UML22 — mustIsolate:

mustIsolate: The wording of UML 2 for StructuredActivityNode.mustIsolate refers to individual nodes instead of all the nodes in the group: If the mustIsolate flag is true for an activity node, then any access to an object by an action within the node must not conflict with access to the object by an action outside the node. A conflict is defined as an attempt to write to the object by one or both of the actions. If such a conflict potentially exists, then no such access by an action outside the node may be interleaved with the execution of any action inside the node. The UML 1.5 wording was better: Because of the concurrent nature of the execution of actions within and across procedures, it can be difficult to guarantee the consistent access and modification of object memory. [Examples snipped] In order to avoid these problems, it is necessary to isolate the effects of a group of actions from the effects of actions outside the group. This is indicated by setting the mustIsolate attribute to "true" on a group action. If a group action is isolated, then any object used by an action within the group cannot be accessed by any action outside the group until the group action as a whole completes. Any concurrent actions that would result in accessing such objects are required to have their execution deferred until the completion of the group action. In the first example above, if the read actions on the temperature and pressure attributes are wrapped in a group action with mustIsolate set to "true", then the temperature and pressure values read are assured to be consistent, since no changes can intervene between the two reads. Similarly, if an isolated group is used for the second action, then the update is assured to be consistent, since no action outside the group can change the list until the update is complete. Note" The term "isolation" is used here in the sense used in traditional transaction terminology. An execution engine may achieve any required isolation using locking mechanisms, or it may simply sequentialize execution to avoid concurrency conflicts. Isolation is different than the property of "atomicity", which is the guarantee that a group of actions either all complete successfully or have no effect at all. Atomicity generally requires a rollback mechanism to prevent committing partial results. This is beyond the scope of what can be guaranteed by the basic action semantics.

see above