Outputs for cross-stack references in AWS - Step-by-Step Execution

Process Flow - Outputs for cross-stack references

Create Stack A

↓

Define Output in Stack A

↓

Deploy Stack A

↓

Create Stack B

↓

Import Output from Stack A in Stack B

↓

Use Imported Value in Stack B

↓

Deploy Stack B

Stack A creates an output value. Stack B imports this output to use it, enabling communication between stacks.

Execution Sample

AWS

Stack A:
Resources:
  MyBucket:
    Type: AWS::S3::Bucket
Outputs:
  MyBucketName:
    Value: !Ref MyBucket
    Export:
      Name: MyBucketNameExport

Stack B:
Resources:
  MyBucketPolicy:
    Type: AWS::S3::BucketPolicy
    Properties:
      Bucket: !ImportValue MyBucketNameExport

Stack A exports a bucket name. Stack B imports that bucket name to attach a policy.

Process Table

Step	Action	Stack	Output/Import Name	Value	Result
1	Create Stack A with bucket resource	Stack A	-	-	Stack A created with bucket
2	Define Output 'MyBucketName' exporting bucket name	Stack A	MyBucketNameExport	my-bucket-123	Output defined and exported
3	Deploy Stack A	Stack A	-	-	Stack A deployed, output available
4	Create Stack B referencing output from Stack A	Stack B	MyBucketNameExport	my-bucket-123	ImportValue resolves to bucket name
5	Use imported value in Stack B resource	Stack B	-	my-bucket-123	Resource configured with imported bucket name
6	Deploy Stack B	Stack B	-	-	Stack B deployed using cross-stack reference

💡 Stack B deployment completes using output value exported by Stack A

Status Tracker

Variable	Start	After Step 2	After Step 4	Final
MyBucketNameExport	undefined	my-bucket-123	my-bucket-123	my-bucket-123
Stack A Deployment Status	not deployed	not deployed	deployed	deployed
Stack B Deployment Status	not deployed	not deployed	not deployed	deployed

Key Moments - 2 Insights

Why do we need to export an output in Stack A before Stack B can import it?

Stack A must explicitly export the output with a name so AWS CloudFormation can share it. Without export, Stack B cannot reference the value (see execution_table step 2 and 4).

What happens if Stack A is not deployed before deploying Stack B?

Stack B's import will fail because the exported value does not exist yet. Stack A must be deployed first to make the output available (see execution_table step 3 and 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'MyBucketNameExport' after Step 2?

A undefined

B not deployed

C my-bucket-123

D -

💡 Hint

Check the 'Value' column in execution_table row for Step 2

At which step does Stack B successfully import the output value?

A Step 4

B Step 3

C Step 2

D Step 6

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for when ImportValue resolves

If Stack A was not deployed, what would happen when deploying Stack B?

A Stack B deploys successfully with default values

B Stack B deploy fails due to missing import

C Stack B ignores the import and continues

D Stack B creates Stack A automatically

💡 Hint

Refer to key_moments about deployment order and import availability

Concept Snapshot

Outputs in one stack can be exported with a name.
Another stack can import these outputs by that name.
Stack A must be deployed first to make outputs available.
Use Export in Outputs section and !ImportValue in other stacks.
This enables sharing resources between stacks safely.

Full Transcript

In AWS CloudFormation, one stack can share values with another stack using outputs and imports. First, Stack A creates a resource and defines an output with an export name. After deploying Stack A, this output becomes available. Stack B can then import this output by referencing the export name. Stack B uses the imported value to configure its resources. This cross-stack reference requires Stack A to be deployed before Stack B. If Stack A is not deployed, Stack B's deployment will fail because the import cannot resolve. This mechanism helps stacks communicate and share resource information safely and cleanly.