The PIF Process Interchange Format and Framework

Jintae Lee, Gregg Yost and the PIF Working Group[1]
Version 1.0
December 22, 1994

4. PIF

A PIF process description consists of a file of objects, such as ACTIVITY, ACTOR, and RESOURCE objects. Each object in the file has a unique id that other objects can use to refer to it. Each object type (or class) has a particular set of attributes defined for it; each attribute describes some aspect of the object. For example, an ACTOR object has a Skills attribute listing the actor's skills. Object classes fall into a class hierarchy (see Figure 1). Each object class has all of the attributes of all of its superclasses, in addition to its own attributes. For example, all PIF objects have a Name attribute, since the class at the root of the PIF hierarchy (ENTITY) has a Name attribute.

When an attribute of one object has another object as its value, the attribute represents a relationship between the two object classes. For example, an ACTOR's Skill attribute takes SKILL objects as values, so Skill represents a relationship between the ACTOR and SKILL classes. Figure 2 depicts the relationships among the PIF classes.

The PIF hierarchy has grown out of the efforts of the PIF Working Group to share process descriptions among the group members' various tools. We have used the following guidelines in developing this hierarchy:

• Generality is preferred over computational efficiency when there is a tradeoff, for the reason that PIF is an interchange language, not a processing language. Therefore, the organization of the object classes is not necessarily well-suited to performing any particular task such as workflow management or process simulation. Instead, our goal has been to define classes that can express a wide variety of processes, and that can be readily translated into other formats that may be more suitable for a particular application.

   

• The PIF constructs should be able to express the constructs of some existing common process representations such as IDEF (SADT) or Petri Nets.

   

• PIF should start with the minimal set of classes and then expand only as it needs to. The minimal set was decided at the first PIF Workshop (October 1993) by examining those constructs common to some major existing process representations and to the process representations used by members of the PIF Working Group.

   

• Additions to the standard PIF classes could be proposed by anybody, but the proposal must be accompanied by concrete examples illustrating the need for the additions. The Working Group decided, through discussions and votes if necessary, whether to accept the proposal. PIF allows groups to define local extensions at will (see Section 4), so new classes or attributes should be added to the standard PIF classes only if they seem to be of sufficiently general usefulness.

The current PIF class hierarchy is by no means complete and is still under development. Section 3 describes all of the current PIF classes.

Figure 1: The PIF class hierarchy.

Figure 2: Relationships among PIF classes.

Primitive value types

The value of an attribute in a PIF object is either the unique identifier of another PIF object or a literal value of one of the following primitive PIF value types. Appendix A describes PIF's syntax, including the syntax of these primitive literals.

• NUMBER: A numeric value. The NUMBER type is subdivided into INTEGER and FLOAT types.

   

• STRING: A sequence of characters.

   

• SYMBOL: Symbols are denoted by character sequences, but have somewhat different properties than strings. PIF symbols are a much-simplified version of symbols in the Lisp programming language (Steele, 1990). In PIF, the main difference between strings and symbols is that symbols are not case-sensitive unless specially quoted, but strings are always case-sensitive.

   

• RESTRICTED-KIF-SENTENCE: A logical expression that may include quantifiers as well as the more common Boolean operators. This type is named as it is because the expressions PIF allows are a subset of those allowed in KIF, a logic-based knowledge interchange format. Appendix A has more details on KIF.

5. Alphabetic Class Reference