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Critical Section Groups

Critical section groups are used to synchronize process instances so that only one process instance executes the grouped activities at any given time. Any concurrently running process instances that contain a corresponding critical section group wait until the process instance that is currently executing the critical section group completes.

Critical Section groups are particularly useful for controlling concurrent access to shared variables (see Synchronizing Access to Shared Variables for more information). However, other situations may occur where you wish to ensure that only one process instance is executing a set of activities at a time.

Synchronization Options

Critical section groups can be used to synchronize all process instances for a particular process definition in a single process engine, or you can synchronize process instances for multiple process definitions, or you can synchronize process instances across multiple process engines.

Single Group

If you wish to synchronize process instances for a single process definition in a single process engine, perform the following:

1.	Create a group around the activities you wish to synchronize.

2.	Specify Critical Section for the Group Action field.

3.	Specify Single Group for the Scope field.

Only one process instance at any given time will execute the activities contained in the Critical Section group.

Multiple Groups

If you wish to synchronize process instances for multiple process definitions, or if you wish to synchronize process instances across multiple process engines, perform the following:

1.	Create a Lock shared configuration resource and specify a name for the resource.

2.	To perform the synchronization across multiple process engines, check the Multi-Engine field of the Lock resource.

When the process instances are executed by the same process engine, locking is performed in memory. When the process instances are executed across multiple engines, the process engines must be configured to use a database for storage, and a database transaction is performed to ensure that only one process instance is executing the critical section group at any given time.

3.	Create a group around the activities you wish to synchronize.

4.	Specify Critical Section for the Group Action Field.

5.	Specify Multiple Groups for the Scope field.

6.	Use the Browse button in the Lock Object field to locate the Lock shared configuration resource you created in Step 1.

7.	Perform steps 3 to 6 for any process definitions you wish to synchronize. Make sure you specify the same Lock shared configuration object for all Critical Section groups.

Usage Guidelines

Because Critical Section groups cause many process instances to wait for one process instance to execute the activities in the group, there may be performance implications when using these groups. In general, you should use the following guidelines when creating critical section groups:

•	Keep the duration of a Critical Section group as short as possible. That is, put only a very few activities in a Critical Section group, and only use activities that execute very quickly.

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Avoid nesting Critical Section groups. If you must use nesting, ensure that Lock shared configuration resources are used in the same order in all process definitions. Deadlocks can occur if you do not specify the Lock resources in the same order in nested Critical Section groups for all process definitions.

•	Do not include any activities that wait for incoming events or have long durations, such as Request/Reply activities, Wait For activities, Sleep activities, or activities that require a long time to execute.

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When using Critical Section groups to retrieve or assign a value to a shared variable across multiple process engines, both the Lock and the Shared Variable shared configuration resources should have the Multi-Engine field checked. You can use a global variable to ensure that the Multi-Engine field is set to the same value for both resources.

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