About modification tracking for configuration changes

For configuration data, when a modification is made to a table in the database, a record for the modification is inserted into the ModLogTable table. One row will be inserted for each separate type of modification, either an insert, update, or delete.

The actual value changes are recorded in the ModLogColumn table. Each column that is modified will result in a row inserted into the ModLogColumn table. The entry in the ModLogColumn table includes both the column value before the change and the new column value.

For example, if you added (inserted) a single analog tag to the system, the following changes would be reflected in the modification tracking tables:

• Two rows would be added to the ModLogTable table, one to track the change to the Tag table and one to track the change to the AnalogTag table.

• One row for each of the columns in both of the Tag and AnalogTag tables will be added to the ModLogColumn table.

As another example, if you updated for a single analog tag the StorageType column in the Tag table and the ValueDeadband and RateDeadband columns in the AnalogTag table, the following changes would be reflected in the modification tracking tables:

• Two rows would be added to the ModLogTable table, one to track the change to the Tag table and one to track the change to the AnalogTag table.

• Three rows would be added to the ModLogColumn table to record the changes to the StorageType, ValueDeadband, and RateDeadband columns.

• Important things to note:

For a tier-2 Historian, modification tracking for a replicated tag appears as being made by the system account that the configuration service is running under, which is typically NT AUTHORITY\SYSTEM. To find out who modified a tag, examine the ModLogTable of the tier-1 Historian.

If modification tracking is enabled and tags are configured in offline mode and modified, only the last known modifications are recorded in the modification tables.