What happens when Hiera looks up a key that does not exist at the top level?

When Hiera looks up a key that does not exist at the top level, it defaults to the common level. Hiera is designed to provide a hierarchical way to manage configuration data and allows users to define different data sources that can be searched in a specific order.

The common level is a fallback that Hiera checks if the specified key isn't found at the top level or more specific levels. This means that if a key is not present in the specific hierarchy, Hiera will look for that key in a broader context (i.e., at the common level) to try and retrieve the appropriate data.

The ability to default to the common level is crucial for ensuring that resources can still be configured even in cases where specific key entries may be missing in a particular node or environment. It facilitates sharing common configurations across various nodes without requiring every node to have its own specific key defined. Thus, this mechanism helps maintain a clean and efficient configuration management process across different environments and applications.