Get exported outputs from a deployed CloudFormation stack

When you have multiple stacks in the same AWS account and Region, you might want to share information between them. This is useful when one stack needs to use resources created by another stack.

For example, you might have one stack that creates network resources, such as subnets and security groups, for your web servers. Other stacks that create the actual web servers can then use the network resources created by the first stack. You don't need to hard code resource IDs in the stack's template or pass IDs as input parameters.

To share information between stacks, you export output values from one stack and import them into another stack. Here's how it works:

1. In the first stack's template (for example, the networking stack), you define certain values for export by using the Export field in the Outputs section. For more information, see Outputs.

2. When you create or update that stack, CloudFormation exports the output values, making them available to other stacks in the same AWS account and Region.

3. In the other stack's template, you use the Fn::ImportValue function to import the exported values from the first stack.

4. When you create or update the second stack (for example, the web server stack), CloudFormation automatically retrieves the exported values from the first stack and uses them.

For a walkthrough and sample templates, see Refer to resource outputs in another CloudFormation stack.

Exporting stack output values versus using nested stacks

A nested stack is a stack that you create within another stack by using the AWS::CloudFormation::Stack resource. With nested stacks, you deploy and manage all resources from a single stack. You can use outputs from one stack in the nested stack group as inputs to another stack in the group. This differs from exporting values.

If you want to isolate information sharing to within a nested stack group, we suggest that you use nested stacks. To share information with other stacks (not just within the group of nested stacks), export values. For example, you can create a single stack with a subnet and then export its ID. Other stacks can use that subnet by importing its ID; each stack doesn't need to create its own subnet. As long as stacks are importing the subnet ID, you can't change or delete it.

For more information about nested stacks, see Embed stacks within other stacks using nested stacks.

Considerations

The following restrictions apply to cross-stack references:

• For each AWS account, Export names must be unique within a Region.

• You can't create cross-stack references across Regions. You can use the intrinsic function Fn::ImportValue to import only values that have been exported within the same Region.

• For outputs, the value of the Name property of an Export can't use Ref or GetAtt functions that depend on a resource.

  Similarly, the ImportValue function can't include Ref or GetAtt functions that depend on a resource.

• After another stack imports an output value, you can't delete the stack that is exporting the output value or modify the exported output value. All the imports must be removed before you can delete the exporting stack or modify the output value.

Listing exported output values

To list exported output values (console)

1. Open the AWS CloudFormation console at https://console.aws.amazon.com/cloudformation.

2. From the left navigation pane, choose Exports.

To list exported output values (AWS CLI)

Use the following list-exports command. Replace us-east-1 with your AWS Region.

aws cloudformation list-exports --region us-east-1

The following is example output.

{
    "Exports": [
        {
            "ExportingStackId": "arn:aws:cloudformation:us-west-2:123456789012:stack/private-vpc/99764070-b56c-xmpl-bee8-062a88d1d800",
            "Name": "private-vpc-subnet-a",
            "Value": "subnet-07b410xmplddcfa03"
        },
        {
            "ExportingStackId": "arn:aws:cloudformation:us-west-2:123456789012:stack/private-vpc/99764070-b56c-xmpl-bee8-062a88d1d800",
            "Name": "private-vpc-subnet-b",
            "Value": "subnet-075ed3xmplebd2fb1"
        },
        {
            "ExportingStackId": "arn:aws:cloudformation:us-west-2:123456789012:stack/private-vpc/99764070-b56c-xmpl-bee8-062a88d1d800",
            "Name": "private-vpc-vpcid",
            "Value": "vpc-011d7xmpl100e9841"
        }
    ]
}

CloudFormation shows the names and values of the exported outputs for the current region and the stack they were exported from. To use an exported output value in another stack's template, you can reference it using the export name and the Fn::ImportValue function.

Listing stacks that import an exported output value

To delete or change exported output values, you must first find out which stacks are importing them.

To list stacks that import an exported output value (console)

1. Open the AWS CloudFormation console at https://console.aws.amazon.com/cloudformation.

2. From the left navigation pane, choose Exports.

3. To see which stacks import a given export value, choose the Export Name for that export value. CloudFormation displays the export details page, which lists all the stacks that are importing the value.

To list stacks that import an exported output value (AWS CLI)

Use the list-imports command. Replace us-east-1 with your AWS Region and private-vpc-vpcid with the name of the exported output value.

aws cloudformation list-imports --region us-east-1 \
    --export-name private-vpc-vpcid

CloudFormation returns a list of stacks that are importing the value.

{
    "Imports": [
        "my-app-stack"
    ]
}

Once you know which stacks are importing a particular exported value, you need to modify those stacks to remove the Fn::ImportValue functions that reference the output values. You must remove all the imports that reference exported output values before you can delete or modify the exported output values.