Parameters, Refs, and Stack Updates
Let's tag the bucket with a purpose. We'll use a parameter to specify the value.
Parameters:
Purpose: <---------------------- add a parameter "Purpose"
Type: String
Resources:
MyS3Bucket:
Type: AWS::S3::Bucket
Properties:
Tags:
- Key: StackPurpose
Value: !Ref Purpose <--- ref the parameter
Parameters, Refs, and Stack Updates
Parameters may be constrained, helping to catch invalid data earlier.
Parameters:
Purpose:
Type: String
Description: What is this stack for? <----- describe it!
Default: MakingMoney <-------------- set a default value
AllowedValues: <--------------- constrain allowed values
- MakingMoney
- Demo
Resources:
MyS3Bucket:
Type: AWS::S3::Bucket
Properties:
Tags:
- Key: StackPurpose
Value: !Ref Purpose
Update Stack
Select the existing stack and choose "Update Stack"
Update Stack
Select new template to update stack
Update Stack
Set the new parameter value
Update Stack
Review the "Change Set" specified by your update
Update Stack
CloudFormation begins updating the stack
Update Stack
Update is complete... review S3 bucket tags
Alternatives to the Console
aws cloudformation create-stack --stack-name demo-stack --template-url ... --parameters ...
AWS SDKs
12 SDKs at time of writing
Integrations
Terraform resource
Cross-References
Refer to parameters, as in tagged bucket example
Parameters:
Purpose: <----------------------- parameter logical name
Type: String
Resources:
MyS3Bucket:
Type: AWS::S3::Bucket
Properties:
Tags:
- Key: StackPurpose
Value: !Ref Purpose <---- cross-reference
Cross-References
Refer to other resources
Resources:
MyS3Bucket: <------------------------ logical name
Type: AWS::S3::Bucket
MyLogGroup:
Type: AWS::Logs::LogGroup
Properties:
LogGroupName: !Ref MyS3Bucket <--- provisioned bucket name
Intrinsic Functions
Compute values at deployment-time, like string joins
Resources:
MyS3Bucket:
Type: AWS::S3::Bucket
MyLogGroup:
Type: AWS::Logs::LogGroup
Properties:
LogGroupName: !Join ["-", [!Ref MyS3Bucket, logs]]
See also Split, Sub, GetAtt, etc.
Pseudo-Parameters
Values that provided by context, used with !Ref
!Ref "AWS::Region" <-- region stack is deployed in
!Ref "AWS::StackName" <-- name of stack being deployed
...plus a few others.
Nested Stacks
CloudFormation's way of reusing and composing stacks
AWS Resources...
Nested Stacks
CloudFormation's way of reusing and composing stacks
...assembled into a stack...
Nested Stacks
CloudFormation's way of reusing and composing stacks
...makes stuff you want
Reusing our Bucket Stack
Our S3 stack can be nested without modification...
Resources:
FirstBucketStack:
Type: AWS::CloudFormation::Stack
Properties:
TemplateURL: https://s3.../01-parameterless-s3-bucket.yaml
SecondBucketStack:
Type: AWS::CloudFormation::Stack
Properties:
TemplateURL: https://s3.../01-parameterless-s3-bucket.yaml
Reusing our Bucket Stack
Our S3 stack can be nested without modification...
Creating a Nested Stack
Creating a Nested Stack
Creating a Nested Stack
Creating a Nested Stack
Nested Stack Usability Tip
Generated names are prefixed with stack context
{stack}-{logical}-{rand}-{logical}-{rand}
How CloudFormation Works
Nested stacks show how CloudFormation works
Nested Stack Performance Tip
Prefer wide and shallow, not narrow and deep!
Nested Stacks
How to reference something in a nested stack?
Step 1: Define the value as an output
Resources:
ImageBucket: ...
Outputs:
BucketName:
Description: User upload image bucket
Value: !Ref ImageBucket
Nested Stacks
How to reference something in a nested stack?
Step 2: Ref the value from the parent template
Resources:
ImageBucketStack: ...
Thumbnailer:
Type: ...
Properties:
BucketName: !GetAtt ImageBucketStack.Outputs.BucketName
Cross-Stack References
Export an output value from one stack
Outputs:
BucketName:
Value: !Ref ImageBucket
Export:
Name: !Sub "${AWS::StackName}-BucketName"
Import it into another with Fn::ImportValue
Parameters:
ImageBucketStackName:...
Resources:
MyResource:
Thumbnailer
"Fn::ImportValue":
!Sub "${ImageBucketStackName}-BucketName"
Cross-Stack References
Cross-Stack References and Changes
Cross-Stack or Nested?
Nested Stack Workflow
Cross-Stack Workflow
Nested Stack Problems
- Opaque to:
- CloudFormation Designer
- CloudFormation Console
- Change Sets
- Unstructured parameters could be limiting
- No libraries or dependency management for templates
- Can't be packaged with the code they need
