AWS CloudFormation
User Guide (API Version 2010-05-15)
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DependsOn Attribute

With the DependsOn attribute you can specify that the creation of a specific resource follows another. When you add a DependsOn attribute to a resource, that resource is created only after the creation of the resource specified in the DependsOn attribute. You can use the DependsOn attribute with any resource. Here are some typical uses:

  • Determine when a wait condition goes into effect. For more information, see Creating Wait Conditions in a Template.

  • Declare dependencies for resources that must be created or deleted in a specific order. For example, you must explicitly declare dependencies on gateway attachments for some resources in a VPC. For more information, see When a DependsOn attribute is required.

  • Override default parallelism when creating, updating, or deleting resources. AWS CloudFormation creates, updates, and deletes resources in parallel to the extent possible. It automatically determines which resources in a template can be parallelized and which have dependencies that require other operations to finish first. You can use DependsOn to explicitly specify dependencies, which overrides the default parallelism and directs CloudFormation to operate on those resources in a specified order.

Syntax

The DependsOn attribute can take a single string or list of strings.

"DependsOn" : [ String, ... ]

Example

The following template contains an AWS::EC2::Instance resource with a DependsOn attribute that specifies myDB, an AWS::RDS::DBInstance. When AWS CloudFormation creates this stack, it first creates myDB, then creates Ec2Instance.

{
    "AWSTemplateFormatVersion" : "2010-09-09",
    "Mappings" : {
        "RegionMap" : {
            "us-east-1" : { "AMI" : "ami-76f0061f" },
            "us-west-1" : { "AMI" : "ami-655a0a20" },
            "eu-west-1" : { "AMI" : "ami-7fd4e10b" },
            "ap-northeast-1" : { "AMI" : "ami-8e08a38f" },
            "ap-southeast-1" : { "AMI" : "ami-72621c20" }
        }
    },
    "Resources" : {
        "Ec2Instance" : {
            "Type" : "AWS::EC2::Instance",
            "Properties" : {
                "ImageId" : {
                   "Fn::FindInMap" : [ "RegionMap", { "Ref" : "AWS::Region" }, "AMI" ]
                }
            },
            "DependsOn" : "myDB"
        },
        "myDB" : {
            "Type" : "AWS::RDS::DBInstance",
            "Properties" : {
               "AllocatedStorage" : "5",
               "DBInstanceClass" : "db.m1.small",
               "Engine" : "MySQL",
               "EngineVersion" : "5.5",
               "MasterUsername" : "MyName",
               "MasterUserPassword" : "MyPassword"
            }
        }
    }
}     

When a DependsOn attribute is required

Some resources in a VPC require a gateway (either an Internet or VPN gateway). If your AWS CloudFormation template defines a VPC, a gateway, and a gateway attachment, any resources that require the gateway are dependent on the gateway attachment. For example, an Amazon EC2 instance with a public IP address is dependent on the VPC-gateway attachment if the VPC and InternetGateway resources are also declared in the same template.

Currently, the following resources depend on a VPC-gateway attachment when they have an associated public IP address and are in a VPC:

  • Auto Scaling group

  • Amazon EC2 instances

  • Elastic Load Balancing load balancers

  • Elastic IP address

A VPN gateway route propagation depends on a VPC-gateway attachment when you have a VPN gateway

The following snippet shows a sample gateway attachment and an Amazon EC2 instance that depends on a gateway attachment:

"GatewayToInternet" : {
  "Type" : "AWS::EC2::VPCGatewayAttachment",
  "Properties" : {
    "VpcId" : { "Ref" : "VPC" },
    "InternetGatewayId" : { "Ref" : "InternetGateway" }
  }
},

"EC2Host" : {
  "Type" : "AWS::EC2::Instance",
  "DependsOn" : "GatewayToInternet",
    "Properties" : {
      "InstanceType" : { "Ref" : "EC2InstanceType" },
      "KeyName"  : { "Ref" : "KeyName" },
      "ImageId"  : { "Fn::FindInMap" : [ "AWSRegionArch2AMI", { "Ref" : "AWS::Region" },
        { "Fn::FindInMap" : [ "AWSInstanceType2Arch", { "Ref" : "EC2InstanceType" }, "Arch" ] } ] },
      "NetworkInterfaces" : [{
        "GroupSet"                 : [{ "Ref" : "EC2SecurityGroup" }],
        "AssociatePublicIpAddress" : "true",
        "DeviceIndex"              : "0",
        "DeleteOnTermination"      : "true",
        "SubnetId"                 : { "Ref" : "PublicSubnet" }
      }]
    }
}