Schema and session database triggers

ABL supports two types of database triggers: schema and session. A schema trigger is a .p procedure that you add, through the Data Dictionary, to the schema of a database. Schema trigger code defined in the database is executed by database clients.

A session trigger is a section of code that you add to a larger, enclosing procedure.

You create schema triggers through the Table or Field Properties dialog box in the Data Dictionary. When you use the Data Dictionary to define a schema trigger for a table or field, the trigger is automatically added to the table or field’s data definitions. OpenEdge allows you to define the trigger while you are creating or modifying a table or field. This trigger definition is stored in a trigger procedure. For information on using the Data Dictionary to create and delete triggers, see OpenEdge Development: Basic Database Tools (Character only) and in graphical interfaces, the online help for the Data Dictionary. For more information schema triggers, see OpenEdge Development: ABL Essentials.

Although their syntax is slightly different, schema and session triggers provide similar functionality. The important difference between them is that schema triggers are independent procedures; whereas session triggers are contained within a larger procedure. Because of this difference, schema triggers always execute when a specified event occurs, regardless of what application initiates the event.

Session triggers are defined as part of a particular application and are only in effect for that particular application. Since session triggers are executed from within an enclosing procedure, they have access to the frames, widgets, and variables defined in the enclosing procedure.

Since schema triggers are compiled separately from the procedure that initiates their execution, they do not have access to the procedure’s frames, widgets, and variables. Use schema triggers for processing that you always want to perform for a specific event. For example, when an order record is deleted, you might always want to delete the corresponding order-line records.

Use session triggers to perform additional or independent processing when the event occurs. Both types of trigger scan return ERRORS that cause the associated event to fail. For more information on the ERROR option of the RETURN statement, see OpenEdge Development: ABL Reference.

You can define a schema and a session trigger for the same table/event or field/event pair. How the triggers interact depends on how you define them.

Ordinarily, both triggers execute, with the session trigger executing first (except for the FIND session trigger, which executes after the FIND schema trigger). In this way, the schema trigger always has precedence over the session trigger. For a WRITE, DELETE, CREATE, or ASSIGN event, the schema trigger can override the session trigger. For a FIND event, the schema trigger can preempt the session trigger.