Pattern 34 (Static Partial Join for Multiple Instances)

FLASH animation of Static Partial Join for Multiple Instances pattern

Description

Within a given process instance, multiple concurrent instances of a task (say m) can be created. The required number of instances is known when the first task instance commences. Once n of the task instances have completed (where n is less than m), the next task in the process is triggered. Subsequent completions of the remaining m-n instances are inconsequential, however all instances must have completed in order for the join construct to reset and be subsequently re-enabled.

Examples

Examine 10 samples from the production line for defects. Continue with the next task when 7 of these examinations have been completed.

Motivation

The Static Partial Join for Multiple Instances pattern is an extension to the Multiple Instances with a priori Runtime Knowledge pattern which allows the process instance to continue once a given number of the task instances have completed rather than requiring all of them to finish before the subsequent task can be triggered.

Overview

The general format of the Static Partial Join for Multiple Instances pattern is illustrated in Figure 52. Transition A corresponds to the multiple instance task. In terms of the operation of this pattern, once the input place i1 is triggered for a case, m instances of the multi-instance task A are initiated concurrently and an "active" status is recorded for the pattern. These instances proceed independently and once n of them have completed, the join can be triggered and a token placed in output place o1 signalling that the thread of control can be passed to subsequent tasks in the process model. Simultaneously with the join firing, the token is removed from the active place allowing the remaining n - m tasks to complete. Once all m instances of task A have finished, the status of the pattern changes to "ready" allowing it to be re-enabled.

Figure 52: Static partial join implementation for multiple instances

There are two variants to this pattern which relax some of the restrictions associated with the form of the pattern described above. First, the Cancelling Partial Join for Multiple Instances pattern removes the need to wait for all of the task instances to complete by cancelling any remaining task instances as soon as the join fires. It is illustrated in Figure 53.

The second, the Dynamic Partial Join for Multiple Instances pattern allows the value of m (i.e. the number of instances) to be determined during the execution of the task instances. In particular, it allows additional task instances to be created "on the fly". This pattern is illustrated in Figure 54.

Context

There are two context conditions associated with this pattern: (1) the number of concurrent task instances (denoted by variable m in Figure 52) is known prior to task commencement and (2) the number of tasks that need to be completed before subsequent tasks in the process model can be triggered (denoted by variable n in Figure 52) is also known prior to task commencement.

Implementation

BPMN and XPDL both appear to offers support for this pattern via the Multiple Instance Loop Activity construct where the MI Flow_Condition attribute is set to complex and ComplexMI_FlowCondition is an expression that evaluates to true when exactly n instances have completed causing a single token to be passed on to the following task. However no detail is provided to explain how the ComplexMI_FlowCondition is specified hence this is considered to constitute partial support for the pattern.

Issues

None identified.

Solutions

N/A.

Evaluation Criteria

An offering achieves full support if it provides a construct that satisfies the description and context criteria for the pattern. It achieves partial support if there is any ambiguity associated with the specification of the join condition.

Product Evaluation

To achieve a + rating (direct support) or a +/- rating (partial support) the product should satisfy the corresponding evaluation criterion of the pattern. Otherwise a - rating (no support) is assigned.