Tri-state locking in database

APPLIES TO: Business Central 2023 release wave 2 (version 23) and later

The tri-state locking feature is aimed at enhancing the performance and concurrency of database transactions. By enabling this feature, AL-based read operations that follow write operations are performed optimistically, rather than with strict consistency and low concurrency. So, users can expect higher levels of concurrency and fewer blocked or failed operations while accessing data.

Note

Explicitly using the LockTable method in code will maintain the same behavior, disabling optimistic reads.

Locking behavior

The Business Central server uses database locks on a table when a session starts to change data in the table. The following examples illustrate the legacy locking behavior, used in versions 22 and earlier, compared to tri-state locking.

// locking behavior in versions 22 (and earlier)
trigger OnAction()
var
    currency1: Record Currency;
    currency2: Record Currency;    
begin
    currency1.FindFirst(); // SQL statement use READUNCOMMITTED

    currency1.Code := 'DKK';
    currency1."ISO Code" := 'DKK';
    currency1.Symbol := 'Kr';
    currency1.Insert();

    currency2.FindLast(); // SQL statement use UPDLOCK (so locks data with an exclusive lock)
end;

Contrast this behavior with how locks are taken with tri-state locking.

// locking behaviour with tri-state locking
trigger OnAction()
var
    currency1: Record Currency;
    currency2: Record Currency;    
begin
    currency1.FindFirst(); // SQL statement use READUNCOMMITTED

    currency1.Code := 'DKK';
    currency1."ISO Code" := 'DKK';
    currency1.Symbol := 'Kr';
    currency1.Insert();

    currency2.FindLast(); // SQL statement use READCOMMITTED (so locks data with a shared lock)
end;

In both cases, the locking behavior is about what happens in a session after a record instance does a data modification (insert/update/delete) on a table.

Properties	Locking behavior in versions 22 (and earlier)	Locking behavior with tri-state locking
Default isolation level for subsequent operations	UpdLock	ReadCommitted
Locking behavior	Session would acquire update lock on data from the table until it committed or rolled back its changes.	Session only acquires a shared lock when reading data.
Consequences	Could cause blocking and contention issues when multiple sessions tried to access or modify the same table.	Allows other sessions to read and write to the same table concurrently, if they don't conflict with each other's changes.

Enable and disable tri-state locking for a tenant

Tri-state locking is enabled by default for Business Central online and on-premises. If you prefer to use legacy locking, you can disable tri-state locking from the Feature Management page as follows:

• In version 25, set the Enabled for field for the Feature: Enable legacy locking scheme in AL to All users:

• In versions 24 and 23, set the Enabled for field for Feature: Enable tri-state locking in AL to None.

Changes take effect on users the next time they sign in to Business Central. Learn more about feature management in Enabling Upcoming Features Ahead of Time.

Note

If you're using Business Central on-premises, the EnableTriStateLocking setting in the server configuration must also be set to true to use tri-state locking. Learn more about configuring the server.

See also

Release plan: Performance gain by reducing locks in the database
Monitoring SQL database locks