Custom resources

As discussed previously in this chapter, it's common for there to be a lengthy wait between the release of a new AWS feature and your ability to use that feature in CloudFormation.

Before custom resources, this led AWS developers down the path of doing over 95 percent of their automation in CloudFormation and then running some CLI commands to fill in the gaps. It was often difficult to tell exactly which resources belonged to which stack, and knowing exactly when your stack had finished execution became a guessing game.

Fast forward to today, and the emerging pattern is to use a custom resource to delegate to a AWS Lambda function. Lambda can fill in the gaps by making API calls on your behalf, and it becomes much easier to track the heritage and completion of these resources.