A typical extension scenario is the development of the extension itself. Whether you are starting your extension from scratch or already have a well-developed version that you need to maintain, the first choice you need to make is to identify the dependencies of your SNOMED CT Extension.
If your Extension extends the SNOMED CT International Edition directly, then you need to pick one of the available International Edition versions:
If you are starting from scratch, it is always recommended to select the latest International Release as the starting point of your Extension.
If you have an existing Extension then you probably already know the International Release version your Extension depends on.
When you have identified the version you need to depend on then you need to import that version (or a later release packages that also includes that version in its FULL RF2 package) first into Snow Owl. Make sure that the createVersion feature of the RF2 import process is enabled, so it will automatically create the versions for each imported RF2 effectiveTime value.
After you have successfully imported all dependencies into Snow Owl, the next step is to create a Code System that represents your SNOMED CT Extension (see Core API). When creating the Code System, besides specifying the namespace and optional modules and languages, you need to enter a Code System shortName, which will serve as the unique identifier of your Extension and select the extensionOf value, which represents the dependency of the Code System.
After you have successfully created the Code System representing your Extension, you can import any existing content from a most recent release or start from scratch by creating the module concept of your extension.
RF2 releases tend to have content issues with the International Edition itself or refer to missing content when you try to import them into Snow Owl via the RF2 Import API. For this reason, the recommended way is to always use the most recent Snapshot RF2 release of a SNOMED CT Extension to form its first representation in Snow Owl. That has a high probability of success without any missing component dependency errors during import. If you are having trouble importing an RF2 Release Package into Snow Owl, feel free to raise a question on our GitHub Issues page.
If your Extension needs to extend another Extension and not the International Edition itself, then you need to identify the version you'd like to depend on in that Extension (that indirectly will select the International Edition dependency as well). When you have identified all required versions, then starting from the International Edition recursively traverse back and repeat the RF2 Import and Code System creation steps described in the previous section until you have finally imported your extension. In the end your extension might look like this, depending on how many Extensions you are depending on.
Setting up a Snow Owl deployment like this is not an easy task. It requires a thorough understanding of each SNOMED CT Extension you'd like to import and their dependencies as well. However, after the initial setup, the maintenance of your Extension becomes straightforward, thanks to the clear distinction from the International Edition and from its other dependencies. The release process is easier and you can choose to publish your Extension as an extension only release, or as an Edition or both (see Release). Additionally, when a new version is available in one of the dependencies, you will be able to upgrade your Extension with the help of automated validation rules and upgrade processes (see Upgrade). From the distribution perspective, this scenario shines when you need to maintain multiple Extensions/Editions in a single deployment.
Pros:
Excellent for authoring and maintenance
Good for distribution
Cons:
Harder to set up the initial deployment
This section describes the use case scenarios present in the world of SNOMED CT and how Snow Owl can be used in those scenarios to maximize its full potential. Each scenario comes with a summary and a pros/cons section to help your decision making process when selecting the appropriate scenario for your use case.
The most common use case to consume a SNOMED CT Release Package is to import it directly into a Terminology Server (like Snow Owl) and make it available as read-only content for both human and machine access (via REST and FHIR APIs).
Since Snow Owl by default comes with a pre-initialized SNOMED CT Code System called SNOMEDCT, it is just a single call to import the official RF2 package using the SNOMED CT RF2 Import API The import by default creates a Code System Version for each SNOMED CT Effective Date available in the supplied RF2 package. After a successful import the content is immediately available via REST and FHIR APIs.
National Release Centers and other Care Providers provide their own SNOMED CT Edition distribution for third-party consumers in RF2 format. Importing their Edition distribution instead of the International Edition directly into the SNOMEDCT pre-initialized SNOMED CT Code System with the same SNOMED CT RF2 Import API makes both the International Edition (always included in Edition packages) and the National Extension available for read-only access.
The single edition scenario without much effort provides access to any SNOMED CT Edition directly on the pre-initialized SNOMEDCT Code System. It is easy to set up and maintain. Because of its flat structure, it is good for distribution and extension consumers. Although it can be used for authoring in certain scenarios, due to the missing distinction between the International Edition and the Extension, it is not the best choice for extension authoring and maintenance.
This scenario can be further extended to support multiple simultaneous Edition releases living on their own dedicated SNOMED CT Code Systems. The Root SNOMEDCT Code System in this case is empty and only serves the purpose of creating other Code Systems "underneath" it. Each SNOMED CT Code System is then imported into its own dedicated branch forming a star-like branch structure at the end (zero-length MAIN branch and content branches). This is useful in distribution scenarios, where multiple Extension Code Systems need to be maintained with their own dedicated set of dependencies and there is no time to set up the proper Extension Scenario (see next section). The only drawback of this setup is the potentially high usage of disk space due to the overlap between the various Editions imported into their own Code Systems (since each of them contains the entire International Release).
Pros:
Good for maintaining the SNOMED CT International Edition
Good for distribution
Simple to set up and maintain
Cons:
Not recommended for extension authoring and maintenance
Not recommended for multi-extension distribution scenarios
On top of single Edition/Extension distribution and authoring, Snow Owl provides full support for multi-SNOMED CT distribution and authoring even if the Extensions depend on different versions of the SNOMED CT International Edition.
To achieve a deployment like this you need to perform the same initialization steps for each desired SNOMED CT Extension as if it were a single extension scenario (see single extension). Development and maintenance of each managed extension can happen in parallel without affecting one or the other. Each of them can have their own release cycles, maintenance and upgrade schedules, and so on.
After you have initialized your Snow Owl instance with the Extensions you'd like to maintain the next steps are:
