PETROV / Russel, N., et al. Workflow Control-Flow Patterns. A Revised View

as a consequence of receiving a trigger on each incoming place. After N incoming triggers have been received for a given process instance (via tokens being received in N distinct input places from i1 to im), the join ¯res and a token is placed in output o1. The completion of the remaining N-M branches has no impact on the join except that it is reset when the last of them is received.

The pattern shares the same advantages over the Structured Partial Join as the

Blocking Discriminator does over the Structured Discriminator, namely greater °exibility as it is able to deal with the situation where a branch is triggered more than once e.g. where the construct exists within a loop. It also shares the same context condition: it can only deal with one case at a time (i.e. once one of the incoming places i1 to in is triggered for a given case, all other incoming triggers that are received are assumed to relate to the same case).

Figure 51: Blocking partial join pattern

There is a variation to this process model where the triggered input place is extended to keep track of both case and enabled input branches and place p4 is extended to keep track of cases where the join has triggered (but not yet reset) is shown in Figure 52. This allows the join to operate in a concurrent environment where it may need to handle multiple cases simultaneously7.

Figure 52: Blocking partial join pattern { extension for concurrent process instances

7This a similar analogy to that noted for the Blocking Discriminator pattern. Figure 51 assumes that only one process instance executes in isolation where as Figure 52 is capable of distinguishing between and managing instances of distinct processes.

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Implementation The approach to implementing this pattern is essentially the same as that for the Blocking Discriminator except that the join ¯res when N incoming branches have triggered rather than just the ¯rst. The Blocking Partial Join is partially supported by BPMN, XPDL and UML 2.0 ADs.

Issues None identi¯ed.

Solutions N/A.

Evaluation Criteria An o®ering achieves full support if it provides a construct that satis¯es the context requirements for the pattern. If there is any ambiguity in how the join condition is speci¯ed, an o®ering is considered to provide partial support for the pattern.

Pattern WCP-32 (Cancelling Partial Join)

Description The convergence of two or more branches into a single subsequent branch following one or more corresponding divergences earlier in the process model. The thread of control is passed to the subsequent branch when N of the incoming branches have been enabled. Triggering the join also cancels the execution of all of the other incoming branches and resets the construct.

Example

{Once the picture is received, it is sent to three art dealers for the examination. Once two of the prepare condition report activities have been completed, the remaining prepare condition report activity is cancelled and the plan restoration activity commences.

Motivation This pattern provides a means of expediting a process instance where a series of incoming branches to a join need to be synchronized but only a subset of those activities associated with each of the branches needs to be completed.

Context The operation of this pattern is shown in Figure 53. It is a context condition of this pattern that only one thread of execution is active for a given process instance in each of the preceding branches to the discriminator. If this is not the case, then the behaviour of the process instance is likely to become unpredictable at a later stage during execution.