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Theselinks indicate similar constraints extending in the opposite direction and in bothdirections, respectively. That is, the top schematic indicates (again, in addition to theactivation semantics of simple precedence) that an instance of B must be preceded by aninstance of A, and the bottom schematic indicates both that any instance of A must befollowed by an instance of B, and that an instance of B must be preceded by an instanceof A. These constraints add a normative component to the description of a system, i.e., acomponent that expresses not just how the system has been observed to behave, but howit ought to behave.
Constrained links are thus particularly useful when IDEF3 is used tomodel a system, not just record beliefs and observations about its behavior.AB12AB12Figure 3-5bFurther Examples of Constrained Precedence LinksClearly, these three links do not exhaust the possible constraints that might holdbetween UOBs. For instance, one might wish to add to the simple precedence semanticsof Figure 3-3 that no more than five minutes can separate the completion of any instanceof A in any activation and the beginning of an instance of B that follows. The finalconstrained precedence link indicates the presence of general constraints of this sort.
Forthis reason, it is called a general constrained precedence link, and is illustrated in Figure3-6. Because the nature of these constraints can vary widely, they must be recordedexplicitly in the precedence link elaboration document. (See Precedence LinkElaboration Document subsection that follows later).27AB12Figure 3-6General Constrained Precedence LinkDashed LinksDashed links carry no predefined semantics. For this reason, they are often referred toas user-defined links or relational links. This type of link highlights the existence of a(possibly constraining) relationship between two UOBs.
For example, the relational linkin Figure 3-7 might indicate the constraint between Sign timesheet and Obtain timesheetapproval that one cannot approve one’s own timesheet. The precise character of therelationship indicated by a relational link is specified in the Relational Link Elaborationdocument.SigntimesheetObtaintimesheetapproval12Figure 3-7Example of a Relational LinkLink NumbersAll links have an elaboration and unique link numbers. Precedence link numbers areprefaced by the letters PL (for “precedence link”).
Relational links are prefaced by theletters DL (for “dashed link”). For example, precedence links may be numbered PL1,PL2, and so on. The uniqueness of link numbers is ensured by using a procedure similarto the UOB numbering scheme. That is, link numbers are assigned sequentially from apool allocated to an author. Displaying link numbers on the process schematics isoptional.Activation Semantics for Nonbranching Process SchematicsBefore introducing junctions (which give IDEF3 the capacity to describe the structureof branching processes), it is useful to generalize the semantics for the different link typesfor larger, nonbranching schematics.
Consider the simple schematic in Figure 3-8 thatdescribes the process of holding a meeting to discuss committee reports.28CallMeetingto Order1DiscussCommitteeReports2CloseMeetingDistributeMinutes34Figure 3-8Nonbranching IDEF3 SchematicAs with IDEF3 schematics generally, the basic semantics of this schematic is to beunderstood in terms of the pattern of possible activations it describes.
In other words, theschematic specifies exactly what counts as a meeting in the given context. As in thesimple two box case, an activation will generally exhibit the following pattern: Aninstance of Call Meeting to Order is followed by an instance of Discuss CommitteeReports, which in turn is followed by instances of Close Meeting and Distribute Minutes,where each instance in the series begins no earlier than its predecessor ends.
As with allnonbranching schematics (and indeed, all schematics without disjunctive branches), thetypical activation pattern for Figure 3-8 is illustrated in the activation plot in Figure 3-9.Call Mtg to OrderDiscuss Committee RptsClose MtgDistribute MinutesFigure 3-9Activation Plot for Figure 3-8The constrained precedence links indicate further constraints on the process:committee reports must not be discussed before the meeting is called to order; and afterthe meeting, minutes must be distributed. The absence of any constraint between thesecond and third UOBs, for example, allows for the possibility of a meeting endingbefore the Discuss Committee Reports UOB completes. In such a case, the truncatedmeeting would be an activation of the described process; it would not violate anyconstraints, and hence would be consistent with the description.
The constraintsindicated by constrained links are to be understood as being independent of anyactivation. So even if the meeting is closed without the UOB Discuss Committee Reportsbeing completed, the constraint between the last two UOBs nonetheless requires thedistribution of minutes after the close of the truncated meeting.29JunctionsJunctions in IDEF3 provide a mechanism to specify the logic of process branching.Additionally, junctions simplify the capture of timing and sequencing relationshipsbetween multiple process paths.Junction TypesIDEF3 schematics are, in general, type-level descriptions of complex processes (i.e.,process types).
Such processes are rarely linear. More typically, they involve any or allof four general sorts of branch points:1.Points at which a process diverges into multiple parallel subprocesses;2.Points at which a process diverges into multiple (possibly nonexclusive)alternative subprocesses;3.Points at which multiple parallel subprocesses converge into a single“thread;” and4.Points at which multiple alternative subprocesses in the process convergeinto a single thread.IDEF3 introduces four general types of junctions to express the four general sorts ofbranch points. The first two sorts are expressed by fan-out junctions: Conjunctive fanout junctions represent points of divergence involving multiple parallel subproceses,while disjunctive fan-out junctions represent points of divergence involving multiplealternative subprocesses.
The last two sorts of branch points are expressed by fan-injunctions: conjunctive fan-in junctions represent points of convergence involvingmultiple parallel subproceses, while disjunctive fan-in junctions represent points ofconvergence involving multiple alternative subprocesses. There is one type ofconjunctive junction, or AND junction, indicated by “&”. There are two types ofdisjunctive junctions: inclusive and exclusive junctions, or OR and XOR junctions,respectively, depending on whether the alternatives in question are mutually exclusive.This classification of junctions is depicted in Figure 3-10.
Their semantics is discussedmore fully in the following sections.30JunctionFan-outFan-inConjunctive DisjunctiveConjunctive DisjunctiveInclusive ExclusiveInclusive ExclusiveFigure 3-10Classification of Junction TypesBasic Junction SyntaxJunctions represent branch points in a general process, points at which either a single“thread” in the process diverges into multiple (parallel or alternative) threads, or multiplethreads converge into one. In IDEF3, such divergence is represented by a single junctionserving as the source of multiple precedence links and convergence by a single junctionserving as the destination of multiple precedence links. Divergence to, and convergencefrom, multiple parallel subprocesses are indicated by the use of an AND junction, asillustrated in Figure 3-11.ABAC&&BCFigure 3-11Diverging and Converging Parallel SubprocessesSimilarly, divergence to, and convergence from, multiple alternative subprocesses areindicated as illustrated in Figure 3-11 except by the use of either an OR or an XORjunction, depending, once again, on whether the alternatives are mutually exclusive.As a convention, the precedence link coming into a fan-out junction (if there is one)will be drawn without an arrow tip, and the outgoing precedence links in a fan-out31junction will be drawn with a single stem, and with rounded rather than sharp corners.Parallel conventions hold for fan-in junctions.
To illustrate, these conventions areapplied to the top two schematics in Figure 3-12, yielding the bottom two schematics.ABACCBBAACCBFigure 3-12Graphical Conventions for Precedence Links Connecting to JunctionsNote that junction symbols are not intrinsically fan-out or fan-in. Rather, a givenoccurrence of a junction symbol in a schematic is fan-out or fan-in depending on whetherit is source or a destination, respectively, of multiple paths.Junction Numbering SchemeTo make unambiguous references to the junctions in an IDEF3 schematic, anidentification scheme for IDEF3 junctions is provided. Recall that precedence links areassigned unique numbers beginning with the letters PL. Junction numbers follow anidentical numbering scheme, except that junction reference numbers start with the letter J,thus: J1, J2, ..., Jn.
As with links, no two distinct junctions can be assigned the samejunction number.Basic Junction SemanticsA fan-out AND junction in a schematic means that, in an activation of the schematicthat reaches the point in the structure of the process represented by that junction, therewill be instances of all UOBs denoted by the UOB boxes that are (immediate) successorsof the junction.
If a synchronous AND junction is used, then, to be an activation of theschematic, those instances must all start simultaneously. Similarly, the intuitive meaningof a fan-in AND junction in a schematic is that, in an activation of the schematic that32traverses that junction, there will be instances of all UOBs denoted by the UOB boxesthat are (immediate) predecessors of the junction. And if a synchronous AND junction isused, then, to be an activation of the schematic, those instances must all endsimultaneously.
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