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cfn-cross-region-export

The problem

When you're building a multi-region infrastructure using CloudFormation, you're often faced with the problem of linking resources from a region to another. For example, if you've created a Route53 hosted zone for your main domain using a stack in the us-east-1 region but you want to create a DNS record from a stack in the ca-central-1 region, you'll need to have access to the HostedZoneId. Usually, if both stacks were in the same region you could to a simple Fn::ImportValue but this isn't going to work this time since this function does not support cross-region referencing. As a workaround, you could decide to use a CloudFormation parameter but this limits the automation that can be done as it requires a manual intervention.

The solution

In a nutshell this project shares the same features as Fn::ImportValue but allows values to be imported from other regions of the same account.

The implementation

The project is divided in 2 parts; the Exporter and the Importer. Only one Exporter stack is needed per region you want outputs to be imported from. The Importer stack on the other hand, need to be instantiated for each region you want to import outputs from.

Here's an example use-case: Let's say you are creating some resources in the ca-central-1 region and you need to import values from the us-east-1 and eu-west-1 regions. You'll need to first provision the Exporter stack in both us-east-1 and eu-west-1 region. You'll then have to provision 2 Importer stacks in the ca-central-1 region, each targeting a specific region.

Usage

Resources: 
  Importer:
    Type: Custom::CrossRegionImporter
    Properties:
      ServiceToken: !ImportValue 'us-east-1:CrossRegionImporterServiceToken'
      Exports:
        Xyz: 'xyz-export-name'
      
  TestImport:
    Type: AWS::SSM::Parameter
    Properties:
      Type: String
      Value: !GetAtt Importer.Xyz

Installation

Important info

If you were using this project in the release v0.1 and before, you need to run the dynamodb key migration script located in migration-script/migrate_dynamo_keys.py. Doing this will migrate rows with the old naming to the new naming.

To run this script you will need to have at least a read access to CloudFormation and a read/write access to the DynamoDB table. the following environment variable also need to be set:

export CROSS_STACK_REF_TABLE_ARN=<THE DYNAMODB TABLE ARN>

Exporter

Start by deploying the Exporter

export AWS_DEFAULT_REGION=<EXPORTER_REGION>
make deploy-exporter SENTRY_ENV=prod SENTRY_DSN=https://...@sentry.io/...

Importer

You can find the CROSS_STACK_REF_TABLE_ARN in the output section of the Exporter stack we've just deployed.

export AWS_DEFAULT_REGION=<IMPORTER_REGION>
make deploy-importer CROSS_STACK_REF_TABLE_ARN=...

Development

Exporter

Create a DynamoDB table. The python script for the Exporter can be ran locally like so:

export SENTRY_DSN=<A SENTRY DSN>
export SENTRY_ENV=<dev|stage|prod|...>
export GENERATED_STACK_NAME='dev-ImportsReplication'
export CROSS_STACK_REF_TABLE_NAME=<THE DYNAMODB TABLE NAME>
python3 exporter/lambda/cross_region_import_replication.py

Just make sure you have these permissions attached to your IAM user (or role):

dynamodb:Scan
cloudformation:CreateStack
cloudformation:UpdateStack
ssm:PutParameter
ssm:DeleteParameter

Importer

Since the script importer/lambda/cross_region_importer.py is expecting to be called in the context of a
CloudFormation custom resource, I suggest to test your modifications using trials and errors. Which means that you edit the script and then deploy it using the method described in the Installation section. You can leverage CloudWatch to help you with the debugging.

TODO

  • Support cross-account imports (using assume-role it should be fairly easy to do)
  • Make the SentryDsn parameter optional