Pattern 12 (Multiple Instances without Synchronization)

FLASH animation of Multiple Instances without Synchronization pattern

Description

Within a given process instance, multiple instances of a task can be created. These instances are independent of each other and run concurrently. There is no requirement to synchronize them upon completion. Each of the instances of the multiple instance task that are created must execute within the context of the process instance from which they were started (i.e. they must share the same case identifier and have access to the same data elements) and each of them must execute independently from and without reference to the task that started them.

Synonyms

Multi threading without synchronization, spawn off facility.

Examples

A list of traffic infringements is received by the Transport Department. For each infringement on the list an Issue-Infringment-Notice task is created. These tasks run to completion in parallel and do not trigger any subsequent tasks. They do not need to be synchronized at completion.

Motivation

This pattern provides a means of creaing multiple instances of a given task. It caters for situations where the number of individual tasks required is known before the spawning action commences, the tasks can execute independently of each other and no subsequent synchronization is required.

Overview

There are two possible variants to the way in which this pattern can operate. The first is illustrated by Figure 14 in which the create instance task runs within a loop and the new task instances are created sequentially. Place p2 indicates the number of instances required and is decremented as each new instance is created. New instances can only be created when the token in p2 has a value greater than zero - the guard on the create instance task ensures this is the case. When all instances have been created, the next task (B) can be enabled - again the guard on task B ensures this is also the case.

Figure 14: Multiple instances without synchronization pattern (variant 1)

In Figure 15, the task instances are all created simultaneously. In both variants, it is a requirement that the number of new instances required is known before the creation task commences. It is also assumed that the task instances can be created that run independently (and in addition to the thread of control which started them) and that they do not require synchronizing as part of this construct.

Figure 15: Multiple instances without synchronization pattern (variant 2)

Context

There is one context condition associated with this pattern: the number of task instances (i.e. numinst) is known as design time and is a fixed value.

Implementation

Most offerings - COSA, iPlanet, BPEL, BPMN, XPDL and UML 2.0 ADs - support the sequential variant of this pattern (as illustrated in Figure 14) with the task creation occuring within a loop. SAP Workflow also does so, but with the limitation that a new process instance is started for each task instance invoked. BPMN also supports the second variant, as do Staffware and FLOWer, and they provide the ability to create the required number of task instances simultaneously.

Issues

None identified.

Solutions

N/A.

Evaluation Criteria

An offering achieves full support if it provides a construct that satisfies the description for the pattern. Where the newly created task instances run in a distinct process instance to the task that started them or it cannot access the same data elements as the parent task, the offering achieves only partial support.

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.