Adding autoscaling to AWS DynamoDB via CloudFormation for already existing table

Question

Our team was quite excited to see autoscaling feature announced for AWS DynamoDB, and when trying it out by adding configuration in web interface we've discovered that's a good match four our applications' needs.

However, attempts to add the same configuration via CF have proved to be a bit more complicated. Following example as provided in this article - http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-dynamodb-table.html#cfn-dynamodb-table-examples-application-autoscaling - below is the result, which does not work. Stack detail:

17:02:25 UTC+0300 UPDATE_ROLLBACK_IN_PROGRESS AWS::CloudFormation::Stack my-stack The following resource(s) failed to create: [WriteCapacityScalableTarget]. 17:02:24 UTC+0300 CREATE_FAILED AWS::ApplicationAutoScaling::ScalableTarget WriteCapacityScalableTarget table/TableName|dynamodb:table:WriteCapacityUnits|dynamodb already exists 17:02:18 UTC+0300 CREATE_IN_PROGRESS AWS::ApplicationAutoScaling::ScalableTarget WriteCapacityScalableTarget
17:02:15 UTC+0300 CREATE_COMPLETE AWS::IAM::Role ScalingRole
17:01:42 UTC+0300 CREATE_IN_PROGRESS AWS::IAM::Role ScalingRole Resource creation Initiated 17:01:42 UTC+0300 CREATE_IN_PROGRESS AWS::IAM::Role ScalingRole
17:01:37 UTC+0300 UPDATE_IN_PROGRESS AWS::CloudFormation::Stack my-stack User Initiated

My CF script is as follows:

{
  "Resources": {
    "MyCustomTableName": {
      "Type": "AWS::DynamoDB::Table",
      "Properties": {
        "TableName": "TableName",
        "AttributeDefinitions": [
          {
            "AttributeName": "someAttribute1:someAttribute2",
            "AttributeType": "S"
          }
        ],
        "KeySchema": [
          {
            "AttributeName": "someAttribute1:someAttribute2",
            "KeyType": "HASH"
          }
        ],
        "ProvisionedThroughput": {
          "ReadCapacityUnits": 1,
          "WriteCapacityUnits": 1
        },
        "StreamSpecification": {
          "StreamViewType": "NEW_AND_OLD_IMAGES"
        }
      }
    },
    "WriteCapacityScalableTarget": {
      "Type": "AWS::ApplicationAutoScaling::ScalableTarget",
      "Properties": {
        "MaxCapacity": 30,
        "MinCapacity": 1,
        "ResourceId": {
          "Fn::Join": [
            "/",
            [
              "table",
              {
                "Ref": "MyCustomTableName"
              }
            ]
          ]
        },
        "RoleARN": {
          "Fn::GetAtt": [
            "ScalingRole",
            "Arn"
          ]
        },
        "ScalableDimension": "dynamodb:table:WriteCapacityUnits",
        "ServiceNamespace": "dynamodb"
      }
    },
    "ScalingRole": {
      "Type": "AWS::IAM::Role",
      "Properties": {
        "AssumeRolePolicyDocument": {
          "Version": "2012-10-17",
          "Statement": [
            {
              "Effect": "Allow",
              "Principal": {
                "Service": [
                  "application-autoscaling.amazonaws.com"
                ]
              },
              "Action": [
                "sts:AssumeRole"
              ]
            }
          ]
        },
        "Path": "/",
        "Policies": [
          {
            "PolicyName": "root",
            "PolicyDocument": {
              "Version": "2012-10-17",
              "Statement": [
                {
                  "Effect": "Allow",
                  "Action": [
                    "dynamodb:DescribeTable",
                    "dynamodb:UpdateTable",
                    "cloudwatch:PutMetricAlarm",
                    "cloudwatch:DescribeAlarms",
                    "cloudwatch:GetMetricStatistics",
                    "cloudwatch:SetAlarmState",
                    "cloudwatch:DeleteAlarms"
                  ],
                  "Resource": "*"
                }
              ]
            }
          }
        ]
      }
    },
    "WriteScalingPolicy": {
      "Type": "AWS::ApplicationAutoScaling::ScalingPolicy",
      "Properties": {
        "PolicyName": "WriteAutoScalingPolicy",
        "PolicyType": "TargetTrackingScaling",
        "ScalingTargetId": {
          "Ref": "WriteCapacityScalableTarget"
        },
        "TargetTrackingScalingPolicyConfiguration": {
          "TargetValue": 70.0,
          "ScaleInCooldown": 60,
          "ScaleOutCooldown": 60,
          "PredefinedMetricSpecification": {
            "PredefinedMetricType": "DynamoDBWriteCapacityUtilization"
          }
        }
      }
    }
  }
}

If anyone can shed some light as to why this is happening, I'd be much obliged :)

Answer 1

Going by the error message

CREATE_FAILED AWS::ApplicationAutoScaling::ScalableTarget WriteCapacityScalableTarget table/TableName|dynamodb:table:WriteCapacityUnits|dynamodb already exists

You are creating a DynamoDB resource with the name "TableName": "TableName". You cannot have two dynamoDB tables with same name within a region.

Go to the DynamoDB console and check if you have any such table and delete it. Post that the template should work fine.

Option 2: If you want to go ahead with the existing table, then you can remove the AWS::DynamoDB::Table resource from your CF Template.

Answer 2

For me this error came while setting up auto-scaling for an already existing dynamo table. Make sure you've removed any manual (via console) auto-scaling that you've set up for that dynamo table. Then re-execute and the stack would reach the UPDATE_COMPLETE stage.