MS-SQL 2000 – A Transaction LOG based Change Data Capture (=CDC) Solution.

Attunity’s CDC solution for MS-SQL 2000 relies on MS-SQL server 2000 transaction LOG files as the origin for captured changes.
SQL Server maintains transaction log files primarily for transaction recovery needs.
Attunity CDC solution accesses the data held there and extract the CDC portions which are propagated to the Staging Area container.
Needless to say that SQL Server activities with respect to the transaction log contents are free of any CDC considerations.

Particularly - MS-SQL data files are subjected to truncations / shrinking.
Under their space management policy, data files are periodically reorganized, either logically (=truncation) or physically (=shrinking).
When either is applied on the transaction LOG, data is not available anymore. Occasional truncation / shrinking of the transaction LOG (manual, BACKUP LOG) can expose the Attunity’s LGR(=Log Reader component) to potential loss of data.
In order to defend against this threat, Attunity’s LGR component is equipped with special programmatic means that protect logged records form being truncated before read.
They takes advantage of the fact that truncation(s) is restricted to non-active LOG space.

For achieving this highest degree of guaranteed delivery and availability, LGR is bundled as a Windows service which is configured to be ‘always up’.
As long as MSSQLserver is running – LGR would poll the transaction LOG for latest new changes, read and parse their contents and eventually save them in a transient storage folder.
Transient storage is later used by the MS-SQL CDC agent as it’s data origin for transmitting changes.