# Refer to resource outputs in another CloudFormation stack
<a name="walkthrough-crossstackref"></a>

This walkthrough shows you how to reference outputs from one CloudFormation stack within another stack to create more modular and reusable templates. 

Instead of including all resources in a single stack, you create related AWS resources in separate stacks. Then, you can refer to required resource outputs from other stacks. By restricting cross-stack references to outputs, you control the parts of a stack that are referenced by other stacks.

For example, you might have a network stack with a VPC, a security group, and a subnet for public web applications, and a separate public web application stack. To ensure that the web applications use the security group and subnet from the network stack, you create a cross-stack reference that allows the web application stack to reference resource outputs from the network stack. With a cross-stack reference, owners of the web application stacks don't need to create or maintain networking rules or assets.

To create a cross-stack reference, use the `Export` output field to flag the value of a resource output for export. Then, use the `Fn::ImportValue` intrinsic function to import the value. For more information, see [Get exported outputs from a deployed CloudFormation stack](using-cfn-stack-exports.md).

**Note**  
CloudFormation is a free service. However, you are charged for the AWS resources that you include in your stacks at the current rate for each one. For more information about AWS pricing, see [the detail page for each product](http://aws.amazon.com).

**Topics**
+ [Use a sample template to create a network stack](#walkthrough-crossstackref-create-vpc-stack)
+ [Use a sample template to create a web application stack](#walkthrough-crossstackref-create-ec2-stack)
+ [Verify the stack works as designed](#walkthrough-crossstackref-verify)
+ [Troubleshoot AMI mapping errors](#walkthrough-crossstackref-troubleshooting-ami)
+ [Clean up your resources](#walkthrough-crossstackref-clean-up)

## Use a sample template to create a network stack
<a name="walkthrough-crossstackref-create-vpc-stack"></a>

Before you begin this walkthrough, check that you have IAM permissions to use all of the following services: Amazon VPC, Amazon EC2, and CloudFormation.

The network stack contains the VPC, security group, and subnet that you will use in the web application stack. In addition to these resources, the network stack creates an Internet gateway and routing tables to enable public access.

You must create this stack before you create the web application stack. If you create the web application stack first, it won't have a security group or subnet.

The stack template is available from the following URL: [https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleNetworkCrossStack.template](https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleNetworkCrossStack.template). To see the resources that the stack will create, choose the link, which opens the template. In the `Outputs` section, you can see the networking resources that the sample template exports. The names of the exported resources are prefixed with the stack's name in case you export networking resources from other stacks. When users import networking resources, they can specify from which stack the resources are imported.

**To create the network stack**

1. Sign in to the AWS Management Console and open the CloudFormation console at [https://console.aws.amazon.com/cloudformation](https://console.aws.amazon.com/cloudformation/).

1. On the **Stacks** page, choose **Create stack** at top right, and then choose **With new resources (standard)**.

1. Choose **Choose an existing template**, and in the **Specify template** section, choose **Amazon S3 URL**.

1. For **Amazon S3 URL**, paste the following URL: **https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleNetworkCrossStack.template**.

1. Choose **Next**.

1. For **Stack name**, type **SampleNetworkCrossStack**, and then choose **Next**.
**Note**  
Record the name of this stack. You'll need the stack name when you launch the web application stack.

1. Choose **Next**. For this walkthrough, you don't need to add tags or specify advanced settings.

1. Ensure that the stack name and template URL are correct, and then choose **Create stack**.

   It might take several minutes for CloudFormation to create your stack. Wait until all resources have been successfully created before proceeding to create the web application stack.

1. To monitor progress, view the stack events. For more information, see [Monitor stack progress](monitor-stack-progress.md).

## Use a sample template to create a web application stack
<a name="walkthrough-crossstackref-create-ec2-stack"></a>

The web application stack creates an EC2 instance that uses the security group and subnet from the network stack. 

You must create this stack in the same AWS Region as the network stack.

The stack template is available from the following URL: [https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleWebAppCrossStack.template](https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleWebAppCrossStack.template). To see the resources that the stack will create, choose the link, which will open the template. In the `Resources` section, view the EC2 instance's properties. You can see how the networking resources are imported from another stack by using the `Fn::ImportValue` function.

**To create the web application stack**

1. From the **Stacks** page, choose **Create stack** at top right, and then choose **With new resources (standard)**.

1. Choose **Choose an existing template**, and in the **Specify template** section, choose **Amazon S3 URL**.

1. For **Amazon S3 URL**, paste the following URL: **https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleWebAppCrossStack.template**.

1. Choose **Next**.

1. For **Stack name**, type **SampleWebAppCrossStack**. In the **Parameters** section, use the default value for the **NetworkStackName** parameter, and then choose **Next**.

   The sample template uses the parameter value to specify from which stack to import values.

1. Choose **Next**. For this walkthrough, you don't need to add tags or specify advanced settings.

1. Ensure that the stack name and template URL are correct, and then choose **Create stack**.

   It might take several minutes for CloudFormation to create your stack.

## Verify the stack works as designed
<a name="walkthrough-crossstackref-verify"></a>

After the stack has been created, view its resources and note the instance ID. For more information on viewing stack resources, see [View stack information from the CloudFormation console](cfn-console-view-stack-data-resources.md).

To verify the instance's security group and subnet, view the instance's properties in the [Amazon EC2 console](https://console.aws.amazon.com/ec2/). If the instance uses the security group and subnet from the `SampleNetworkCrossStack` stack, you have successfully created a cross-stack reference.

Use the console to view the stack outputs and the example website URL to verify that the web application is running. For more information, see [View stack information from the CloudFormation console](cfn-console-view-stack-data-resources.md).

## Troubleshoot AMI mapping errors
<a name="walkthrough-crossstackref-troubleshooting-ami"></a>

If you receive the error `Template error: Unable to get mapping for AWSRegionArch2AMI::[region]::HVM64`, the template doesn't include an AMI mapping for your AWS Region. Instead of updating the mapping, we recommend using Systems Manager public parameters to dynamically reference the latest AMIs:

1. Download the `SampleWebAppCrossStack` template to your local machine from: [https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleWebAppCrossStack.template](https://s3.amazonaws.com/cloudformation-examples/user-guide/cross-stack/SampleWebAppCrossStack.template).

1. Delete the entire `AWSRegionArch2AMI` mapping section.

1. Add the following Systems Manager parameter:

   ```
   "LatestAmiId": {
     "Description": "The latest Amazon Linux 2 AMI from the Parameter Store",
       "Type": "AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>",
       "Default": "/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2"
     }
   ```

1. Replace the existing `ImageId` reference:

   ```
   "ImageId": { "Fn::FindInMap": [ "AWSRegionArch2AMI", { "Ref": "AWS::Region" } , "HVM64" ] }, 
   ```

   With:

   ```
   "ImageId": { "Ref": "LatestAmiId" },
   ```

   This parameter automatically resolves to the latest Amazon Linux 2 AMI for the region where you deploy the stack. 

   For other Linux distributions, use the appropriate parameter path. For more information, see [Discovering public parameters in Parameter Store](https://docs.aws.amazon.com/systems-manager/latest/userguide/parameter-store-finding-public-parameters.html) in the *AWS Systems Manager User Guide*.

1. Upload the modified template to an S3 bucket in your account:

   ```
   aws s3 cp SampleWebAppCrossStack.template s3://amzn-s3-demo-bucket/
   ```

1. When creating the stack, specify your S3 template URL instead of the example URL.

## Clean up your resources
<a name="walkthrough-crossstackref-clean-up"></a>

To ensure that you are not charged for unwanted services, delete the stacks.

**To delete the stacks**

1. In the CloudFormation console, choose the `SampleWebAppCrossStack` stack.

1. Choose **Actions**, and then choose **Delete stack**.

1. In the confirmation message, choose **Delete**.

1. After the stack has been deleted, repeat the same steps for the `SampleNetworkCrossStack` stack.
**Note**  
Wait until CloudFormation completely deletes the `SampleWebAppCrossStack` stack. If the EC2 instance is still running in the VPC, CloudFormation won't delete the VPC in the `SampleNetworkCrossStack` stack.