Up until recently, JSON was your only option. We actually encourage you to adopt YAML, and we'll be using it for all of the examples that are shown in this book. Some of the reasons for this are as follows:

• It's just nicer to look at. It's less syntax-heavy, and should you choose to go down the path of generating your CloudFormation templates, pretty much every language has a YAML library of some kind.
• The size of your templates will be much smaller. This is more practical from a developer's point of view, but it also means that you're less likely to run into the CloudFormation size limit on template files (50 KB).
• The string-substitution features are easier to use and interpret.
• Your EC2 UserData (the script that runs when your EC2 instance boots) will be much easier to implement and maintain.