TORUS: Tracing Complex Requirements for Large Cyber-physical Systems
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Dowdeswell, Barry Robert
Supervisor
Sinha, Roopak
Haemmerle, Enrico
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Auckland University of Technology
Abstract
Cyber-Physical Systems are embedded computers that control complex, physical processes
via autonomous peripherals while cooperating as agents in distributed networks.
Due to the scale and complexity of the interactions that occur within cyber-physical
systems, tracing system requirements accurately and appropriately is extremely hard.
The literature confirms that they are even harder to maintain and keep up-to-date during
the life of the project.
However, the information that requirements traceability provides is a crucial part of
determining the completeness of an application. Existing requirements management
systems do not scale well and traceability is difficult in such highly heterogeneous
environments.
This research presents TORUS (Traceability Of Requirements Using Splices), a
novel traceability framework that operates outside of, yet connects to, diverse requirements
and development environments. Our approach introduces Splices, autonomous
traceability data structures that persist trace information through the inevitable changes
that occur during system design and development.
A Design Science research methodology was adopted to show how the TORUS
framework can be applied to cyber-physical systems that employ the IEC 61499 Function
Block Architecture. A mechanical item sorting machine is modeled, the requirements
of which are described initially using CESAR (Cost-efficient Methods and Processes for Safety-relevant Embedded Systems) requirement templates. These templates
help to formalize the pre-Requirement Specification’s free-form text into less
ambiguous requirements statements. A domain ontology is defined before modeling the
requirements further within the Sparx Enterprise Architect Requirements Management
system. Enterprise Architect uses SysML diagrams to capture each requirement in
context with its acceptance tests, non-functional and safety requirements while the
model can be persisted for later use.
Formal mathematical models of requirements, function blocks and splices are
presented to show how this trace information can be mined, delivering important project
metrics to stakeholders. By capturing not only the current state of the system but also
by preserving historic traces, TORUS allows project teams to see a much richer view of
their system’s artifacts.
In parallel with the creation of these models, prototypes of TORUS were created
in Java to explore the proposed splice metadata model. These demonstrated that it is
possible to extract trace information directly from both Enterprise Architect models and
the nxtStudio IEC 61499 object repository. Using the relationships expressed by these formalisms, the resulting metadata information model for splices is extended to demonstrate how these entities can capture the status of each requirement. We define a set of splices as being the Skein of the system;
the set of traces that connect the model and application artifacts together like warp and
weft of the threads in a tapestry. Information aggregated in this way is important since
it provides quantifiable metrics that can be used to provide an empirically-determined
overall state of the system under examination.
The results indicate that the TORUS framework scales well and that the skein and
splices can provide metrics that should allow us to perform code-level validation and
completeness checking in the future.
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Keywords
Cyber-physical Systems, Traceability, Software requirements, IEC 61499 function blocks, TORUS: Traceability of Requirements Using Splices