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. This means that if it cannot find the requested data at the specified hierarchy level, it will automatically check the common level, which is designed to provide default values for a given key. This feature allows for a fallback mechanism, ensuring that the absence of data at a higher level does not result in an error or failure in data retrieval.

In a multi-tier data structure, having a common level provides a safety net for your configurations, ensuring that your Puppet modules can still function properly, even if specific values aren't defined at higher levels. This hierarchical approach enhances the manageability of configurations, allowing for centralized defaults without requiring explicit specifications at every level. Thus, the design of Hiera supports flexibility and robustness in managing configuration data.