Each Code System in Snow Owl is managed in a repository where every state change of an artifact is stored as a revision to provide a full change history over time. When the changes to the Code System reach a particular state to be marked, the revision can be distinguished by a tag through a process called versioning.
Commonly used Code Systems are frequently extended by the national health authorities even if very little is specified by the group maintaining the particular terminology on how to manage national or local extensions. Snow Owl provides a common mechanism for extension management where work performed on a particular extension is isolated from other extensions or the Code System being extended. Extension content is stored on a branch that is forked from a particular version of the Code System to be extended. The extension branch has access to all of the content that is stored on its parent branch up to the point of the fork. Note, that extensions to a Code System are also registered as a Code System hence a repository can maintain more than one Code System.
Using this approach, multiple extensions can be maintained even if these extensions depend on different versions of the base Code System. The figure below depicts a scenario where Extension1 is extending Version1 of the base Code System while Extension2 extending Version2 of the same base Code System.
As Code System extensions are organized into hierarchies, a dependency chain can be expressed between Code Systems where the dependency Code System is the Code System being extended, whereas the extension Code System is a Code System that extends the code system in question. As an example, the dependency chain for the above depicted scenario would look like this:
Base
Extensions
Extension1
Extension2
Versions
Version1
Version1
Extension1
Dependencies
Base - Version1
Versions
Ext1 version1
Extension2
Dependencies
Base - Version2
Versions
Ext2 version1
