AWS CloudFormation Interview Questions
Core Concepts
Q: What is CloudFormation? vs Terraform?
CloudFormation is AWS's native IaC service. Define JSON/YAML templates, AWS provisions resources and manages state.
|---|---|---|
| Cloud | AWS only | Multi-cloud |
|---|
| State | AWS manages (S3+DynamoDB) | You manage |
| Rollback | Automatic on failure | Manual |
| New services | Usually earlier support (AWS builds it internally) (needs verification — "same-day" varies by service, not guaranteed) | Depends on provider release cadence |
---
Q: Template structure.
yaml
AWSTemplateFormatVersion: '2010-09-09'
Parameters: # Dynamic inputs
Env: {Type: String, AllowedValues: [dev, prod]}
Conditions:
IsProd: !Equals [!Ref Env, prod]
Resources: # Required — what to create
MyBucket:
Type: AWS::S3::Bucket
DeletionPolicy: Retain
Properties:
BucketName: !Sub "my-bucket-${Env}"
Outputs:
BucketArn:
Value: !GetAtt MyBucket.Arn
Export:
Name: !Sub "${AWS::StackName}-BucketArn"
---
Q: Key intrinsic functions.
yaml
!Ref MyParam # Reference parameter or resource
!GetAtt Resource.Attr # Get resource attribute (e.g., Arn, Endpoint)
!Sub "string-${Var}" # String substitution
!Join [':', [a, b, c]] # Join: "a:b:c"
!If [Cond, TrueVal, FalseVal]
!FindInMap [Map, Key1, Key2]
!ImportValue ExportName # Cross-stack reference
---
Q: ChangeSets and nested stacks.
bash
# Preview changes before applying (like terraform plan)
aws cloudformation create-change-set \
--stack-name my-stack --template-body file://template.yaml \
--change-set-name preview
aws cloudformation describe-change-set --stack-name my-stack --change-set-name preview
aws cloudformation execute-change-set --stack-name my-stack --change-set-name preview
Nested stacks: modular templates — parent references child via AWS::CloudFormation::Stack with TemplateURL.
---
Q: Deletion policies and rollback.
yaml
DeletionPolicy: Retain # Keep resource after stack delete
DeletionPolicy: Snapshot # Take snapshot before delete (RDS/EBS)
UpdateReplacePolicy: Retain # Keep old resource when replacement needed
On failure: CloudFormation rolls back entire stack by default. Stack enters ROLLBACK_COMPLETE — must delete before retrying.
Revision Notes
CFN: AWS-native IaC. JSON/YAML -> AWS manages state. Auto-rollback on failure.
vs Terraform: CFN=AWS-only, auto-rollback | TF=multi-cloud, self-managed state
TEMPLATE: Parameters | Conditions | Resources (required) | Outputs
FUNCTIONS: !Ref | !GetAtt | !Sub | !Join | !If | !ImportValue
CHANGESET: preview before apply (like terraform plan)
create-change-set -> describe -> execute
DELETION POLICY: Retain | Snapshot | UpdateReplacePolicy
ROLLBACK: automatic on failure -> ROLLBACK_COMPLETE -> must delete to retry
