Event handlers, whether for scheduled events or for system event handling, contain compound statements, and are similar in many ways to stored procedures. You can add loops, conditional execution, and so on, and you can use the SQL Anywhere debugger to debug event handlers.
After each execution of an event handler, a COMMIT occurs if no errors occurred. A ROLLBACK occurs if there was an error.
One difference between event handlers and stored procedures is that event handlers do not take any arguments. Certain information about the context in which an event was triggered is available through the EVENT_PARAMETER function, which supplies information about the connection that caused an event to be triggered (connection ID, user ID), as well as the event name and the number of times it has been executed. See EVENT_PARAMETER function [System].
During development, you want event handlers to be triggered at convenient times. You can use the TRIGGER EVENT statement to explicitly cause an event to execute, even when the trigger condition or scheduled time has not occurred. However, TRIGGER EVENT does not cause disabled event handlers to be executed. See TRIGGER EVENT statement.
While it is not good practice to develop event handlers on a production database, you can disable event handlers from Sybase Central or explicitly using the ALTER EVENT statement.
It can be useful to use a single set of actions to handle multiple events. For example, you may want to take a notification action if disk space is limited on any of the devices holding the database or log files. To do this, create a stored procedure and call it in the body of each event handler, passing any needed context information as parameters to the procedure.
Debugging event handlers is very similar to debugging stored procedures. The event handlers appear in the events list.
For more information and step-by-step instructions, see Debugging an event handler.
You can use the SET HIDDEN clause to hide the definition of an event handler. Specifying the SET HIDDEN clause results in the permanent obfuscation of the event handler definition stored in the action column of the ISYSEVENT system table. See ALTER EVENT statement.
You can also determine how many instances of a particular event handler are currently active using the NumActive event parameter. This function is useful if you want to limit an event handler so that only one instance executes at any given time.
For more information about the NumActive event parameter, see EVENT_PARAMETER function [System].
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