Is there a way to run initial SQL when creating an RDS database instance using CloudFormation?

Solution 1:

No, it's not possible. However, you could have an EC2 instance connect to your RDS instance to do it. I'd probably store a .sql file in S3 and use a cloud-init script on the EC2 instance to download the file and execute it.

Solution 2:

It would also be possible to create a CloudFormation custom resource. There is a good discussion about how to build one using SNS here; it is also possible to build one using Lambda. Custom resources are essentially just RPCs, so it wouldn't be difficult to create one to initialize a database with a schema, for example.

Solution 3:

CloudFormation still doesn't hold any solutions for us, but hopefully they will add Database Migration Service support soon.

In the meantime, there is great solution if you're using CodePipeline: create a migration stage that invokes a Lambda function to run your migration. I stumbled across this guide for invoking Lambda from CodePipeline that may be helpful for those unfamiliar.