CloudFormation, apply Condition on DependsOn

Question

The task that I need to do is make CDN depend on a S3 bucket. But we want to make it use the existing bucket rather than creating a new one.

Here is the sample code that I am trying:

"Parameters" : {
  "UseExistingBucket" : {
    "Description" : "Yes/No",
    "Default" : "yes",
    "Type" : "String",
    "AllowedValues" : [ "yes", "no" ]
  }
},
"Conditions" : {
  "CreateS3Resources" : {"Fn::Equals" : [{"Ref" : "UseExistingBucket"}, "no"]}
},
"Resources" : {
  "StaticBucket" : {
    "Type" : "AWS::S3::Bucket",
    "Condition" : "CreateS3Resources",
    "Properties" : {
      "BucketName" : { "Fn::Join": [ "-", [ "app",  { "Ref": "EnvType" }, "static" ] ] }
    },
    "DeletionPolicy": "Retain"
  },
  "MyStaticDistribution": {
    "Type": "AWS::CloudFront::Distribution",
    "Properties": {
      "DistributionConfig": {
        "Origins": [
          {
            "DomainName": {
              "Fn::If" : [
                "CreateS3Resources",
                { "Fn::Join": [ "-", [ "app",  { "Ref": "EnvType" }, "static" ] ] },
                {"Fn::GetAtt": [ "StaticBucket", "DomainName" ] }
              ]
            },
            "Id": "S3Origin",
          }
        ]
      }
    },
    "DependsOn": [{
      "Fn::If" : [
        "CreateS3Resources",
        { "Fn::Join": [ "-", [ "app",  { "Ref": "EnvType" }, "static" ] ] },
        ""
      ]
    }]
  }
}

Please suggest to me any more details, if they are required (atleast stackoverflow does wants more details, but I have not specified any :-P)

Answer 1

You can do this by using Fn:GetAtt wrapped in a conditional Fn:If. Using Fn:GetAtt implies a dependency, so CloudFormation will automatically wait once it reaches that function, the same as if you were using a DependsOn.

Example

The code snippet below shows this by conditionally retrieving the name of a nested stack that has not yet been created but only does so if the condition UseNestedStack is set to true. If UseNestedStack is false it will not wait and instead retrieve a local variable name.

{
"Fn::If": ["UseNestedStack", {
    "Fn::GetAtt": ["NestedStack", "Outputs.Name"]
}, {
    "Ref": "LocalName"
}]

How Do I know This? (Another Example)

Unfortunately there is no official documentation officially stating this but it was AWS that told me to do it this way and in their code examples you can see that when order matters they use Fn:GetAtt. I've tried this numerous times and it works every time. Try it yourself on a simple stack. Here is some more pseudo proof from an AWS lambda example that I tweaked and have used myself. The stack below could not possibly work if the AMI function is created after the resource AMI info, AMI Info needs output of the AMI function, so AWS has chained them together using Fn: GetAtt. To see this scroll to the bottom and look at the resource AMIInfo and you will see it references AMIFunction via fn: Gett. CloudFormation sees this and goes back to AMIFunction to create it first.

"AMIInfoFunction": {
  "DependsOn":"SourceStack",
  "Type": "AWS::Lambda::Function",
  "Properties": {
    "Code": {
      "S3Bucket": { "Ref": "DeploymentBucket" },
      "S3Key": {"Fn::Join": [
        "",
        [
          {
            "Ref": "ApplicationName"
          },
          "/amilookup.zip"
        ]
      ]}
    },
    "Handler": "amilookup.handler",
    "Runtime": "nodejs",
    "Timeout": "30",
    "Role": { "Fn::GetAtt" : ["LambdaExecutionRole", "Arn"] },
    "VpcConfig": {
      "SecurityGroupIds": [ {"Ref": "InstanceSecurityGroup"}],
      "SubnetIds": [ {"Ref":"PrivateSubnetA"},{"Ref":"PrivateSubnetB"} ]
    }
  }
},
"AMIInfo": {
  "Type": "Custom::AMIInfo",
  "Properties": {
    "ServiceToken": { "Fn::GetAtt" : ["AMIInfoFunction", "Arn"] },
    "StackName": { "Ref":"SourceStack" }
  }
}

Update: This is now documented (Fn::GetAtt) in the documentation for dependent stacks. Thank you to idoimaging in the comments for pointing that out. https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-dependson.html

Answer 2

In your template you do not need to add the DependsOn attribute to your MyStaticDistribution resource as you already have a reference to the StaticBucket resource.

This is documented in the Specifying Dependencies section of the Optimize AWS CloudFormation Templates blog post: https://aws.amazon.com/blogs/devops/optimize-aws-cloudformation-templates/

When you need CloudFormation to wait to provision one resource until another one has been provisioned, you can use the DependsOn attribute.

You can also introduce references between elements by using either the { "Ref": "MyResource" } or the { "Fn::GetAtt" : [ "MyResource" , "MyAttribute" ] } functions. When you use one of these functions, CloudFormation behaves as if you’ve added a DependsOn attribute to the resource.

Answer 3

For yaml users, you can also use:

Conditions:
  CreateConfigRecorder: !Equals [ !Ref ConfigRecorderExists, 'false' ]

Resource:
#my 1st AWS Resource
  ConfigRecorder: 
    Condition: CreateConfigRecorder
    Type: AWS::Config::ConfigurationRecorder
    *more codes below*

#added, since DependsOn: !If is not possible, trigger by WaitCondition if CreateConfigRecorder is true
#Hacks: https://garbe.io/blog/2017/07/17/cloudformation-hacks/
  ConfigRecorderWaitHandle: 
    Condition: CreateConfigRecorder
    DependsOn: ConfigRecorder
    Type: "AWS::CloudFormation::WaitConditionHandle"
#added, since DependsOn: !If is not possible, trigger by WaitCondition if CreateConfigRecorder is false
  WaitHandle: 
    Type: "AWS::CloudFormation::WaitConditionHandle"
#added, since DependsOn: !If is not possible
  WaitCondition: 
    Type: "AWS::CloudFormation::WaitCondition"
    Properties: 
      Handle: !If [CreateConfigRecorder, !Ref ConfigRecorderWaitHandle, !Ref WaitHandle]
      Timeout: "1"
      Count: 0
#my 2nd AWS Resource that requires DependsOn Attribute
  AWSConfigRule:
    Type: AWS::Config::ConfigRule
    DependsOn: WaitCondition #added, since DependsOn: !If is not possible
    *more codes below*

Basically my 2nd resource only has DependsOn attribute if my 1st resource is non-existent, before running the CFN. I got this from: https://garbe.io/blog/2017/07/17/cloudformation-hacks/