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Walkthrough: Create a scalable, load\-balancing web server

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This template creates a sample web site that uses Amazon EC2 Auto Scaling and Elastic Load Balancing and is configured to use multiple Availability Zones. The template also contains CloudWatch alarms that execute scaling policies to add or remove instances from the Auto Scaling group when the defined thresholds are exceeded.

This template creates one or more Amazon EC2 instances. You will be billed for the AWS resources used if you create a stack from this template.

The template assumes that your account supports the EC2-VPC platform. In other words, you have a default VPC that allows instances to access the Internet. If you don't have a default VPC, you can create one. For more information, see Amazon EC2 and Amazon Virtual Private Cloud in the *Amazon EC2 User Guide for Linux Instances*.

Amazon EC2 and Amazon Virtual Private Cloud

You can get the latest version of this sample template at https://s3\.amazonaws\.com/cloudformation\-templates\-us\-east\-1/AutoScalingMultiAZWithNotifications\.template.

https://s3.amazonaws.com/cloudformation-templates-us-east-1/AutoScalingMultiAZWithNotifications.template

Auto scaling multi\-AZ template

{
  "AWSTemplateFormatVersion" : "2010-09-09",

  "Description" : "AWS CloudFormation Sample Template AutoScalingMultiAZWithNotifications: Create a multi-az, load balanced and auto-scaled sample web site running on an Apache Web Server. The application is configured to span all Availability Zones in the region and is auto-scaled based on the CPU utilization of the web servers. Notifications will be sent to the operator email address on scaling events. The instances are load balanced with a simple health check against the default web page. **WARNING** This template creates one or more Amazon EC2 instances and an Elastic Load Balancer. You will be billed for the AWS resources used if you create a stack from this template.",

  "Parameters" : {
    "InstanceType" : {
      "Description" : "WebServer EC2 instance type",
      "Type" : "String",
      "Default" : "t2.small",
      "AllowedValues" : [ 
        "t1.micro", 
        "t2.nano", 
        "t2.micro", 
        "t2.small", 
        "t2.medium", 
        "t2.large", 
        "m1.small", 
        "m1.medium", 
        "m1.large", 
        "m1.xlarge", 
        "m2.xlarge", 
        "m2.2xlarge", 
        "m2.4xlarge", 
        "m3.medium", 
        "m3.large", 
        "m3.xlarge", 
        "m3.2xlarge", 
        "m4.large", 
        "m4.xlarge", 
        "m4.2xlarge", 
        "m4.4xlarge", 
        "m4.10xlarge", 
        "c1.medium", 
        "c1.xlarge", 
        "c3.large", 
        "c3.xlarge", 
        "c3.2xlarge", 
        "c3.4xlarge", 
        "c3.8xlarge", 
        "c4.large", 
        "c4.xlarge", 
        "c4.2xlarge", 
        "c4.4xlarge", 
        "c4.8xlarge", 
        "g2.2xlarge", 
        "g2.8xlarge", 
        "r3.large", 
        "r3.xlarge", 
        "r3.2xlarge", 
        "r3.4xlarge", 
        "r3.8xlarge", 
        "i2.xlarge", 
        "i2.2xlarge", 
        "i2.4xlarge", 
        "i2.8xlarge", 
        "d2.xlarge", 
        "d2.2xlarge", 
        "d2.4xlarge", 
        "d2.8xlarge", 
        "hi1.4xlarge", 
        "hs1.8xlarge", 
        "cr1.8xlarge", 
        "cc2.8xlarge", 
        "cg1.4xlarge"
      ],
      "ConstraintDescription" : "must be a valid EC2 instance type."
    },

    "OperatorEMail": {
      "Description": "EMail address to notify if there are any scaling operations",
      "Type": "String",
      "AllowedPattern": "([a-zA-Z0-9_\\-\\.]+)@((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.)|(([a-zA-Z0-9\\-]+\\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\\]?)",
      "ConstraintDescription": "must be a valid email address."
    },

    "KeyName" : {
      "Description" : "The EC2 Key Pair to allow SSH access to the instances",
      "Type" : "AWS::EC2::KeyPair::KeyName",
      "ConstraintDescription" : "must be the name of an existing EC2 KeyPair."
    },

    "SSHLocation" : {
      "Description" : "The IP address range that can be used to SSH to the EC2 instances",
      "Type": "String",
      "MinLength": "9",
      "MaxLength": "18",
      "Default": "0.0.0.0/0",
      "AllowedPattern": "(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})/(\\d{1,2})",
      "ConstraintDescription": "must be a valid IP CIDR range of the form x.x.x.x/x."
    }
  },

  "Mappings" : {
    "AWSInstanceType2Arch" : {
      "t1.micro"    : { "Arch" : "HVM64"  },
      "t2.nano"     : { "Arch" : "HVM64"  },
      "t2.micro"    : { "Arch" : "HVM64"  },
      "t2.small"    : { "Arch" : "HVM64"  },
      "t2.medium"   : { "Arch" : "HVM64"  },
      "t2.large"    : { "Arch" : "HVM64"  },
      "m1.small"    : { "Arch" : "HVM64"  },
      "m1.medium"   : { "Arch" : "HVM64"  },
      "m1.large"    : { "Arch" : "HVM64"  },
      "m1.xlarge"   : { "Arch" : "HVM64"  },
      "m2.xlarge"   : { "Arch" : "HVM64"  },
      "m2.2xlarge"  : { "Arch" : "HVM64"  },
      "m2.4xlarge"  : { "Arch" : "HVM64"  },
      "m3.medium"   : { "Arch" : "HVM64"  },
      "m3.large"    : { "Arch" : "HVM64"  },
      "m3.xlarge"   : { "Arch" : "HVM64"  },
      "m3.2xlarge"  : { "Arch" : "HVM64"  },
      "m4.large"    : { "Arch" : "HVM64"  },
      "m4.xlarge"   : { "Arch" : "HVM64"  },
      "m4.2xlarge"  : { "Arch" : "HVM64"  },
      "m4.4xlarge"  : { "Arch" : "HVM64"  },
      "m4.10xlarge" : { "Arch" : "HVM64"  },
      "c1.medium"   : { "Arch" : "HVM64"  },
      "c1.xlarge"   : { "Arch" : "HVM64"  },
      "c3.large"    : { "Arch" : "HVM64"  },
      "c3.xlarge"   : { "Arch" : "HVM64"  },
      "c3.2xlarge"  : { "Arch" : "HVM64"  },
      "c3.4xlarge"  : { "Arch" : "HVM64"  },
      "c3.8xlarge"  : { "Arch" : "HVM64"  },
      "c4.large"    : { "Arch" : "HVM64"  },
      "c4.xlarge"   : { "Arch" : "HVM64"  },
      "c4.2xlarge"  : { "Arch" : "HVM64"  },
      "c4.4xlarge"  : { "Arch" : "HVM64"  },
      "c4.8xlarge"  : { "Arch" : "HVM64"  },
      "g2.2xlarge"  : { "Arch" : "HVMG2"  },
      "g2.8xlarge"  : { "Arch" : "HVMG2"  },
      "r3.large"    : { "Arch" : "HVM64"  },
      "r3.xlarge"   : { "Arch" : "HVM64"  },
      "r3.2xlarge"  : { "Arch" : "HVM64"  },
      "r3.4xlarge"  : { "Arch" : "HVM64"  },
      "r3.8xlarge"  : { "Arch" : "HVM64"  },
      "i2.xlarge"   : { "Arch" : "HVM64"  },
      "i2.2xlarge"  : { "Arch" : "HVM64"  },
      "i2.4xlarge"  : { "Arch" : "HVM64"  },
      "i2.8xlarge"  : { "Arch" : "HVM64"  },
      "d2.xlarge"   : { "Arch" : "HVM64"  },
      "d2.2xlarge"  : { "Arch" : "HVM64"  },
      "d2.4xlarge"  : { "Arch" : "HVM64"  },
      "d2.8xlarge"  : { "Arch" : "HVM64"  },
      "hi1.4xlarge" : { "Arch" : "HVM64"  },
      "hs1.8xlarge" : { "Arch" : "HVM64"  },
      "cr1.8xlarge" : { "Arch" : "HVM64"  },
      "cc2.8xlarge" : { "Arch" : "HVM64"  }
    },

    "AWSInstanceType2NATArch" : {
      "t1.micro"    : { "Arch" : "NATHVM64"  },
      "t2.nano"     : { "Arch" : "NATHVM64"  },
      "t2.micro"    : { "Arch" : "NATHVM64"  },
      "t2.small"    : { "Arch" : "NATHVM64"  },
      "t2.medium"   : { "Arch" : "NATHVM64"  },
      "t2.large"    : { "Arch" : "NATHVM64"  },
      "m1.small"    : { "Arch" : "NATHVM64"  },
      "m1.medium"   : { "Arch" : "NATHVM64"  },
      "m1.large"    : { "Arch" : "NATHVM64"  },
      "m1.xlarge"   : { "Arch" : "NATHVM64"  },
      "m2.xlarge"   : { "Arch" : "NATHVM64"  },
      "m2.2xlarge"  : { "Arch" : "NATHVM64"  },
      "m2.4xlarge"  : { "Arch" : "NATHVM64"  },
      "m3.medium"   : { "Arch" : "NATHVM64"  },
      "m3.large"    : { "Arch" : "NATHVM64"  },
      "m3.xlarge"   : { "Arch" : "NATHVM64"  },
      "m3.2xlarge"  : { "Arch" : "NATHVM64"  },
      "m4.large"    : { "Arch" : "NATHVM64"  },
      "m4.xlarge"   : { "Arch" : "NATHVM64"  },
      "m4.2xlarge"  : { "Arch" : "NATHVM64"  },
      "m4.4xlarge"  : { "Arch" : "NATHVM64"  },
      "m4.10xlarge" : { "Arch" : "NATHVM64"  },
      "c1.medium"   : { "Arch" : "NATHVM64"  },
      "c1.xlarge"   : { "Arch" : "NATHVM64"  },
      "c3.large"    : { "Arch" : "NATHVM64"  },
      "c3.xlarge"   : { "Arch" : "NATHVM64"  },
      "c3.2xlarge"  : { "Arch" : "NATHVM64"  },
      "c3.4xlarge"  : { "Arch" : "NATHVM64"  },
      "c3.8xlarge"  : { "Arch" : "NATHVM64"  },
      "c4.large"    : { "Arch" : "NATHVM64"  },
      "c4.xlarge"   : { "Arch" : "NATHVM64"  },
      "c4.2xlarge"  : { "Arch" : "NATHVM64"  },
      "c4.4xlarge"  : { "Arch" : "NATHVM64"  },
      "c4.8xlarge"  : { "Arch" : "NATHVM64"  },
      "g2.2xlarge"  : { "Arch" : "NATHVMG2"  },
      "g2.8xlarge"  : { "Arch" : "NATHVMG2"  },
      "r3.large"    : { "Arch" : "NATHVM64"  },
      "r3.xlarge"   : { "Arch" : "NATHVM64"  },
      "r3.2xlarge"  : { "Arch" : "NATHVM64"  },
      "r3.4xlarge"  : { "Arch" : "NATHVM64"  },
      "r3.8xlarge"  : { "Arch" : "NATHVM64"  },
      "i2.xlarge"   : { "Arch" : "NATHVM64"  },
      "i2.2xlarge"  : { "Arch" : "NATHVM64"  },
      "i2.4xlarge"  : { "Arch" : "NATHVM64"  },
      "i2.8xlarge"  : { "Arch" : "NATHVM64"  },
      "d2.xlarge"   : { "Arch" : "NATHVM64"  },
      "d2.2xlarge"  : { "Arch" : "NATHVM64"  },
      "d2.4xlarge"  : { "Arch" : "NATHVM64"  },
      "d2.8xlarge"  : { "Arch" : "NATHVM64"  },
      "hi1.4xlarge" : { "Arch" : "NATHVM64"  },
      "hs1.8xlarge" : { "Arch" : "NATHVM64"  },
      "cr1.8xlarge" : { "Arch" : "NATHVM64"  },
      "cc2.8xlarge" : { "Arch" : "NATHVM64"  }
    }
,
    "AWSRegionArch2AMI" : {
      "us-east-1"        : {"HVM64" : "ami-0ff8a91507f77f867", "HVMG2" : "ami-0a584ac55a7631c0c"},
      "us-west-2"        : {"HVM64" : "ami-a0cfeed8", "HVMG2" : "ami-0e09505bc235aa82d"},
      "us-west-1"        : {"HVM64" : "ami-0bdb828fd58c52235", "HVMG2" : "ami-066ee5fd4a9ef77f1"},
      "eu-west-1"        : {"HVM64" : "ami-047bb4163c506cd98", "HVMG2" : "ami-0a7c483d527806435"},
      "eu-west-2"        : {"HVM64" : "ami-f976839e", "HVMG2" : "NOT_SUPPORTED"},
      "eu-west-3"        : {"HVM64" : "ami-0ebc281c20e89ba4b", "HVMG2" : "NOT_SUPPORTED"},
      "eu-central-1"     : {"HVM64" : "ami-0233214e13e500f77", "HVMG2" : "ami-06223d46a6d0661c7"},
      "ap-northeast-1"   : {"HVM64" : "ami-06cd52961ce9f0d85", "HVMG2" : "ami-053cdd503598e4a9d"},
      "ap-northeast-2"   : {"HVM64" : "ami-0a10b2721688ce9d2", "HVMG2" : "NOT_SUPPORTED"},
      "ap-northeast-3"   : {"HVM64" : "ami-0d98120a9fb693f07", "HVMG2" : "NOT_SUPPORTED"},
      "ap-southeast-1"   : {"HVM64" : "ami-08569b978cc4dfa10", "HVMG2" : "ami-0be9df32ae9f92309"},
      "ap-southeast-2"   : {"HVM64" : "ami-09b42976632b27e9b", "HVMG2" : "ami-0a9ce9fecc3d1daf8"},
      "ap-south-1"       : {"HVM64" : "ami-0912f71e06545ad88", "HVMG2" : "ami-097b15e89dbdcfcf4"},
      "us-east-2"        : {"HVM64" : "ami-0b59bfac6be064b78", "HVMG2" : "NOT_SUPPORTED"},
      "ca-central-1"     : {"HVM64" : "ami-0b18956f", "HVMG2" : "NOT_SUPPORTED"},
      "sa-east-1"        : {"HVM64" : "ami-07b14488da8ea02a0", "HVMG2" : "NOT_SUPPORTED"},
      "cn-north-1"       : {"HVM64" : "ami-0a4eaf6c4454eda75", "HVMG2" : "NOT_SUPPORTED"},
      "cn-northwest-1"   : {"HVM64" : "ami-6b6a7d09", "HVMG2" : "NOT_SUPPORTED"}
    }

  },
 
  "Resources" : {
    "NotificationTopic": {
      "Type": "AWS::SNS::Topic",
      "Properties": {
        "Subscription": [ { "Endpoint": { "Ref": "OperatorEMail" }, "Protocol": "email" } ]
      }
    },

    "WebServerGroup" : {
      "Type" : "AWS::AutoScaling::AutoScalingGroup",
      "Properties" : {
        "AvailabilityZones" : { "Fn::GetAZs" : ""},
        "LaunchConfigurationName" : { "Ref" : "LaunchConfig" },
        "MinSize" : "1",
        "MaxSize" : "3",
        "LoadBalancerNames" : [ { "Ref" : "ElasticLoadBalancer" } ],
        "NotificationConfiguration" : {
          "TopicARN" : { "Ref" : "NotificationTopic" },
          "NotificationTypes" : [ "autoscaling:EC2_INSTANCE_LAUNCH",
                                  "autoscaling:EC2_INSTANCE_LAUNCH_ERROR",
                                  "autoscaling:EC2_INSTANCE_TERMINATE",
                                  "autoscaling:EC2_INSTANCE_TERMINATE_ERROR"]
        }
      },
      "CreationPolicy" : {
        "ResourceSignal" : {
          "Timeout" : "PT15M",
          "Count"   : "1"
        }
      },
      "UpdatePolicy": {
        "AutoScalingRollingUpdate": {
          "MinInstancesInService": "1",
          "MaxBatchSize": "1",
          "PauseTime" : "PT15M",
          "WaitOnResourceSignals": "true"
        }
      }
    },

    "LaunchConfig" : {
      "Type" : "AWS::AutoScaling::LaunchConfiguration",
      "Metadata" : {
        "Comment" : "Install a simple application",
        "AWS::CloudFormation::Init" : {
          "config" : {
            "packages" : {
              "yum" : {
                "httpd" : []
              }
            },

            "files" : {
              "/var/www/html/index.html" : {
                "content" : { "Fn::Join" : ["\n", [
                  "<img src=\"", {"Fn::FindInMap" : ["Region2Examples", {"Ref" : "AWS::Region"}, "Examples"]}, "/cloudformation_graphic.png\" alt=\"AWS CloudFormation Logo\"/>",
                  "<h1>Congratulations, you have successfully launched the AWS CloudFormation sample.</h1>"
                ]]},
                "mode"    : "000644",
                "owner"   : "root",
                "group"   : "root"
              },

              "/etc/cfn/cfn-hup.conf" : {
                "content" : { "Fn::Join" : ["", [
                  "[main]\n",
                  "stack=", { "Ref" : "AWS::StackId" }, "\n",
                  "region=", { "Ref" : "AWS::Region" }, "\n"
                ]]},
                "mode"    : "000400",
                "owner"   : "root",
                "group"   : "root"
              },

              "/etc/cfn/hooks.d/cfn-auto-reloader.conf" : {
                "content": { "Fn::Join" : ["", [
                  "[cfn-auto-reloader-hook]\n",
                  "triggers=post.update\n",
                  "path=Resources.LaunchConfig.Metadata.AWS::CloudFormation::Init\n",
                  "action=/opt/aws/bin/cfn-init -v ",
                  "         --stack ", { "Ref" : "AWS::StackName" },
                  "         --resource LaunchConfig ",
                  "         --region ", { "Ref" : "AWS::Region" }, "\n",
                  "runas=root\n"
                ]]}
              }
            },

            "services" : {
              "sysvinit" : {
                "httpd"    : { "enabled" : "true", "ensureRunning" : "true" },
                "cfn-hup" : { "enabled" : "true", "ensureRunning" : "true",
                              "files" : ["/etc/cfn/cfn-hup.conf", "/etc/cfn/hooks.d/cfn-auto-reloader.conf"]}
              }
            }
          }
        }
      },
      "Properties" : {
        "KeyName" : { "Ref" : "KeyName" },
        "ImageId" : { "Fn::FindInMap" : [ "AWSRegionArch2AMI", { "Ref" : "AWS::Region" },
                                          { "Fn::FindInMap" : [ "AWSInstanceType2Arch", { "Ref" : "InstanceType" }, "Arch" ] } ] },
        "SecurityGroups" : [ { "Ref" : "InstanceSecurityGroup" } ],
        "InstanceType" : { "Ref" : "InstanceType" },
        "UserData"       : { "Fn::Base64" : { "Fn::Join" : ["", [
             "#!/bin/bash -xe\n",
             "yum update -y aws-cfn-bootstrap\n",

             "/opt/aws/bin/cfn-init -v ",
             "         --stack ", { "Ref" : "AWS::StackName" },
             "         --resource LaunchConfig ",
             "         --region ", { "Ref" : "AWS::Region" }, "\n",

             "/opt/aws/bin/cfn-signal -e $? ",
             "         --stack ", { "Ref" : "AWS::StackName" },
             "         --resource WebServerGroup ",
             "         --region ", { "Ref" : "AWS::Region" }, "\n"
        ]]}}
      }
    },

    "WebServerScaleUpPolicy" : {
      "Type" : "AWS::AutoScaling::ScalingPolicy",
      "Properties" : {
        "AdjustmentType" : "ChangeInCapacity",
        "AutoScalingGroupName" : { "Ref" : "WebServerGroup" },
        "Cooldown" : "60",
        "ScalingAdjustment" : "1"
      }
    },
    "WebServerScaleDownPolicy" : {
      "Type" : "AWS::AutoScaling::ScalingPolicy",
      "Properties" : {
        "AdjustmentType" : "ChangeInCapacity",
        "AutoScalingGroupName" : { "Ref" : "WebServerGroup" },
        "Cooldown" : "60",
        "ScalingAdjustment" : "-1"
      }
    },

    "CPUAlarmHigh": {
     "Type": "AWS::CloudWatch::Alarm",
     "Properties": {
        "AlarmDescription": "Scale-up if CPU > 90% for 10 minutes",
        "MetricName": "CPUUtilization",
        "Namespace": "AWS/EC2",
        "Statistic": "Average",
        "Period": "300",
        "EvaluationPeriods": "2",
        "Threshold": "90",
        "AlarmActions": [ { "Ref": "WebServerScaleUpPolicy" } ],
        "Dimensions": [
          {
            "Name": "AutoScalingGroupName",
            "Value": { "Ref": "WebServerGroup" }
          }
        ],
        "ComparisonOperator": "GreaterThanThreshold"
      }
    },
    "CPUAlarmLow": {
     "Type": "AWS::CloudWatch::Alarm",
     "Properties": {
        "AlarmDescription": "Scale-down if CPU < 70% for 10 minutes",
        "MetricName": "CPUUtilization",
        "Namespace": "AWS/EC2",
        "Statistic": "Average",
        "Period": "300",
        "EvaluationPeriods": "2",
        "Threshold": "70",
        "AlarmActions": [ { "Ref": "WebServerScaleDownPolicy" } ],
        "Dimensions": [
          {
            "Name": "AutoScalingGroupName",
            "Value": { "Ref": "WebServerGroup" }
          }
        ],
        "ComparisonOperator": "LessThanThreshold"
      }
    },

    "ElasticLoadBalancer" : {
      "Type" : "AWS::ElasticLoadBalancing::LoadBalancer",
      "Properties" : {
        "AvailabilityZones" : { "Fn::GetAZs" : "" },
        "CrossZone" : "true",
        "Listeners" : [ {
          "LoadBalancerPort" : "80",
          "InstancePort" : "80",
          "Protocol" : "HTTP"
        } ],
        "HealthCheck" : {
          "Target" : "HTTP:80/",
          "HealthyThreshold" : "3",
          "UnhealthyThreshold" : "5",
          "Interval" : "30",
          "Timeout" : "5"
        }
      }
    },

    "InstanceSecurityGroup" : {
      "Type" : "AWS::EC2::SecurityGroup",
      "Properties" : {
        "GroupDescription" : "Enable SSH access and HTTP from the load balancer only",
        "SecurityGroupIngress" : [ {
          "IpProtocol" : "tcp",
          "FromPort" : "22",
          "ToPort" : "22",
          "CidrIp" : { "Ref" : "SSHLocation"}
        },
        {
          "IpProtocol" : "tcp",
          "FromPort" : "80",
          "ToPort" : "80",
          "SourceSecurityGroupOwnerId" : {"Fn::GetAtt" : ["ElasticLoadBalancer", "SourceSecurityGroup.OwnerAlias"]},
          "SourceSecurityGroupName" : {"Fn::GetAtt" : ["ElasticLoadBalancer", "SourceSecurityGroup.GroupName"]}
        } ]
      }
    }
  },

  "Outputs" : {
    "URL" : {
      "Description" : "The URL of the website",
      "Value" :  { "Fn::Join" : [ "", [ "http://", { "Fn::GetAtt" : [ "ElasticLoadBalancer", "DNSName" ]}]]}
    }
  }
}

Template walkthrough

The example template contains an Auto Scaling group with a LoadBalancer, a security group that defines ingress rules, CloudWatch alarms, and scaling policies.

The template has three input parameters: InstanceType is the type of EC2 instance to use for the Auto Scaling group and has a default of m1.small; WebServerPort is the TCP port for the web server and has a default of 8888; KeyName is the name of an EC2 key pair to be used for the Auto Scaling group. KeyName must be specified at stack creation (parameters with no default value must be specified at stack creation).

The AWS::AutoScaling::AutoScalingGroup resource WebServerGroup declares the following Auto Scaling group configuration:

AWS::AutoScaling::AutoScalingGroup

Fn::GetAZs

The AWS::AutoScaling::LaunchConfiguration resource LaunchConfig declares the following configurations to use for the EC2 instances in the WebServerGroup Auto Scaling group:

AWS::AutoScaling::LaunchConfiguration

AWS::EC2::SecurityGroup

GetAtt

Manage security groups in Amazon EC2-Classic

Manage security groups in Amazon VPC

The AWS::ElasticLoadBalancing::LoadBalancer resource ElasticLoadBalancer declares the following LoadBalancer configuration:

AWS::ElasticLoadBalancing::LoadBalancer

The AWS::AutoScaling::ScalingPolicy resource WebServerScaleUpPolicy is a simple scaling policy that scales up the Auto Scaling group WebServerGroup. The `AdjustmentType` property is set to ChangeInCapacity. This means that the `ScalingAdjustment` represents the number of instances to add (if `ScalingAdjustment` is positive, instances are added; if negative, instances are deleted). In this example, `ScalingAdjustment` is 1; therefore, the scaling policy increments the number of EC2 instances in the group by 1 when the policy is executed. The Cooldown property specifies that Amazon EC2 Auto Scaling waits 60 seconds before starting any other scaling activities due to simple scaling policies.

AWS::AutoScaling::ScalingPolicy

The AWS::CloudWatch::Alarm resource CPUAlarmHigh specifies the simple scaling policy WebServerScaleUpPolicy as the action to execute when the alarm is in an `ALARM` state (AlarmActions). The alarm monitors the EC2 instances in the WebServerGroup Auto Scaling group (Dimensions). The alarm measures the average (Statistic) EC2 instance CPU utilization (Namespace and MetricName) of the instances in the WebServerGroup (Dimensions) over a 300 second interval (Period). When this value (average CPU utilization over 300 seconds) remains greater than 90 percent (ComparisonOperator and Threshold) for 2 consecutive periods (EvaluationPeriod), the alarm will go into an `ALARM` state and CloudWatch will execute the WebServerScaleUpPolicy policy (AlarmActions) described above scale up the WebServerGroup.

AWS::CloudWatch::Alarm

The CPUAlarmLow alarm measures the same metrics but has an alarm that triggers when CPU utilization is less than 75 percent (ComparisonOperator and Threshold) and executes the WebServerScaleDownPolicy policy to remove 1 EC2 instance from the Auto Scaling group WebServerGroup.