1.1 Choosing a terminology server view

SNOMED CT Release Format 2 is designed to enable the distribution and use of a full historical view of SNOMED CT from its first release in 2002 up to its most recent release. This allows terminology servers to provide a range of different views of SNOMED CT . However, it does not require that all terminology servers support the full range of views.
identifies three options for the views that a SNOMED CT terminology server may support. The simplest of these is the single snapshot view which provides access to a single release version . This closely matches the view provided by the original SNOMED CT release format ( RF1 ). The most powerful full view which allows the server to provide access to any selected version of SNOMED CT from a single representation of the SNOMED CT resource. This makes full use of the version features in RF2 . Alternatively a server may provide a selected set of snapshots representing versions of known interest to its users.
People designing a terminology server need to decide whether their server will only provide access to a single current view of the SNOMED CT resource or will also support retrospective views of earlier versions of the terminology. The single snapshot view is simplest to implement and matches the service most vendors offered with original SNOMED CT release format ( RF1 ). A more complete view is now possible using Release format 2 and this offers several significant advantages. It supports incremental updates allowing smoother transition as new versions become available. It also allows changes between versions to be detected more easily and can be used to evaluate queries against an earlier version for comparative purposes.
People choosing a terminology server need to consider whether a server that only supports a single snapshot view of the current version meets their requirements. If they require access to previous versions a server that supports the full view is likely to be the best long term solution. A server that allows access to multiple discrete snapshots may provide a reasonable interim solution but may be less flexible and less easy to maintain.

Table 1. SNOMED CT views that may be supported by terminology servers
View	Description
Snapshot view	A snapshot view terminology service provides access to the content of the current state of all the components of the International Release and any chosen Extension Releases .
Multi- snapshot view	A "multi- snapshot view " terminology service provides access to: the content of the current state and content of all components of the International release and any chosen Extension Releases ; the content of one or more additional snapshot views , each of which represents the state of all components at a different fixed point in time. A "multi- snapshot view " terminology server may provide access to delta views that report the differences between two snapshot views . This is limited to comparisons of specific points represented by the available snapshot views .
Full view	A Full view terminology service provides access to: the complete content of the full International release and any chosen Extension Releases ; the state and content of all components as they were at any specified point in time. A full view terminology server should also provide access to views that show the changes to components between any two specified points in time.

The full view is required to support some SNOMED CT use cases but many requirements can be adequately met by providing access to a current Snapshot view . The multi- snapshot view is an approach that may meet some requirements that are not met by a single snapshot without requiring support for the Full view .
Note: terminology servers that do not support the Full view still need to be able to import from a Full release as Extension providers are not required to provide the snapshot or delta releases ( Importing release types )

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1.1 Choosing a terminology server view