Introduction

Each Subject in a BIS Repository can have one PhysicalPartition child Element, under which the PhysicalModels pertaining to the Subject will be organized using mechanisms described in Model Hierarchy. The Model Hierarchy is constrained by Modeling Perspective, but within the Physical Perspective, it is desirable to further organize Models according to Sites, Facilities, Systems, and Components to make the hierarchy of Models understandable by software and users. This section describes “Model Affinity” (a way of specifying “constraints” on the ModelContainsElements relationship) and the best-practice for using them to organize the Physical Model Hierarchy.

Organization Strategy

Motivations

BIS defines a data model that is shared by a growing set of applications and services. Many of these applications and services read and/or write PhysicalModel data. There are two choices to ensure that these applications and services will be coordinated:

• Require every application and service to work with any data organization.
• Specify a data organization which applications and services should read and write.

The second option has been chosen for BIS as it is the more practical solution.

The strategy and organization described in this page may seem overly complex to domain developers who just want to model a widget network. These developers naturally want a simple organization that has a network of widgets in the top-most PhysicalModel. The problem with this widget-centric data organization is that there will be users who want to model widget networks, thingamajig systems and doohickey facilities and coordinate between them; how can these users do that if the top model is dedicated to widgets?

BIS has been created to facilitate multi-discipline coordination, and that naturally adds some complexity to single-discipline use cases.

Predictability vs Flexibility / Strong vs Weak Type Safety

A choice must be made between strong type-safety and weak type-safety.

Strong type-safety (e.g. RetainingWall can only be contained in a StructuralSystem Model), provides strict checking to ensure that type-related rules are always followed, and hence results in very predictable models that software can easily process. However, strong type-safety limits flexibility and can cause frustration (e.g. I have to create a StructuralSystem Model just to create a single RetainingWall?). Another concern with strong type-safety is that real-project experience with BIS is very limited, so the schema designers understanding of real-world workflows with BIS data is limited; there is a risk of schema designers making strong type-safety decisions that are proved over time to be incorrect.

Therefore, Weak type-safety has been selected for the PhysicalModel hierarchy organization.

Validation rules will be created to determine if PhysicalElements reside in appropriate Models and the PhysicalModel hierarchy is appropriate, but there will be no prohibitions that ensure that the PhysicalModel hierarchy meets certain standards.

This page provides an overview of the basis of those validation rules and the mechanisms for defining them.

PhysicalModels and the Elements that they Model

As described in Model Hierarchy, every Model breaks-down an Element. The Model and the Element represent the same real-world Entity, but the Model provides more granular information about the Entity.

Breakdown Models are weakly-typed in BIS. To understand the real-world Entity that a Model is modeling, it is necessary to look at the Element which the Model is breaking down. PhysicalModel should not be subclassed. The few PhysicalModel subclasses that exist are deprecated and should not be used. When terms such as "Site Model" are used, they indicate "a Model that breaks down a Site", but do not indicate a strongly-typed SiteModel.

ModelAffinity Custom Attribute

While strong-typing is not used, there is still a desire to provide validation of the organization of the data in a PhysicalModel hierarchy. To run this validation, the software must be able to determine which Models are appropriate containers for each Element. This is determined through the ModelAffinity custom attribute.

The ModelAffinity custom attribute can be applied to any PhysicalElement subclass. It declares for the class:

• Which Elements' breakdown Models this Element has an affinity for (multiple can be declared).
• The strength of that affinity.
• The rationale for the affinity.

There are three levels of affinity strength:

• Required - not residing in an appropriate Model should trigger a validation error.
• Recommended - not residing in an appropriate Model should trigger a validation warning.
• Suggested - not residing in an appropriate Model should trigger a validation note.

In the future, ModelAffinity information may be used to provided improved UI tools.

Example Usage

Here is an example of the ModelAffinity custom attribute in use:

    <ECEntityClass typeName="StructuralElement" displayLabel="Structural Element" modifier="Abstract">
        <BaseClass>bis:PhysicalElement</BaseClass>
        <ECCustomAttributes>
            <ClassHasHandler xmlns="BisCore.01.00" />
            <ModelAffinity>
              <AffinityStrength>Recommended</AffinityStrength>
              <Rationale>Structural analysis software may not be able to analyze StructuralElements that do not reside in a StructuralSystem breakdown Model.</Rationale>
              <BreakdownModels>
                <string>StructuralPhysical:StructuralSystem</string>
              </BreakdownModels>
            </ModelAffinity/>
        </ECCustomAttributes>
    </ECEntityClass>

Inherited ModelAffinity

ModelAffinity cannot be overridden in subclasses, but it can be narrowed. The effective affinity setting for a class is the sum of all the defined ModelAffinity attributes in the class and all of its superclasses.

For example, if:

• There is a Pipe class with a SewerPipe subclass.
• Pipe has an affinity to ISystem.
• SewerPipe has an affinity to SewerSystem

Then the validation for SewerPipe would confirm:

• SewerPipe instances reside in ISystem Models.
• SewerPipe instances reside in SewerSystem Models.

A SewerPipe placed in a Site Model would generate two warnings (or errors) during validation; one for not being in an ISystem Model and one for not being in a SewerSystem Model.

When an ancestor class has already defined a ModelAffinity, it only makes sense to define a narrowing ModelAffinity in a subclass. Defining a conflicting ModelAffinity is a symptom of the design of the subclass conflicting with the design of the superclass.

Syntax Details

TODO: Remove this section after discussion and implementation

This custom attribute is written so it could be applied to other Elements (not just PhysicalElements) in the future.

<ECEnumeration typeName="ModelAffinityStrength" backingTypeName="string" isStrict="true">
  <ECEnumerator value="Required" description="Element not residing in this breakdown Model should trigger a validation error."/>
  <ECEnumerator value="Recommended"  description="Element not residing in this breakdown Model should trigger a validation warning."/>
  <ECEnumerator value="Suggested"  description="Element not residing in this breakdown Model should trigger a validation note."/>
</ECEnumeration>

<ECCustomAttributeClass typeName="ModelAffinity" description="Applied to an Element subclass to indicate that the subclass has an affinity to certain types of breakdown Models (i.e. Models breaking down a certain class of Element). This affinity is a "loose" constraint can be used for validation and UI enhancements. Inherited ModelAffinities are also applied." appliesTo="EntityClass">
  <ECProperty propertyName="AffinityStrength" typeName="ModelAffinityStrength" description="The strength of the affinity between the Element and the BreakdownModels in which it may reside."/>
  <ECProperty propertyName="Rationale" typeName="string" description="The reason why the affinity exists."/>
  <ECArrayProperty propertyName="BreakdownModels" typeName="string" description="List of Element subclasses whose breakdown Models are appropriate containers for the class that this custom attribute is applied to. Use format '[SchemaName]:[ClassName]', e.g. 'Fruit:Banana'" minOccurs="1" maxOccurs="unbounded"/>
</ECCustomAttributeClass>

Mixin Classes

ModelAffinity custom attribute instances can refer to mixin classes, but cannot be applied to mixin classes.

The mixin classes ISite, IFacility and ISystem are used in the ModelAffinity custom attribute for many key PhysicalModel hierarchy classes.

Top PhysicalModel

The top PhysicalModel (the one directly owned by the PhysicalPartition) should contain a single PhysicalElement that corresponds to the Subject (that owns the PhysicalPartition). As Subjects define their subject in text only, the subject may be vague (Is "Burj Khalifa" only the building, or is it the building and the surrounding site?"). This vagueness is clarified by the PhysicalElement that is in the top PhysicalModel.

The top PhysicalModel most often contains an ISite, IFacility, or ISystem PhysicalElement.

Top PhysicalModels with More than One PhysicalElement

There is no strict requirement limiting the top PhysicalModel to contain only a single PhysicalElement. iModels that are generated from other repositories will sometimes have top PhysicalModels with multiple PhysicalElements as that best matches the organization of the source data. Legacy data may also have a non-standard organization.

PhysicalModel Hierarchy Strategy

While ModelAffinity provides a mechanism to declare and enforce a PhysicalModel hierarchy, it does not define a coordinated strategy. The strategy for organizing the hierarchy relies on classifying PhysicalElements into five types:

• Site - An area of land and its contents.
• Facility - A cross-disciplinary Entity such as a building, bridge, tunnel, wharf, etc.
• System - A single discipline physical system, such as structural system, electrical system, sewer system, etc.
• System Component - A component for a particular system.
• General-Use Component - A component that does not have any ModelAffinity.

The overall PhysicalModel hierarchy strategy is defined in the following table:

Classification	Affinity to	Example Classes
ISite	ISite (declared in implementing classes)	Site
IFacility	ISite, IFacility, ISystem (declared in implementing classes)	Building, Bridge, Tunnel
ISystem	ISite, IFacility, ISystem (declared in implementing classes)	SewerSystem, StructuralSystem, etc.
*System	ISite, IFacility, ISystem	SewerSystem, StructuralSystem
*SystemComponent	*System	SewerPipe, Beam
General-Use Component	(none)	Bolt, Chair

These types and their behaviors are explained below, along with the behaviors of their breakdown PhysicalModels if they have breakdown Models.

ISite

ISite is mixin class that represents an region of land and all that is contained on, above and below that land. The ISite mixin applies only to PhysicalElement classes.

In conversation (and writing) the Models that break down Site Elements are referred to as Site Models, even thought there is not a strongly-typed SiteModel class. By convention, the top-most Model in a PhysicalModel hierarchy is considered a Site Model, even though it breaks down PhysicalPartition.

ISite Models typically contain ISite, IFacility, ISystem and General-Use PhysicalElements.

ISite Elements should only be placed in ISite Models (and the top PhysicalModel). ISite Elements placed in any other Model will generate validation warnings.

To implement this behavior every class that includes the ISite mixin must include a ModelAffinity custom attribute that indicates a "Recommended" affinity to PhysicalPartition and ISite.

IFacility

IFacility is a mixin class that represents a significant multi-disciplinary infrastructure Entity that corresponds well to user concepts. Buildings, bridges, tunnels, wharves, and towers are all examples of multi-discipline Entities that are modeled with a PhysicalElement subclass that includes the IFacility mixin.

In conversation (and writing) the Models that break down IFacility Elements are referred to as Facility Models, even thought there is not a strongly-typed IFacilityModel class. These Models are usually referred to by their more-specific Element types (e.g. Building Models are Models that break down Building Elements.)

IFacility Models typically contain IFacility, ISystem and General-Use PhysicalElements.

IFacility Elements may be placed in ISite, IFacility and ISystem Models. IFacility Elements placed in any other Model will generate validation warnings.

To implement this behavior every class that includes the IFacility mixin must include a ModelAffinity custom attribute that indicates a "Recommended" affinity to PhysicalPartition, ISite, IFacility and ISystem.

ISystem

ISystem is a mixin class that represents a significant discipline-specific arrangement of Entities intended to fulfill one or more functions. Sewers, roadways, HVAC and fire-suppression systems are all examples of real-world Entities that are modeled with ISystem subclasses. ISystem subclasses tend to be suffixed with 'System' (e.g. StructuralSystem, SewerSystem); 'Network' is another commonly used suffix for these classes (e.g. RoadNetwork).

Architecture is also considered an ISystem, as it is discipline-specific and has the same general behaviors at the other ISystems.

In conversation (and writing) the Models that break down ISystem Elements are referred to as System Models, even thought there is not a strongly-typed ISystemModel class. These Models are usually referred to by their more-specific Element types (e.g. SewerSystem Models are Models that break down SewerSystem PhysicalElements.)

ISystem Models may contain IFacility, ISystem and General-Use PhysicalElements, as well as components specific to the particular ISystem (e.g. SewerPipe in SewerSystem)

ISystem Elements may be placed in ISite, IFacility and ISystem Models. ISystem Elements placed in any other Model will generate validation warnings.

To implement this behavior every class that includes the ISystem mixin must include a ModelAffinity custom attribute that indicates a "Recommended" affinity to PhysicalPartition, ISite, IFacility and ISystem.

System Components

System Component represents an Entity that is part of a specific ISystem and generally does not makes sense outside of the specific system (e.g. sewer pipe outside of sewer system). System Component is not a BIS class, nor mixin.

System Components are identified by their ModelAffinity with a specific ISystem subclass.

System Components rarely have breakdown Models.

System Component Elements should only be placed in their related System Models. System Component Elements placed in any other Model will generate validation warnings.

To implement this behavior every System Component class must define or inherit a ModelAffinity custom attribute that indicates a "Recommended" (or stronger) affinity to the particular ISystem class that is appropriate for the System Component.

General-Use Components

General-Use Components represents Entities such as bolts and chairs which are not specific to any discipline or facility.

General-Use Components are identified by their lack of any ModelAffinity.

General-Use Components rarely have breakdown Models.

General-Use Elements can be placed in any PhysicalModel. General-Use Components never generate validation warnings.

Example

The following diagram illustrates all variations of the PhysicalModel hierarchy breakdown that will not generate warnings or errors.

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Discussion

This section is for discussion of both the content of the documentation and the BIS strategies and policies that back that content.

Changes Since June 22 2018

Site has been changed to ISite. Resulting in moderated changes (including one image change).

The top PhysicalModel has been given its own behavior, recommended use and clarified relationship to its Subject.

Site Class

We need a Site class that implements ISite. Should it be Sealed?

Implementation Tasks

1. Create ModelAffinity Custom Attribute
2. Create schema that defines ISite, IFacility and ISystem.
3. Create Site class
4. Provide validation of ModelAffinity usage?
5. Provide validation code/capability that uses ModelAffinity data.

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Last Updated: 02 February, 2022