Using Full and Snapshot Releases

Meeting Version Access Requirements

A key decision when designing any solution intended to provide to SNOMED CT is whether it should support import files from the full release, the current snapshot release or both. The determining factor when considering these options is the range of SNOMED CT versions that to which the solution needs to provide access.  Table 4.2-1 identifies the release type import options that can be used to meet particular requirements to for access to different sets of SNOMED CT versions. The release type options are summarized in the sections following this table.

Table 4.2-1: Release Type Import Options to Support Version Access Requirements

Import Current Snapshot Release Only

Description

• A single set of tables is created for all the release files that need to be imported.
• Data is imported into these table from the current snapshot release.
• The database is optimized by appropriate additional indexes.
• Access to common combinations of data from multiple tables may be facilitated by creating database views, procedures and functions.

Advantages

• Simple solution which performs well.
• Supports to access the current snapshot release and current delta view.

Disadvantages

• No access to previous snapshot views.
• Cannot access to the previous state of components in the current delta view. 

Import Current and Previous Snapshot Releases

Description

• Two sets of tables are created for all the release files that need to be imported.
• Data from the current snapshot release is imported into one of these sets of tables
• Data from the previous snapshot release is imported into the other set of tables. 
• The database is optimized  by appropriate additional indexes.
• Access to common combinations of data from multiple tables may be facilitated by creating a sets of database views, procedures and functions applicable to each set of tables.
• Access to information about changes to data between the two snapshot release may be facilitated by additional views, procedures and functions that combine or compare data from the two versions.

Advantages

• Simple duplication of the single snapshot view which performs well.
• Supports to access the current and previous snapshot releases and current and previous delta views.
• Supports access to previous state of components in the current delta view.

Disadvantages

• No access to snapshot views prior to the previous version.
• Heavy use of disk space as a result of duplication of rows that are identical in both snapshots. Each snapshot takes more than 80% of the space required by the full release.
• Not extensible because this approach is not realistic for multiple versions due to a linear increase in redundant use of disk space.

Import Full Release Only

Description

• A single set of tables is created for all the release files that need to be imported.
• Data is imported into these table from the full release.
• The database is optimized  by appropriate additional indexes.

• Access to data in specific versions is facilitated by virtual snapshot views that can be accessed in the same way as database tables³ .

• Access to common combinations of data from snapshot views of tables may be facilitated by creating database views, procedures and functions.
• Access to information about changes to data between any two snapshot release may be facilitated by additional views, procedures and functions that combine or compare data from those versions.

Advantages

• Access to the complete release history of all versions of a SNOMED CT Edition.
• Efficient use of disk space requiring only 20% more disk space than importing a single snapshot release.
• Able to access snapshot views for any date.
• Able to access delta views for any date range.

Disadvantages

• Virtual views performs less well than a native database tables because the content of a view is the result of query on a database table.

Import Full and Current Snapshot Releases

• Two set of tables are created for all the release files that need to be imported.
• Data is imported into these table from the full release and current snapshot release.
• The database is optimized  by appropriate additional indexes.

• Access to data in specific versions is facilitated by virtual snapshot views that can be accessed in the same way as database tables³ .

• Access to common combinations of data from the snapshot tables or from the snapshot views of full release tables may be facilitated by creating database views, procedures and functions.
• Access to information about changes to data between any two snapshot release may be facilitated by additional views, procedures and functions that combine or compare data from those versions.

Advantages

• Access to the complete release history of all versions of a SNOMED CT Edition.
• High performance access to the current snapshot view using the snapshot release table rather than a virtual snapshot view. 
  • This is a significant advantage as the current snapshot is the most commonly used view.
• Able to access snapshot views for any date.
• Able to access delta views for any date range.

Disadvantages

• Requires approximately 80% more disk space than only importing the full release.
  • However, unlike the use of multiple snapshot tables. this disk space increase with each new release is determined by numbers of additions and changes rather and the only redundancy is a single version snapshot.

Importing the Delta Release

There is no need to import the delta release when importing either the full or snapshot releases. This is because rows that have changed in a release can be identified using their effectiveTimes. So it is possible to extract the most recent delta view from a snapshot release and all delta views can be extracted from a full release.

There is one potential use case for importing a delta release and that is to update a database containing the previous release to current release.  A full release table for a specified previous release date can be updated by simply inserting all the rows from a the relevant delta release file for the period starting immediately after the previous release date up to and including a new release date.

In practice, it may be more efficient to start with an empty database and import the latest full release. However, the delta release update option may be useful where there is requirement to retain the operational integrity of the database during the update process. Future plans for more frequent or "continuous" updates to the SNOMED CT International Edition may also benefit from updates using delta releases of small number of changes or additions.