache for Redis Auto Scaling. Currently, ElastiCache for Redis supports the following predefined metric in Elasticache Replicas Auto Scaling:

ElastiCacheReplicaEngineCPUUtilization – The average value of the EngineCPUUtilization metric in CloudWatch across all replicas in the ElastiCache for Redis cluster. The average value of the EngineCPUUtilization metric in CloudWatch across all replicas in the ElastiCache for Redis cluster. You can find the aggregated metric value in CloudWatch under ElastiCache for Redis ReplicationGroupId, Role for required ReplicationGroupId and Role Replica.

To use a predefined metric in your scaling policy, you create a target tracking configuration for your scaling policy. This configuration must include a PredefinedMetricSpecification for the predefined metric and a TargetValue for the target value of that metric.

Using a custom metric

By using custom metrics, you can define a target-tracking scaling policy that meets your custom requirements. You can define a custom metric based on any ElastiCache for Redis metric that changes in proportion to scaling. Not all ElastiCache for Redis metrics work for target tracking. The metric must be a valid utilization metric and describe how busy an instance is. The value of the metric must increase or decrease in proportion to the number of replicas in the cluster. This proportional increase or decrease is necessary to use the metric data to proportionally increase or decrease the number of replicas.

{"TargetValue": 50,
    "CustomizedMetricSpecification":
    {"MetricName": "EngineCPUUtilization",
        "Namespace": "AWS/ElastiCache",
        "Dimensions": [
            {"Name": "RelicationGroup","Value": "my-db-cluster"},
            {"Name": "Role","Value": "REPLICA"}
        ],
        "Statistic": "Average",
        "Unit": "Percent"
    }
}

Using cooldown periods

You can specify a value, in seconds, for ScaleOutCooldown to add a cooldown period for scaling out your cluster. Similarly, you can add a value, in seconds, for ScaleInCooldown to add a cooldown period for scaling in your cluster. For more information about ScaleInCooldown and ScaleOutCooldown, see TargetTrackingScalingPolicyConfiguration in the Application Auto Scaling API Reference. The following example describes a target-tracking configuration for a scaling policy. In this configuration, the ElastiCacheReplicaEngineCPUUtilizationpredefined metric is used to adjust an ElastiCache for Redis cluster based on an average CPU utilization of 40 percent across all replicas in that cluster. The configuration provides a scale-in cooldown period of 10 minutes and a scale-out cooldown period of 5 minutes.

{"TargetValue": 40.0,
    "PredefinedMetricSpecification":
    {"PredefinedMetricType": "ElastiCacheReplicaEngineCPUUtilization"
    },
    "ScaleInCooldown": 600,
    "ScaleOutCooldown": 300
}

Disabling scale-in activity

You can prevent the target-tracking scaling policy configuration from scaling in your ElastiCache for Redis cluster by disabling scale-in activity. Disabling scale-in activity prevents the scaling policy from deleting replicas, while still allowing the scaling policy to add them as needed.

You can specify a Boolean value for DisableScaleIn to enable or disable scale in activity for your cluster. For more information about DisableScaleIn, see TargetTrackingScalingPolicyConfiguration in the Application Auto Scaling API Reference.

{"TargetValue": 40.0,
    "PredefinedMetricSpecification":
    {"PredefinedMetricType": "ElastiCacheReplicaEngineCPUUtilization"
    },
    "DisableScaleIn": true
}

Applying a scaling policy to an ElastiCache for Redis cluster

After registering your cluster with ElastiCache for Redis auto scaling and defining a scaling policy, you apply the scaling policy to the registered cluster. To apply a scaling policy to an ElastiCache for Redis cluster, you can use the Amazon CLI or the Application Auto Scaling API.

Using the Amazon CLI

To apply a scaling policy to your ElastiCache for Redis cluster, use the put-scaling-policy command with the following parameters:

For Linux, OS X, or Unix:

aws application-autoscaling put-scaling-policy \
    --policy-name myscalablepolicy \
    --policy-type TargetTrackingScaling \
    --resource-id replication-group/myscalablecluster \
    --service-namespace elasticache \
    --scalable-dimension elasticache:replication-group:Replicas \
    --target-tracking-scaling-policy-configuration file://config.json

{"TargetValue": 40.0,
    "PredefinedMetricSpecification":
    {"PredefinedMetricType": "ElastiCacheReplicaEngineCPUUtilization"
    },
    "DisableScaleIn": true
}

For Windows:

aws application-autoscaling put-scaling-policy ^
    --policy-name myscalablepolicy ^
    --policy-type TargetTrackingScaling ^
    --resource-id replication-group/myscalablecluster ^
    --service-namespace elasticache ^
    --scalable-dimension elasticache:replication-group:Replicas ^
    --target-tracking-scaling-policy-configuration file://config.json

Using the API

To apply a scaling policy to your ElastiCache for Redis cluster with the Application Auto Scaling API, use the PutScalingPolicy Application Auto Scaling API operation with the following parameters:

POST / HTTP/1.1
Host: autoscaling.us-east-2.amazonaws.com
Accept-Encoding: identity
Content-Length: 219
X-Amz-Target: AnyScaleFrontendService.PutScalingPolicy
X-Amz-Date: 20160506T182145Z
User-Agent: aws-cli/1.10.23 Python/2.7.11 Darwin/15.4.0 botocore/1.4.8
Content-Type: application/x-amz-json-1.1
Authorization: AUTHPARAMS
{
    "PolicyName": "myscalablepolicy",
    "ServiceNamespace": "elasticache",
    "ResourceId": "replication-group/myscalablecluster",
    "ScalableDimension": "elasticache:replication-group:Replicas",
    "PolicyType": "TargetTrackingScaling",
    "TargetTrackingScalingPolicyConfiguration": {
        "TargetValue": 40.0,
        "PredefinedMetricSpecification":
        {
            "PredefinedMetricType": "ElastiCacheReplicaEngineCPUUtilization"
        }
    }
}