1. Requirements

4.2.3 The project manager shall develop and record the software architecture.

1.1 Notes

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

1.2 Applicability Across Classes

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

2. Rationale

Experience confirms that the quality and longevity of a software-reliant system is largely determined by its architecture. (See lessons learned NASA Study of Flight Software Complexity.

A software architecture:

• Formalizes precise subsystem decompositions.
• Defines and formalizes the dependencies among software work products within the integrated system.
• Serves as the basis for evaluating the impacts of proposed changes.
• Maintains rules for use by subsequent software engineers that assure a consistent software system as the work products evolve.
• Provides a stable structure for use by future groups through the documenting of the architecture, its views and patterns, and its rules.

3. Guidance

Architectural design is defined as "the process of defining a collection of hardware and software components and their interfaces to establish the framework for the development of a computer system."

The software architecture of a program or computing system is the structure or structures of the system, which comprise software components, the properties of those components, and the relationships between them. Documenting software architecture facilitates communication between stakeholders, documents early decisions about high-level design, and allows reuse of design components and patterns between projects.

The software architecture is drafted during the early life-cycle phases of a project and baselined during Preliminary Design Review (PDR) (see SWE-019 and Topic 7.8 - Maturity of Life-Cycle Products at Milestone Reviews ). The drafting begins when the top-level (systems) requirements are collected and organized. The project's operational concepts document is prepared based on these top-level requirements. From this point the project development team develops, decomposes, and sub-allocates these requirements to multiple and more narrowly focused activities. (Tarullo

NASA/SP-2007-6105, NASA Systems Engineering Handbook,

As the software development team starts its effort, it organizes the activities based on these allocated and derived requirements. The key step is to transform these requirements into a logical and cohesive software architecture that supports the overall systems architecture for the NASA project. The team develops a software architecture to serve as guidance for the development of the components and systems level software work products through a process known as architectural design.

Software architecture is commonly organized using the concepts of "views" and "patterns." A view is a representation of a set of system components and the relationships among them. Views are used to describe the system from the viewpoint of different stakeholders, such as end-users, developers or project managers.

The resulting software architecture also allows for the following: The verification of the software components, the integration of work products into systems, and the integration of the software systems into the rest of the project's systems.

SWE-057 calls for the software architecture to be documented. The required content for the Software Design Description document includes the CSCI architectural design. The actual format for recording and describing the architectural concept is left to the software project team (all projects are different!). As a minimum, include the following:

• An assessment of architectural alternatives.
• A description of the chosen architecture.
• Adequate description of the subsystem decomposition.
• Definition of the dependencies between the decomposed subsystems.
• Methods to measure and verify architectural conformance.
• Characterization of risks inherent to the chosen architecture.
• Documented rationale for architectural changes (if made).
• Evaluation and impact of proposed changes.

See Topic 7.7 - Software Architecture Description for additional information on the recommended kinds of content that usually appear in software architecture descriptions and for examples from a number of sources of outlines for documenting software architecture descriptions.

In situations where the software architecture does need to be changed, dependency models now offer the potential for maintaining the architecture over successive revisions during the software life cycle by specifying rules explicitly that define the acceptable and unacceptable dependencies between subsystems. The dependency structure model is an example of a compact representation that lists all constituent subsystems/activities and the corresponding information exchange and dependency patterns.

The Software Architecture Review Board, a software engineering sub-community of practice available to NASA users via the NASA Engineering Network (NEN), is a good resource of software design information including sample documents, reference documents, and expert contacts.

NASA-specific software measurement usage information and resources are available in Software Processes Across NASA (SPAN), accessible to NASA users from the SPAN tab in this Handbook. 

Additional guidance related to the software architecture development and documentation may be found in the following related requirements in this handbook:

4. Small Projects

Software architecture is one of those non-coding activities that can improve the quality of the software. Small projects may want a less-formal, more-affordable method of development. In general, if software development involves a low-risk and highly precedented system, the project can skimp on architecture. If the development involves high-risk and novel systems, the project must pay more attention to it.

5. Resources

6. Lessons Learned

A documented lesson from the NASA Lessons Learned database notes the following:

NASA Study of Flight Software Complexity. Lesson number: 2050: This March 2009 study identified numerous factors that led to the accelerating growth of flight software size and complexity that in turn lead to flight software development problems. In particular the lesson learned states "Good software architecture is the most important defense against incidental complexity in software designs..."