Requirements

All SNOMED CT release packages must include a set of full release files for a SNOMED CT edition or extension. A full release includes the complete history of the content of that edition or extension. This allows a terminology server to enable access to versioned views of the content of this edition or extension as it was at any point between its first release and the current release. However, the extent to which a terminology service supports access to different versioned views may vary.
Terminology service providers must ensure that their services optimize the performance of access to the current snapshot view of a selected edition or extension. They should also provide services that enable access to earlier snapshot views and delta views to support the analysis of data entered using earlier versions of the same edition or extension and terminology change management when updating to a newer version.
Healthcare application providers must ensure that their applications can use terminology services that provide access to the versioned views of SNOMED CT required by their users. Services that support access to the snapshot view of the versioned edition currently in use should be selected by default and should be optimized. Services that enable access to a new version of the same edition should also be accessible to meet user requirements for managing updates to a newer versioned edition¹. Access to earlier versions of the same edition should also be accessible to meet requirements for analysis of existing data recorded using terminology content that has subsequently been inactivated.
Terminology service users must procure applications and terminology services that optimize access to the snapshot view of a selected versioned edition that they are currently using. They must also ensure that the applications and terminology services that they procure meet their requirements for managing updates to a newer versioned edition¹. They should also ensure that the application and terminology services can meet their requirement for analysis of previously entered data recorded using terminology content that has subsequently been inactivated.

Types of Versioned Views

A versioned view is a set of component versions and reference set member versions defined by characteristics of their effectiveTimes.

Notes

Versioned views and release types are closely related. A release type is a physical representation of a particular versioned view.
Some versioned views are not instantiated as release types but all valid versioned views of a SNOMED CT edition can be generated from a full release of that edition.

A delta view is a view of SNOMED CT that contains only rows that represent changes to components and reference set members since a specified date or between two specified dates in the past.
A full view is a view of SNOMED CT that includes all versions of all components and reference set members in a full release.
A snapshot view is a view of SNOMED CT that includes the most recent version of all components and reference set members at a specified point in time.

There are three types of versioned views:

A snapshot view is a view of SNOMED CT that includes the most recent version of all components and reference set members at a specified point in time.

- The current snapshot view is most important versioned view. It will be in constant use during day to day use of the terminology.
- When updating to a new version, access to the previous earlier snapshot view is required to determine the type of change made to each component and reference set member (see 4.9 Identify Changes to the Terminology).
- When reviewing, analyzing or reporting data collected while using earlier versions, access to snapshot views of the terminology at the time of data collection may assist interpretation of any unexpected variations.

A delta view is a view of SNOMED CT that contains only rows that represent changes to components and reference set members since a specified date or between two specified dates in the past.

- When updating to a new version, a delta view identifies components and reference set members that were added, updated or inactivated between to releases. to terminology content (see 4.9 Identify Changes to the Terminology).

A full view is a view of SNOMED CT that includes all versions of all components and reference set members in a full release.

- The ability to access the current full view may be useful for an organization that does not apply every update of the edition that they use. For example, if they receive data from another organization that includes content that is not in the snapshot view they are using.
- In exceptional circumstances, access to earlier versions of the full view may also be useful for detailed forensic review of data and resolution of anomalies.

Terminology Service Versioned View Options

Table 5.3.2-1 identifies options for the snapshot views that a SNOMED CT terminology server may support and Table 5.3.2-2 identifies the types of delta views that may be supported.

Table 5.3.2-1: Snapshot views that may be supported by terminology servers

View	Description
Single snapshot view of an edition	Some terminology services may only support access to a single snapshot view of a specified SNOMED CT edition at a one preset point in time. The specified time in this case is the current version in use by the client application. Individual service requests cannot specify a different version. When a decision is made to update to a newer version the server would be reconfigured to change the specified versioned view. In most cases, a terminology service that supports a single snapshot view of a specified edition only needs to import the relevant snapshot release files for that edition². A single snapshot view of an edition: Meets requirements for current data entry and display. This includes the display of data entered using earlier versions of the selected edition. Even if a concepts is now inactive, it will still have at least one active description. Does not meet requirements for change management when assessing and updating to a more recent version of the terminology. Does not meet the requirements for analysis of retrospective data that may have been recorded and previously analysed when an earlier version of the terminology was in use. Does not meet requirements for access to terminology data in SNOMED CT modules that are not part of the specified SNOMED CT edition.
Multiple snapshot views of an edition	Some terminology services may support access to snapshot views of a specified SNOMED CT edition for a limited number of specified points in time. The specified times in this case would typically include the current version in use by the client application and the version in use prior to the most recent update. Individual service requests can specify any of the supported versions. When a decision is made to update to a newer version the server would be reconfigured to add the new versioned view to the set of supported views. A terminology service that supports multiple versioned views of a specified edition can do this in one of two ways: Importing the snapshot release for each version of the edition that needs to be accessed²; or Importing the full release for the most recent version of the edition and then optimizing access to the required snapshot views³. Access to multiple snapshot views of an edition: Meets requirements for current data entry and display. This includes the display of data entered using earlier versions of the selected edition. Even if a concept if now inactive, it will still have at least one active description. Meets requirements for change management when assessing and updating to a more recent version of the terminology. Partially meets requirements for analysis of retrospective data that may have been recorded and previously analysed when an earlier version of the terminology was in use. This is limited to data that was originally recorded using one of the accessible versioned views. Does not meet requirements for access to terminology data in SNOMED CT modules that are not part of the specified SNOMED CT edition.
All snapshot views of an edition	Some terminology services may support access to snapshot views of a specified SNOMED CT edition at any specified specified point in time. The required version is specified by the client application as part of each terminology service call. A terminology service that supports all snapshot views of a specified edition can do this in one of three ways: Importing the snapshot release for all versions of the edition²; or Importing the full release for the most recent version of the edition and then optimizing access to all snapshot views; or Importing the full release for the most recent version of the edition to enable access to all snapshot views and specifically optimizing access to the current snapshot view and any other versioned that is likely to be frequently accessed³. A terminology service that supports all snapshot views of a specified edition needs to import the full release files for that edition. Access to all snapshot views of an edition: Meets requirements for data entry and display. Meets requirements for change management when assessing and updating to a more recent version of the terminology. Full change management functionality requires services that support comparisons of a component between versions. Meets requirements for analysis of retrospective data that may have been recorded and previously analysed when an earlier version of the terminology was in use. Does not meet requirements for access to terminology data in SNOMED CT modules that are not part of the specified SNOMED CT edition.
All snapshot views of a range of different editions ⁴	Some terminology services may support access to snapshot views of a range of different SNOMED CT editions at any specified specified point in time. The required edition and version is specified by the client application as part of each terminology service call. A terminology service that supports a range of snapshot views of a specified edition needs to import the full release files for all the modules required by one or more of the supported editions. Access to all snapshot views of a range of different editions: Meets requirements for data entry and display. Meets requirements for change management when assessing and updating to a more recent version of the terminology. Full change management functionality requires services that support comparisons of a component between versions. Meets requirements for analysis of retrospective data that may have been recorded and previously analysed when an earlier version of the terminology was in use. Meets requirements for access to terminology data in SNOMED CT modules that are part of any of the supported SNOMED CT editions.

Table 5.3.2-2: Delta views that may be supported by terminology servers

View	Description
Single delta view of changes between current version and previous version of an edition	Some terminology services may only support a single delta view of changes between current version and previous version of an edition. This is useful for reviewing the most recent changes to the terminology. However, a complete review of the impact of those changes also requires access the snapshot view of the previous release, as this is the baseline for the changes shown in the delta. (see 4.9 Identify Changes to the Terminology).
Multiple delta views of changes between a selected version and the preceding version of the same edition	Multiple delta views of changes between a specified version and the preceding version of the same edition. This is useful for reviewing the changes between earlier releases. It provides an opportunity to retrospectively remedy issues arising from change. However, complete review of the impact of those changes also requires access the snapshot view of the release prior to each delta view, as this is the baseline for the changes shown in the delta. (see 4.9 Identify Changes to the Terminology).
Configurable delta views of changes to an edition	Configurable delta views of changes to an edition between any two specified points provide a flexible solution that can identify a sequence of changes occurring to one or more components. Fully configurable delta views require access to data for a full release of the edition. This also provides the authoritative source for the starting point for all the changes shown in the delta.
Configurable delta views of a range of available editions	Configurable delta views of changes to one of a number of editions between any two specified points in time provide additional value in cases where more than one edition is in use.

Creating and Updating Versioned Views

Creating and Updating Full Views

To enable terminology services to access a full view of a SNOMED CT edition, a server must be able to import a full release of that edition into an appropriate datastore.

To enable access to a more recent full view of a SNOMED CT edition, a server may:

Import the latest full release as a replacement for the previous full release data; or
Import the latest delta release and append this data to the previous full release data.

Creating and Updating Snapshot Views

To provide access to the current snapshot view of a SNOMED CT edition, a server may:

Import the current snapshot release for that edition;
Selectively import data from the current full release for that edition, including only the most recent versions of each component or reference set member up to and including a specified snapshot time; or
Enable access to the full view of that edition to be filtered to include only the most recent version of each component prior to a specified snapshot time;

To provide access to more recent snapshot views of a SNOMED CT edition, a server may:

Use any of the above methods for providing access to a snapshot view for a different date, either in addition to or as a replacement for a previously imported snapshot view; or
Import the latest delta release and use this to update the previous snapshot view by appending new components and replacing existing components with updated versions.

Creating and Updating Delta Views

To provide access to the delta view for the current version of a SNOMED CT edition, a server may:

Import the delta release for that edition into an appropriate datastore;
Selectively import data from the current full release for that edition, including only component versions with effectiveTime values that are after the previous release date and are also on or before the current release date;
Enable access to the full view of that edition to be filtered to include only versions of components with effectiveTime values that are after the previous release date and on or before the current release date.

To provide access to other delta views of a SNOMED CT edition, a server may:

Use any of the above methods to create delta views between other release dates. In all cases the effectiveTime must be after one release date and on or before the next release date;
Import the full release and provide delta views by selectively filtering data to ensure the effectiveTime is greater than a specified start date and less than or equal to a specified end date.
- This method can be used to generate configurable delta views between specified dates that may not be directly related to adjacent release dates. In this type of delta view, there may be more than one version of a component with the specified date range. In this case, it may be useful to consider two possible delta views. One including only the most recent change to each component in the specified period and the other including all changes during that period.

Footnotes

1

For more information about version access requirements to support version update see 3.7 Terminology Change Management.

2

Organizations responsible fornational editions and extensions are only required to distribute a full release of their content. Therefore, terminology services that only support access to a specified set of snapshot views may need to enable importing of those snapshots from a full release. However, in practice most organizations also distribute a snapshot releases so this requirement may only occur in exceptional circumstances.

3

The snapshot release files for the 2020-01-31 International Edition requires 1.25 Gb of storage while the full release files requires just under 2 Gb of storage. Therefore, services that provide access to two or more separate snapshot views are likely to require more storage than services that use the full release to provide access to a complete range of snapshot views. However, storing and indexing the data for individual snapshots is likely to deliver significant performance advantages as it avoids the need to generate the snapshot in response to each service request.

4

It is theoretically possible for a terminology service to support any number of different SNOMED CT editions. However, as a large number of organizations are licensed to create SNOMED CT extensions, it is unlikely that a single instance of any terminology service would be required to provide access to every SNOMED CT edition.

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5.3.2 Supporting Versioned Views