Pattern 5 (Simple Merge)

FLASH animation of Simple Merge pattern

Description

The convergence of two or more branches into a single subsequent branch such that each enablement of an incoming branch results in the thread of control being passed to the subsequent branch.

Synonyms

XOR-join, exclusive OR-join, asynchronous join, merge.

Examples

At the conclusion of either the bobcat-excavation or the D9-excavation tasks, an estimate of the amount of earth moved is made for billing purposes.

After the case-payment or provide-credit tasks, initiate the product-receipt task.

Motivation

The Simple Merge pattern provides a means of merging two or more distinct branches without synchronizing them. As such, this presents the opportunity to simplify a process model by removing the need to explicitly replicate a sequence of tasks that is common to two or more branches. Instead, these branches can be joined with a simple merge construct and the common set of tasks need only to be depicted once in the process model.

Overview

Figure 5 illustrates the behaviour of this pattern. Immediately after either task A or B is completed, task C will be enabled. There is no consideration of synchronization.

Figure 5: Simple merge pattern

Context

There is one context condition associated with the pattern: the place at which the merge occurs (i.e. place p1 fin Figure 5) is safe and can never contain more than one token.

Implementation

Similar to patterns WCP2-WCP4, this pattern can either be represented explicitly or implicitly. Staffware, SAP Workflow and UML 2.0 ADs provide specific join constructs for this purpose where as it is represented implicitly in WebSphere MQ, FLOWer, COSA and BPEL. BPMN and XPDL allow it to be represented in both ways.

Issues

One issue that can arise with the use of this pattern occurs where it cannot be ensured that the incoming place to the merge (p1) is safe.

Solutions

In this situation, the context conditions for the pattern are not met and it cannot be used, however there is an alternative pattern - the Multi-Merge (WCP8) - that is able to deal with the merging of branches in potentially unsafe process instances.

Evaluation Criteria

Full support for this pattern is demonstrated by any offering which provides a construct which satisfies the description when used in a context satisfying the context assumption.

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.