Using its native data storage, Sentences can show us data values as they were at any past point in time.

We used this diagram to illustrate the associative model's transaction commit mechanism, whereby all updates are accomplished by adding items and links to the database.

This mechanism also allows users to see data as it was at any time in the past by specifying a date and time at which we wish to view or query the data.


For example, if we query Google's balance and specified that we wanted to see its value as at 10:15 on 10-May-2011, it would say $100,000. If we queried its value as at 18:45 on 11-Jun-2011, it would say $125,000.

This is because the database's visibility layer screens out any transaction whose timestamp is later than our chosen date/time. Any item or link whose transaction is not visible is likewise not visible.

The identical mechanism applies also to changes to the database's schema, so schema and data will always be in step.


Of course, the database also provides archiving mechanisms so that genuinely old data may be removed from the active chapters.   The user may revisit this data at any time simply by adding the archive chapter to their profile.
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