Package com is a user-facing interface to the object registry. Since the only part of the API you need to use is Register, the rest of the API for interacting with an object registry is in its own subpackage. This is used by other tooling built around com, for example the config subpackage. When you register an object, it will populate fields based on the com struct tags used on them. The object will then also be used to populate fields of other objects in the registry where the type or interface matches. In the above example component, it has fields with all three possible struct tags: Singleton will pick the first object in the registry that implements that interface. You can also use pointers to concrete types, for example to other component types. Extpoint is going to be a slice of all objects in the registry that implement that interface. Config is not populated, but is allowed to be populated via the registry API. If you're using the config package, it will do this for you and populate it based on configuration. In this case, the key would be "DB" and the value could be the name of any registered component that implements api.Store.