Around the 6-month mark of CloudFormation use, everyone hits the same wall. "I want to auto-create Slack channels, but there's no AWS::Slack::Channel", "I want to register GitHub webhooks, but CFN has no resource", "Newly launched AWS services aren't supported in CFN yet." The tool filling this gap, introduced in 2015, is Custom Resource, and built on top of that in 2021 was CloudFormation Hooks, a pre-change validation mechanism. Though these tools do different work on the surface, they share the same deep philosophy — turning CloudFormation from a closed resource set into an extensible workflow engine.