SysML Effort About to Bear Fruit - Software Development Times On The Web

SysML Effort About to Bear Fruit
Method for modeling complex systems expected to be adopted by OMG
By Alan Moore

May 1, 2006 — The goal of creating a customized version of UML 2 designed to address the specific needs of system engineers is in sight. On April 26, Object Management Group was expected to vote for the adoption of SysML version 0.99 at its technical meeting in St. Louis. The vote brings to fruition an effort spearheaded by the SysML Merge Team (SMT), chaired by Sanford Friedenthal of Lockheed Martin.

The completion of the SMT’s SysML specification was the result of unprecedented cooperation among some of the foremost tool vendors, leading industry users, government agencies and professional organizations. Creating the SysML 0.99 specification, for which I was the specification architect, has been a mammoth task in which more than 100 man-years of effort has been invested.

SysML is a visual modeling language that extends UML 2 in order to support the specification, analysis, design, verification and validation of complex systems that include components for hardware, software, data, personnel, procedures and facilities. SysML reuses a subset of UML 2 concepts and diagrams and augments them with some new diagrams and constructs appropriate for systems modeling.

One of the goals of the SysML submitters was to try to reuse UML 2 as much as possible and avoid making changes unless absolutely necessary. The SMT defined a subset of UML 2, called UML4SysML, to remove nonessential concepts and then built the SysML profile using UML4SysML as its reference metamodel. In the SysML Diagram Taxonomy, many UML diagrams are directly reused, others are modified, and two diagrams are essentially new—the Requirements Diagram and the Parametric Diagram.

Requirements in SysML
One of the two principal extensions to SysML is support for requirements. The ?requirement? stereotype extends class to specify the textual “shall” statement and capture the requirement id, often used for integration with external Requirements Management systems. The UML containment relationship is used to decompose a requirement into its constituent requirements. A requirement is related to other key modeling artifacts via a set of stereotyped dependencies. The ?deriveReqt? and ?satisfy? dependencies describe the derivation of requirements from other requirements and the satisfaction of requirements by design, respectively. The ?verify? dependency shows the link from a test case to the requirement or requirements it verifies. Two key late additions were the inclusion of the UML ?refine? dependency, used to indicate that a SysML model element is a refinement of a textual requirement, and ?copy?, used to show reuse of some standard requirement within the current requirement hierarchy.

Only the most basic attributes of a requirement are covered by the SysML specification. More specialized requirement types can be specified using specialization of the ?requirement? stereotype. Typical examples are operational, functional, interface, control, performance, physical and storage requirements. These stereotypes may restrict the types of model elements that can satisfy or refine the requirement.

The requirements model can be shown in graphical, tree structure or tabular format. The graphical format is called a Requirements Diagram.

Building With Blocks
The major structural extension in SysML is the Block, which extends UML Structured Class; it is a general-purpose hierarchical structuring mechanism that abstracts away much of the software-specific detail implicit in UML structured classes. Blocks can represent any level of the system hierarchy, including the top-level system, a subsystem, or logical or physical component of a system or environment. A SysML Block describes a system as a collection of parts, and connections between them that enable communication and other forms of interrelationships. Ports are a special kind of part that give access to internal structure from the outside of a composite object for use when the block is used within a larger structure. There are two types of ports: Service Ports, which support client/server communication; and Flow Ports, which define flows into or out of a block.

Two diagrams are used to describe block relationships, the Block Definition Diagram (bdd), which is similar to a traditional class diagram, is used to describe relationships that exist between blocks; the Internal Block Diagram (ibd) is used to describe block internals.

Parametric Models
Parametric models are used to describe system properties and their interrelationships. In order to support this type of modeling, a new structural feature, called a ConstraintBlock, has been introduced into SysML. A ConstraintBlock defines a set of parameters and one or more expressions that state how a change to the value of one parameter impacts the value of other parameters. By default, these parameters are nondirectional and so have no notion of causality. These ConstraintBlocks are then bound via their parameters to system properties to enforce system constraints. Based on the reusable concept of a block, new ConstraintBlocks can be built by reusing more primitive ConstraintBlocks such as basic mathematical operators.

SysML also defines a model of value types that can take units and dimensions; these value types can then be used to type parameters in parametric models.

ConstraintBlocks may be used to express mathematical equations, such as ‘F=m*a’ and ‘a = dv/dt’; or statistical values and utility functions, such as might be used in trade studies.

A special diagram, the Parametric Diagram, has been introduced to facilitate this parametric viewpoint. It is a specialized variant of an internal block diagram that restricts diagram elements to represent constraint blocks, their parameters and the block properties that they bind to. Both parameters and properties may be represented as small “pin-like” boxes to help make the diagrams more scalable.

In addition, SysML provides extensions to activities to describe the distributed flow of material, energy or information through a system. These extensions allow the modeler to impose restrictions on the rate at which items flow along edges in an activity, or in and out of parameters of behaviors.

More than three years from the initial request for proposal, version 0.99 of SysML looks set. The extensions that are being made to UML 2 by SysML will help systems engineers more adequately model their systems, and at the same time the reuse of UML at SysML’s core will enable much smoother flow down from systems engineering to software engineering than at present.

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