Updatable ledger tables
Updatable ledger tables are system-versioned tables on which users can perform updates and deletes while also providing tamper-evidence capabilities. When updates or deletes occur, all earlier versions of a row are preserved in a secondary table, known as the history table. The history table mirrors the schema of the updatable ledger table. When a row is updated, the latest version of the row remains in the ledger table, while its earlier version is inserted into the history table by the system, transparently to the application.
Both updatable ledger tables and temporal tables are system-versioned tables, for which the database engine captures historical row versions in secondary history tables. Either technology provides unique benefits. Updatable ledger tables make both the current and historical data tamper evident. Temporal tables support querying the data stored at any point in time instead of only the data that's correct at the current moment in time. You can use both technologies together by creating tables that are both updatable ledger tables and temporal tables.
You can create an updatable ledger table by specifying the
LEDGER = ON argument in your CREATE DATABASE (Transact-SQL) statement.
LEDGER = ON is optional when creating updatable ledger tables in a ledger database. By default, each table is an updatable ledger table in a ledger database.
For information on options available when you specify the
LEDGER argument in your T-SQL statement, see CREATE TABLE (Transact-SQL).
After a ledger table is created, it can't be reverted to a table that isn't a ledger table. As a result, an attacker can't temporarily remove ledger capabilities on a ledger table, make changes, and then reenable ledger functionality.
Updatable ledger table schema
An updatable ledger table needs to have the following GENERATED ALWAYS columns that contain metadata noting which transactions made changes to the table and the order of operations by which rows were updated by the transaction. This data is useful for forensics purposes in understanding how data was inserted over time.
If you don't specify the required
GENERATED ALWAYS columns of the ledger table and ledger history table in the CREATE TABLE (Transact-SQL) statement, the system automatically adds the columns and uses the following default names. For more information, see examples in Creating an updatable ledger table.
|Default column name||Data type||Description|
|ledger_start_transaction_id||bigint||The ID of the transaction that created a row version|
|ledger_end_transaction_id||bigint||The ID of the transaction that deleted a row version|
|ledger_start_sequence_number||bigint||The sequence number of an operation within a transaction that created a row version|
|ledger_end_sequence_number||bigint||The sequence number of an operation within a transaction that deleted a row version|
The history table is automatically created when an updatable ledger table is created. The history table captures the historical values of rows changed because of updates and deletes in the updatable ledger table. The schema of the history table mirrors that of the updatable ledger table it's associated with.
When you create an updatable ledger table, you can either specify the name of the schema to contain your history table and the name of the history table or you have the system generate the name of the history table and add it to the same schema as the ledger table. History tables with system-generated names are called anonymous history tables. The naming convention for an anonymous history table is
For every updatable ledger table, the system automatically generates a view, called the ledger view. The ledger view is a join of the updatable ledger table and its associated history table. The ledger view reports all row modifications that have occurred on the updatable ledger table by joining the historical data in the history table. This view enables users, their partners, or auditors to analyze all historical operations and detect potential tampering. Each row operation is accompanied by the ID of the acting transaction, along with whether the operation was a
DELETE or an
INSERT. Users can retrieve more information about the time the transaction was executed and the identity of the user who executed it and correlate it to other operations performed by this transaction.
For example, if you want to track transaction history for a banking scenario, the ledger view provides a chronicle of transactions over time. By using the ledger view, you don't have to independently view the updatable ledger table and history tables or construct your own view to do so.
For an example of using the ledger view, see Create and use updatable ledger tables.
The ledger view's schema mirrors the columns defined in the updatable ledger and history table, but the GENERATED ALWAYS columns are different than those of the updatable ledger and history tables.
Ledger view schema
The ledger view column names can be customized when you create the table by using the
<ledger_view_option> parameter with the CREATE TABLE (Transact-SQL) statement. For more information, see ledger view options and the corresponding examples in CREATE TABLE (Transact-SQL).
|Default column name||Data type||Description|
|ledger_transaction_id||bigint||The ID of the transaction that created or deleted a row version.|
|ledger_sequence_number||bigint||The sequence number of a row-level operation within the transaction on the table.|