Organizing Models and Elements
Information Hierarchy explains the mechanics of how
Elements, and parent-child relationships among
Elements can be used together to model Entities in the real-world from various Modeling Perspectives and at various Modeling Granularities.
Top of the World explains how and why Elements are structured within the RepositoryModel to describe what the BIS Repository is “about”. The ‘leaf nodes’ in the
RepositoryModel are InformationPartitionElements (henceforth “partitions”) that establish a Modeling Perspective for a given Subject.
This topic reviews those fundamentals and provides guidance to Domain authors and client developers on how and why Models and Elements are organized under partitions. Before diving into detailed explanations, we summarize the rules reviewed or established in this topic.
A Summary of Principles and Rules for Modeling with BIS
These principles and rules govern how
Elements are organized to model real-world Objects.
- BIS conceives that a real-world Object consists of multiple Entities, each from a different Modeling Perspective.
Elementmodels an Entity.
Elementcan only be placed in a
Modelof compatible Modeling Perspective.
- A sub-Model of an
Elementmodels the same Entity as the
Element, but at a finer granularity.
Models can only sub-model
Elements of a compatible Modeling Perspective.
- A parent
Elementidentifies an Entity and may model a part of it, but it together with its child
Elements model the Entity.
Elements and Child
Elements must be in the same
- A single party should be responsible for all
Elements in a given
- A single party can be responsible for multiple
- Modeling the physical perspective:
PhysicalElements primarily around the implementations of physical systems (that implement functions). These will tend to align with responsibility boundaries.
- Where a physical Entity is part of multiple physical systems, it should be modeled by a
Modelowned by the primary responsible party. Other parties can “share” the
- Within the constraints above, the domain author can impose additional organization for domain-specific reasons.
- Domain authors should give users flexibility to further-partition for arbitrary reasons, so what could logically be modeled in a single
Modelcould be partitioned into two, with one “including” the other.
- “Include” relationships should never be recursive.
- Software should not be too “rigid” in its expectations of how
Elements are organized in to
Models, especially in the physical perspective. Functional and analytical
Models can be more rigid, because they are highly-specialized.
The Top-Model for Each Perspective
A top-Model models a top-level Entity from a specified perspective
As described in Top of the World the root
Subject identifies the real-world Object modeled in a
BIS Repository. When modeling an Object with BIS it is split into multiple Entities, one for each modeling perspective. Each modeling perspective is represented by a partition
Element added as a child of the
Subject. A top-Model is created for each partition to model the Object from the Entities perspective.
- Subject: mentions/identifies the highest-level Object to be modeled.
- Partition: establishes the perspective-specific Entity of the Object to be modeled.
- Top-Model: models the Entity from the given perspective.
Contents of a top-Model
Model models the complete Entity established by the
Subject and partition.
Model should contain one
Element for each discrete Object included in the
Subject. When the modeled
Subject refers to a single discrete Object, the top-
Model will contain only a single
Element that corresponds directly to a perspective on that Object. In this case, the Code and DisplayLabel of that
Element may be the same or similar to that of the
Subject (particularly if this is the [Primary Perspective] for the Object.) because they are “about” the same thing.
When the modeled
Subject is ‘compound’ (referring to multiple discrete Objects), there will be multiple
Elements in the top-
Model. An example compound-
Subject might be “Deliverables of Phase II of Project X”. The project’s contract will contain a list of the top-level things-to-be-built, and the top-
Model would contain one
Element for each thing-to-be-built in that list. This honors the Single Responsible Party Principal (SRPP) because one General Contractor will ultimately be responsible for all of the deliverables.
If you find that a top-
Model is unable to follow the SRPP, then the modeled-
Subject needs to be decomposed into child
Subjects. For example, with a compound-
Subject like “The Dock and the largest boat that must be able to dock there”. The top-
Model for that
Subject would contain separate
Elements for the dock and the boat, but a single party is unlikely to be responsible for both. Creating “Dock” and “Boat” child
Subjects solves this problem.
Modeling from the “Top-Model” on down
Model is where modeling from a given perspective really begins. As a domain author, you are faced with many options for modeling your domain, which are described below.
Multiple domain contexts
There will be distinct Digital Twins for design phases like Design, Construction, and Operations, and for major categories of infrastructure like Cities or Buildings or other facilities. There should be as much commonality among these as possible, but the domain author must have an awareness of the kinds of Digital Twins and the range of workflows where their domain is relevant. The modeling decisions discussed below occur in the context of those Digital Twins and workflows.
Domain authors should study examples of other domain schemas to see how others have addressed modeling challenges.
The many considerations
The domain author has a tough job. They must consider:
- Object boundaries: Consider the real-world and conceptually carve it up into Objects
- Entity boundaries: Carve those Objects up into Entities.
- Whole-part relationships: Consider the whole-part relationships among the Entities (and how to model them.)
- Other Relationships: Consider other relationships among Entities (and how to model them).
- Attributes: Consider the attributes of the Entities (and how to model them).
The “Object boundaries” consideration is business-as-usual for data modelers and will not be covered here.
The “Entity boundaries” consideration is covered in discussions of Modeling Perspectives.
The remaining considerations will be covered in the following sections.
Each Entity will be modeled with an
Element. Does the Entity have “parts” that are worth modeling individually in the given context? The answer to that question will lead to one of three choices:
- “Atomic” Element: There are no “parts” to model. Atomic
Elements can still model significant internal structure using the GeometryStream, Properties, and
- Parent-Child Modeling: The whole Entity is modeled as “parts” consisting of a parent
Elementand its child
Elements. The “sum” of the parent and child
Elements models the whole Entity. These
IParentElement. See Parent-Child Relationships.
- Sub-modeling: The “parts” are modeled in a sub-
Element. The sub-modeled
Elementrepresents the whole Entity. The “sum” of all
Elements in the sub-
Elementalso represents the whole Entity, but at a finer granularity. These
In the picture below, Entity 0 is modeled with an
Element P-0 represents the whole Entity 0, and its sub-
Model also represents the whole Entity 0, at a different granularity. Entity 4 is modeled as an
Elements P-4, P-5, and P-6 collectively represent Entity 4 and its parts.
Use these questions to determine whether to use parent-child modeling vs sub-modeling:
That last question needs elaboration. What are typical reasons why users need to both model the Entity as-a-whole and also model it as a collection of fine-grained parts? Reasons include:
- According to the SRPP, if one party determines that X should exist in a particular place, but delegates responsibility for modeling X in detail to a second party, the delegate would need to do their work in their own
- If some workflows want to deal with the Entity-as-a-whole, but other workflows (e.g. fabrication for construction) need to deal with it as a set of parts, the fabricator would want a
Modelfocused on the parts used to fabricate the whole.
As a boundary case, if one of the parts is clearly a “lead” part, e.g. a “Beam” with specific additions (welded on) or subtractions (cut out) then maybe parent-child is more appropriate. The decision to use parent-child modeling vs sub-modeling can be quite tricky. You should examine schemas from comparable domains and consult experts, if possible.
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Last Updated: 21 May, 2020