Concurrency requires the activities of concurrently running methods to be synchronized. Critical data must be safeguarded so diverse methods on other activities do not perform concurrent updates. Guarded methods satisfy both these needs.
A guarded method combines
the UNGUARDED option of the ::METHOD directive or the SETUNGUARDED method of the Method class with the GUARD instruction.
The UNGUARDED option and the SETUNGUARDED method both provide unconditional concurrency. Including a GUARD instruction in a method makes concurrency conditional:
GUARD ON WHEN expression
If the
expression on the GUARD instruction evaluates to
1 (true), the method continues to run. If the
expression on the GUARD instruction evaluates to
0 (false), the method does not continue running. GUARD reevaluates the
expression whenever the value of an exposed object variable changes. When the expression evaluates to
1, the method resumes running. You can use GUARD to block running any method when proceeding is not safe. (See
Section 2.9, “GUARD” for details about GUARD.)
It is important to ensure that you use an expression that can be fulfilled. If the condition expression cannot be met, GUARD ON WHEN puts the program in a continuous wait condition. This can occur in particular when several activities run concurrently. In this case, a second activity can make the condition expression invalid before GUARD ON WHEN can use it.
To avoid this, ensure that the GUARD ON WHEN statement is executed before the condition is set to true. Keep in mind that the sequence of running activities is not determined by the calling sequence, so it is important to use a logic that is independent of the activity sequence.
