Pattern 18 (Milestone)

FLASH animation of Milestone pattern

Description

A task is only enabled when the process instance (of which it is part) is in a specific state (typically a parallel branch). The state is assumed to be a specific execution point (also known as a milestone) in the process model. When this execution point is reached the nominated task can be enabled. If the process instance has progressed beyond this state, then the task cannot be enabled now or at any future time (i.e. the deadline has expired). Note that the execution does not influence the state itself, i.e. unlike normal control-flow dependencies it is a test rather than a trigger.

Synonyms

Test arc, deadline, state condition, withdraw message.

Examples

Most budget airlines allow the routing of a booking to be changed providing the ticket has not been issued;

The enrol student task can only execute whilst new enrolments are being accepted. This is after the open enrolment task has completed and before the close off enrolment task commences.

Motivation

The Milestone pattern provides a mechanism for supporting the conditional execution of a task or sub-process (possibly on a repeated basis) where the process instance is in a given state. The notion of state is generally taken to mean that control-flow has reached a nominated point in the execution of the process instance (i.e. a Milestone). As such, it provides a means of synchronizing two distinct branches of a process instance, such that one branch cannot proceed unless the other branch has reached a specified state.

Overview

The nominal form of the Milestone pattern is illustrated by Figure 24. Task A cannot be enabled when it received the thread of control unless the other branch is in state p1 (i.e. there is a token in place p1). This situation presumes that the process instance is either in state p1 or will be at some future time. It is important to note that the repeated execution of A does not influence the top parallel branch.

Figure 24: Milestone pattern

Note that A can only occur if there is a token in p1. Hence a Milestone may cause a potential deadlock. There are at least two ways of avoiding this. First of all, it is possible to define an alternative task of A which takes a token from the input place(s) of A without taking a token from p1. One can think of this task as a time-out or a skip task. This way the process does not get stuck if C occurs before A. Moreover, it is possible to delay the execution of C until the lower branch finishes. Note that in both cases A may be optional (i.e. not execute at all) or can occur multiple times because the token in p1 is only tested and not removed.

Context

There are no specific context conditions for this pattern.

Implementation

The necessity for an inheritent notion of state within the process model means that the Milestone pattern is not widely supported. Of the offerings examined, only COSA is able to directly represent it. FLOWer offers indirect support for the pattern through the introduction of a data element for each situation in which a Milestone is required. This data element can be updated with a value when the Milestone is reached and the branch which must test for the Milestone achievement can do so using the FLOWer milestone construct. Note that this is only possible in a data-driven system like FLOWer. It is not possible to use variables this way in a classical control-flow driven system because a "busy wait" would be needed to constantly inspect the value of this variable.

(Note that FLOWer only re-evaluates the state after each change with respect to data elements).

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. It receives a partial support rating if there is not a specific construct for the Milestone but it can be achieved indirectly.

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.