How can I quickly and effectively debug CloudFormation templates?

Question

CloudFormation is a powerful AWS offering that allows the programmatic creation of AWS resource stacks, such as the web tier of an application, a high performance computing cluster, or an entire application stack, with a single API call. It is immensely powerful. Using it is surely considered a good AWS practice, especially when it's combined with Chef, Puppet, or cloud-init. Debugging it drives me to vice.

Take a production example: The stock mongodb cluster templates won't work for me. I don't particularly know why. I'm sure it's something simple as it almost always is. My problem isn't that I can't figure out what's wrong. It's that it takes the stack between 20 and 30 minutes to fail, and then another three or four minutes to delete, assuming it deletes the resources properly at all.

What am I missing? I know about the --disable-rollback flag and use it like oxygen. I learned long ago to wrap exit messages with cfn-signal and to throw them like ballast off a sinking ship. How can I make the template debugging process faster, or am I stuck forever noticing my mistakes half an hour after I make them?

Answer 1

Use the aws cloudformation validate-template command in the AWS CLI tool. It only validates whether your template is valid JSON or YAML, not whether your keys and values are correct (for example doesn't check for typos in keys)

Answer 2

Another option, a year later, is to abstract these templates to a 3rd party library, such as troposphere. That library constructs the JSON payload for you, and does a lot of validation along the way. This also solves the "Wow managing a 1000-line JSON file sure is sad" problem.