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

Conditions

The optional Conditions section contains statements that define the circumstances under which entities are created or configured. For example, you can create a condition and then associate it with a resource or output so that AWS CloudFormation only creates the resource or output if the condition is true. Similarly, you can associate the condition with a property so that AWS CloudFormation only sets the property to a specific value if the condition is true. If the condition is false, AWS CloudFormation sets the property to a different value that you specify.

You might use conditions when you want to reuse a template that can create resources in different contexts, such as a test environment versus a production environment. In your template, you can add an EnvironmentType input parameter, which accepts either prod or test as inputs. For the production environment, you might include Amazon EC2 instances with certain capabilities; however, for the test environment, you want to use reduced capabilities to save money. With conditions, you can define which resources are created and how they're configured for each environment type.

Conditions are evaluated based on predefined pseudo parameters or input parameter values that you specify when you create or update a stack. Within each condition, you can reference another condition, a parameter value, or a mapping. After you define all your conditions, you can associate them with resources and resource properties in the Resources and Outputs sections of a template.

At stack creation or stack update, AWS CloudFormation evaluates all the conditions in your template before creating any resources. Resources that are associated with a true condition are created. Resources that are associated with a false condition are ignored. AWS CloudFormation also re-evaluates these conditions at each stack update before updating any resources. Resources that are still associated with a true condition are updated. Resources that are now associated with a false condition are deleted.

Important

During a stack update, you cannot update conditions by themselves. You can update conditions only when you include changes that add, modify, or delete resources.

How to Use Conditions Overview

Depending on the entity you want to conditionally create or configure, you must include statements in the following template sections:

Parameters section

Define the inputs that you want your conditions to evaluate. The conditions evaluate to true or false based on the values of these input parameters. If you want your conditions to evaluate pseudo parameters, you don't need to define the pseudo parameters in this section; pseudo parameters are predefined by AWS CloudFormation.

Conditions section

Define conditions by using the intrinsic condition functions. These conditions determine when AWS CloudFormation creates the associated resources.

Resources and Outputs sections

Associate conditions with the resources or outputs that you want to conditionally create. AWS CloudFormation creates entities that are associated with a true condition and ignores entities that are associated with a false condition. Use the Condition key and a condition's logical ID to associate it with a resource or output. To conditionally specify a property, use the Fn::If function. For more information, see Condition Functions.

Syntax

The Conditions section consists of the key name Conditions. Each condition declaration includes a logical ID and intrinsic functions that are evaluated when you create or update a stack. The following pseudo template outlines the Conditions section:

JSON

"Conditions" : { "Logical ID" : {Intrinsic function} }

YAML

Conditions: Logical ID: Intrinsic function

Condition Intrinsic Functions

You can use the following intrinsic functions to define conditions:

  • Fn::And

  • Fn::Equals

  • Fn::If

  • Fn::Not

  • Fn::Or

For the syntax and information about each function, see Condition Functions.

Note

Fn::If is only supported in the metadata attribute, update policy attribute, and property values in the Resources section and Outputs sections of a template.

Examples

The following sample template includes an EnvType input parameter, where you can specify prod to create a stack for production or test to create a stack for testing. For a production environment, AWS CloudFormation creates an Amazon EC2 instance and attaches a volume to the instance. For a test environment, AWS CloudFormation creates only the Amazon EC2 instance.

The CreateProdResources condition evaluates to true if the EnvType parameter is equal to prod. In the sample template, the NewVolume and MountPoint resources are associated with the CreateProdResources condition. Therefore, the resources are created only if the EnvType parameter is equal to prod.

JSON

{ "AWSTemplateFormatVersion" : "2010-09-09", "Mappings" : { "RegionMap" : { "us-east-1" : { "AMI" : "ami-0ff8a91507f77f867", "TestAz" : "us-east-1a" }, "us-west-1" : { "AMI" : "ami-0bdb828fd58c52235", "TestAz" : "us-west-1a" }, "us-west-2" : { "AMI" : "ami-a0cfeed8", "TestAz" : "us-west-2a" }, "eu-west-1" : { "AMI" : "ami-047bb4163c506cd98", "TestAz" : "eu-west-1a" }, "sa-east-1" : { "AMI" : "ami-07b14488da8ea02a0", "TestAz" : "sa-east-1a" }, "ap-southeast-1" : { "AMI" : "ami-08569b978cc4dfa10", "TestAz" : "ap-southeast-1a" }, "ap-southeast-2" : { "AMI" : "ami-09b42976632b27e9b", "TestAz" : "ap-southeast-2a" }, "ap-northeast-1" : { "AMI" : "ami-06cd52961ce9f0d85", "TestAz" : "ap-northeast-1a" } } }, "Parameters" : { "EnvType" : { "Description" : "Environment type.", "Default" : "test", "Type" : "String", "AllowedValues" : ["prod", "test"], "ConstraintDescription" : "must specify prod or test." } }, "Conditions" : { "CreateProdResources" : {"Fn::Equals" : [{"Ref" : "EnvType"}, "prod"]} }, "Resources" : { "EC2Instance" : { "Type" : "AWS::EC2::Instance", "Properties" : { "ImageId" : { "Fn::FindInMap" : [ "RegionMap", { "Ref" : "AWS::Region" }, "AMI" ]} } }, "MountPoint" : { "Type" : "AWS::EC2::VolumeAttachment", "Condition" : "CreateProdResources", "Properties" : { "InstanceId" : { "Ref" : "EC2Instance" }, "VolumeId" : { "Ref" : "NewVolume" }, "Device" : "/dev/sdh" } }, "NewVolume" : { "Type" : "AWS::EC2::Volume", "Condition" : "CreateProdResources", "Properties" : { "Size" : "100", "AvailabilityZone" : { "Fn::GetAtt" : [ "EC2Instance", "AvailabilityZone" ]} } } }, "Outputs" : { "VolumeId" : { "Value" : { "Ref" : "NewVolume" }, "Condition" : "CreateProdResources" } } }

YAML

AWSTemplateFormatVersion: "2010-09-09" Mappings: RegionMap: us-east-1: AMI: "ami-0ff8a91507f77f867" TestAz: "us-east-1a" us-west-1: AMI: "ami-0bdb828fd58c52235" TestAz: "us-west-1a" us-west-2: AMI: "ami-a0cfeed8" TestAz: "us-west-2a" eu-west-1: AMI: "ami-047bb4163c506cd98" TestAz: "eu-west-1a" sa-east-1: AMI: "ami-07b14488da8ea02a0" TestAz: "sa-east-1a" ap-southeast-1: AMI: "ami-08569b978cc4dfa10" TestAz: "ap-southeast-1a" ap-southeast-2: AMI: "ami-09b42976632b27e9b" TestAz: "ap-southeast-2a" ap-northeast-1: AMI: "ami-06cd52961ce9f0d85" TestAz: "ap-northeast-1a" Parameters: EnvType: Description: Environment type. Default: test Type: String AllowedValues: - prod - test ConstraintDescription: must specify prod or test. Conditions: CreateProdResources: !Equals [ !Ref EnvType, prod ] Resources: EC2Instance: Type: "AWS::EC2::Instance" Properties: ImageId: !FindInMap [RegionMap, !Ref "AWS::Region", AMI] MountPoint: Type: "AWS::EC2::VolumeAttachment" Condition: CreateProdResources Properties: InstanceId: !Ref EC2Instance VolumeId: !Ref NewVolume Device: /dev/sdh NewVolume: Type: "AWS::EC2::Volume" Condition: CreateProdResources Properties: Size: 100 AvailabilityZone: !GetAtt EC2Instance.AvailabilityZone Outputs: VolumeId: Condition: CreateProdResources Value: !Ref NewVolume