1. Requirements

3.3.6 The project shall provide and maintain bidirectional traceability from software design to the software code.

1.1 Notes

NPR 7150.2, NASA Software Engineering Requirements, does not include any notes for this requirement.

1.2 Applicability Across Classes

Classes C through E and Safety Critical are labeled, "SO." This means that this requirement applies to the safety-critical aspects of the software.

Class F and Class G are labeled "X (not OTS)." This means that this requirement does not apply to off-the-shelf software for these classes.

2. Rationale

Code (software) is based on a software design which is based on a set of software requirements. Some measure of assurance is needed to show that the created code fulfills the design upon which it is based without leaving out any design elements or adding any new functionality not present in the design.

Tracing elements of the code such as functions, subsystems, modules, CSCI (Computer Software Configuration Item)s, etc. to design elements helps provide the basis of this assurance.

Traceability matrices help ensure that each design element, typically documented in a Software Design Document (SDD), is implemented in the resulting software, giving a reason for the code implementation through the design back to the software requirements. 

Traceability links between individual requirements and other system elements, including, but not limited to, source code, are helpful tools when evaluating the impact of changing or deleting a requirement. When a requirement is changed, traceability can help identify the affected products, including design, documentation, source code, tests, etc.  (NASA-GB-8719.13, NASA Software Safety Guidebook ²⁷⁶).

Tracing source code to design elements also provides the ability to demonstrate traceability of safety-critical software functions and safety-critical software controls to detailed design specifications.

Bidirectional traceability is defined as a traceability chain that can be traced in both the forward and backward directions as illustrated below (Bidirectional Requirements Traceability, Westfall, 2006 ³⁵⁶).  It is important because it can point out software design elements that are not fulfilled in the code (i.e., missing or incomplete functionality) as well as source code that does not have a parent design element (i.e., extra functionality). Ideally, the trace does not identify any source code elements that have no source design element, but if such "orphan" source code elements are discovered in the trace, they need to be discussed by the project team and assurance personnel to determine if the "orphan" elements are necessary. If they are determined to be necessary, any missing design elements and source requirements are added to the project.

3. Guidance

Before starting the traceability activity, it is assumed that the documents being traced (e.g., requirements, design, code, test data, etc.) have been approved.

Using a matrix, such as the one shown below (Bidirectional Requirements Traceability, Westfall, 2006 ³⁵⁶), allows a single exercise to show traceability both forwards and backwards. The matrix is completed left to right early in the appropriate phase in the project life cycle. As each column is completed, the forward trace is extended to the next set of products. Simply starting with a column, such as the LLD (low-level design) Section, and looking at the data in the columns to the left shows the backward traceability from a LLD element to its parent HLD (high-level design) element and back to the parent requirements.

While traceability matrices are not the only method for capturing bidirectional traceability, they are the most common. Traceability matrices can be included in the documents, such as the Software Requirements Specification (SRS), to which they apply or they can be combined into a single matrix covering higher level requirements, software requirements, design, code, and verification. General guidance for creating a bidirectional traceability matrix includes the following suggested actions:

• Create the matrix at the beginning of the project.
• Uniquely identify the elements in the matrix (requirements identifiers, design document identifiers and paragraph numbers, code module identifiers, etc.).
• Keep the matrix maintained throughout the life of the project.
• Assign responsibility for creating and maintaining the matrix to a project team member, since managing the links/references can be a labor-intensive process that needs to be tracked and monitored.
• Maintain the matrix as an electronic document to make maintenance and reporting easier.
• Create the matrix such that it may be easily sorted to achieve/convey bi-directional traceability.
• Ensure a review of the matrix at major phases / key reviews of the project.

If the software development team is not the same as the software design team, collaboration may be needed to ensure proper bidirectional traceability between source code and design. Early collaboration is valuable since a one-to-one subsystem to design package relationship (or as close as possible), makes the traceability activity between design and source code much simpler.

Key aspects of tracing source code include:

• Clear tracing of safety requirements to the source code that implements them (typically via comments in the code or via the traceability matrix using requirements traced to design to code) (NASA-GB-8719.13, NASA Software Safety Guidebook ²⁷⁶).
• For traceability purposes, safety-critical code and data is to be commented as such (NASA-STD-8719.13B, NASA Software Safety Standard ²⁷¹).
• Trace source code to the detailed design specifications.
• Map each low-level function or data specification to the requirements it fulfills. ⁰⁴⁷
• Trace unit tests to source code and to design specifications. ¹²⁷

Additional guidance related to bidirectional traceability may be found in the following related requirements in this Handbook:

4. Small Projects

For small projects without access to a tracing tool for requirements, design, and code and with budget limitations preventing them from acquiring a new tool and associated training, tracing may be done with a spreadsheet (such as Excel), a simple database (such as Access) or a textual document. It is very important that the project be diligent about keeping such traces up to date as these methods do not include automatic updates when requirements, design elements, etc. change (NASA-STD-8719.13B, NASA Software Safety Standard ²⁷¹).

In some instances, value-based requirements tracing may be a prudent deviation/waiver request against SWE-064 which would provide partial relief from this requirement. However, no matter how small the budget is, traceability of safety-critical requirements all the way through design and code is always a priority. 

5. Resources

5.1 Tools

Tools to aid in compliance with this SWE, if any, may be found in the Tools Library in the NASA Engineering Network (NEN).

NASA users find this in the Tools Library in the Software Processes Across NASA (SPAN) site of the Software Engineering Community in NEN.

The list is informational only and does not represent an “approved tool list”, nor does it represent an endorsement of any particular tool. The purpose is to provide examples of tools being used across the Agency and to help projects and centers decide what tools to consider.

6. Lessons Learned

No Lessons Learned have currently been identified for this requirement.