Why does DB2 FORCE APPLICATION not work sometimes?

Question

This article will explain why a DB2 FORCE APPLICATION command may not work even though you get a successful return code from the execution of the command.

Cause

There is a delay being experienced when executing this command.

Answer

A DB2 FORCE APPLICATION command is an interrupt request.

When an agent executes a force, it will set a force flag for the application that it will force and depending on what the coordinator agent is doing, it will react accordingly by either waiting or sending him an interrupt.

In some cases, we will be in a wait mode. This is due to a encountering a higher priority.

When an interrupt is provided, each interrupt has priorities. The general rule of thumb DB2 will only interrupt the target if the target is working on a lower priority.

For example: If an application is in the middle of a rollback and if the user executes a DB2 FORCE APPLICATION command to force it, the database can go into a inconsistent state and therefore, we do not want that to happen. So DB2 will not interrupt the rollback. This control is implemented by the interrupt priority

In this example, just prior to the rollback, the priority will be increased so a lower priority agent in this case the force application will not interrupt him. Also, the force operation is an asynchronous operation. So if the application is in rollback processing, the agent doing the force will only mark the force flag of the application, but will not interrupt the application immediately. After the coordinator agent finishes the rollback, DB2 restores to the lower interrupt level (likely back to 0), and on various places in the code path, the agent checks for interrupt or force, so at this time, DB2 will notice the force and then react to it.

In conclusion, the delay is due to DB2 evaluating priorities prior to executing.

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