page_title: Object Process Methodology
section_title: Structural links
hierarchical_section_title: Object Process Methodology / Syntax and semantics / Links / Structural links
caption_attribution_description: English: OPM State-Specified Structural Relations and Links
context_page_description: Object Process Methodology is a conceptual modeling language and methodology for capturing knowledge and designing systems, specified as ISO/PAS 19450. Based on a minimal universal ontology of stateful objects and processes that transform them, OPM can be used to formally specify the function, structure, and behavior of artificial and natural systems in a large variety of domains.
  OPM was conceived and developed by Dov Dori. The ideas underlying OPM were published for the first time in 1995. Since then, OPM has evolved and developed.
  In 2002, the first book on OPM was published, and on December 15, 2015, after six years of work by ISO TC184/SC5, ISO adopted OPM as ISO/PAS 19450. A second book on OPM was published in 2016.
  Since 2019, OPM has become a foundation for a Professional Certificate program in Model-Based Systems Engineering - MBSE at EdX. Lectures are available as web videos on Youtube.
context_section_description: Structural links specify static, time-independent, long-lasting relations in the system. A structural link connects two or more objects or two or more processes, but not an object and a process, except in the case of an exhibition-characterization link.
  Unidirectional tagged structural link
  Has a user-defined semantics regarding the nature of the relation from one thing to the other. Graphically, an arrow with an open arrowhead. Along the tagged structural link, the modeler should record a meaningful tag in the form of a textual phrase that expresses the nature of the structural relation between the connected objects (or processes) and makes sense when placed in the OPL sentence whose syntax follows.
  Unidirectional null-tagged structural link
  A unidirectional tagged structural link with no tag. In this case, the default unidirectional tag is used. The modeler has the option of setting the default unidirectional tag for a specific system or a set of systems. If no default is defined, the default tag is "relates to".
  Bidirectional tagged structural link
  When the tags in both directions are meaningful and not just the inverse of each other, they may be recorded by two tags on either side of a single bidirectional tagged structural link. Each tag shall align on the side of the arrow with the harpoon edge sticking out of the arrowhead unambiguously determining the direction in which each relation applies. The syntax of the resulting tagged structural link is two separate tagged structural link OPL sentences, one for each direction. Graphically, a line with harpoon shaped arrowheads on opposite sides at both ends of the link's line shall.
  Reciprocal tagged structural link
  A reciprocal tagged structural link is a bidirectional tagged structural link with no more than one tag. In either case, reciprocity indicate that the tag of a bidirectional structural link has the same semantics for its forward and backward directions. When no tag appears, the default tag shall be "are related". The syntax of the reciprocal tagged structural link with only one tag shall be: Source-thing and destination thing are reciprocity-tag. The syntax of the reciprocal tagged structural link with no tag is: Source thing and Destination-thing are related.
  Fundamental structural relations
  The most prevalent structural relations among OPM things and are of particular significance for specifying and understanding systems. Each of the fundamental relations is elaborate or refine one OPM thing, the source thing, or refinee, into a collection of one or more OPM things, the destination thing or things, or refineables. The fundamental structural relations are; Aggregation, Exhibition-characterization, Generalization-specialization, and Classification-instantiation.
  Aggregation-participation link
  A refinee—the whole—aggregates one or more other refineables—the parts. Graphically, a black solid (filled in) triangle with its apex connecting by a line to the whole and the parts connecting by lines to the opposite horizontal base shall denote the aggregation-participation relation link.
  Exhibition-characterization link
  A thing exhibits, or is characterized by, another thing. The exhibition-characterization relation binds a refinee—the exhibitor—with one or more refineables, which shall identify features that characterize the exhibitor Graphically, a smaller black triangle inside a larger empty triangle with that larger triangle's apex connecting by a line to the exhibitor and the features connecting to the opposite (horizontal) base defines the exhibition-characterization relation link.
  Generalization-specialization and inheritance
  These are structural relations which provide for abstracting any number of objects or process classes into superclasses, and assigning attributes of superclasses to subordinate classes.
  Generalization-specialization link: The refinee—the general—generalizes the refineables, which are specializations of the general. Binds one or more specializations with the same persistence attribute value as the general, such that either the general and a
caption: OPM state-specified structural relations and links
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