1. Requirements

1.1 Notes

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

1.2 History

1.3 Applicability Across Classes

This requirement applies based on the selection criteria defined in this requirement.

2. Rationale

Software architecture deals with the fundamental organization of a system, as embodied in its components and their relationships to each other and the environment.

3. Guidance

Per the FAQ section of the NASA Software Architecture Review Board (SARB) sub-community of practice, accessible to NASA users on the NASA Engineering Network (NEN) 

“Software architecture should be evaluated concerning the problem it is trying to solve. That's why a project's problem statement is an essential part of review preparation; it describes the problem to be solved and lists success criteria held by various stakeholders. Software architecture should also be evaluated concerning common concerns within its problem domain [e.g.,] flight software...

“During a review, it's important to keep in mind the distinction between software architecture and software architecture description. A system can have great architecture and a poor description, or vice versa, or any of the other two combinations. In a review, it's important to note where each weakness lies: in architecture or description." 

From Systems and software engineering - Software life cycle processes 

a) Traceability to the system requirements.

b) Consistency with the system requirements.

c) Appropriateness of design standards and methods used.

d) Feasibility of the software items fulfilling their allocated requirements.

e) Feasibility of operation and maintenance.

NOTE System architecture traceability to the system requirements should also provide for traceability to the stakeholder requirements baseline.” 

Software architecture reviews are held during the software architecture formulation phase before the software architecture is finalized.  Generally, a software architecture review should be held before the software preliminary design review (PDR). The earlier the review board is involved in the software architecture development process, the more effective the inputs will be to the architecture development.

The SARB checklist found on the SARB sub-community of practice, accessible to NASA users on NEN, provides the following advice for software architecture reviews:

• Some of the basic questions to be answered during the review include (excerpted from the SARB checklist found on the SARB sub-community of practice, accessible to NASA users on NEN):
  
  • Is it clear what the architecture needs to do? 
  • Is the architecture capable of supporting what the system must do? 
  • How does the architecture compare with previously implemented architectures, both successful and unsuccessful?  In particular, does the architecture display superior qualities that should be incorporated on other missions and/or at other Centers?
  • Have all the requirements been defined?
  • Have all the stakeholders been identified to provide the essential design input specifying how the software is to be operated, how it is to execute its functions, and how it is to provide its products?
  • Have all the relevant quality attributes been identified and prioritized, and have they been characterized relative to the nature of the mission?
  • Has the source of all software elements been identified? Is it clear which portions of the system are custom-built, which are heritage/reused, and which are commercial off-the-shelf (COTS)?  Does the rationale exist explaining the project’s respective choices? 
  • Is it clear whether the architecture will be compatible with the other system components? 
  • Has the architecture been properly documented for future reference?
  • Does the architecture induce unnecessary complexity when addressing what must be done?
  • Is the proposed fault management approach appropriate for the system? 
  • Is the architecture flexible enough to support the maturation of the architects’ understanding of what the system must do?
  • Does the architecture support efficient code development and testing?
  • Have safety-critical elements of the architecture been identified properly?
  • Will the architecture support future systems that are natural extensions or expansions of the current system?
  • Does the architecture support performance requirements, including processing rates, data volumes, downlink constraints, etc.? 
  • To enable effective reuse, does the proposed architecture support some degree of scalability? 

• When putting together the review team, choose reviewers sufficiently familiar with the type of mission being supported to know what basic functions the software must implement.

• The general guidance for architecture reviews:
  
  • Have a standard approach/format for presenting the architecture; use a format similar to a formal design review, for example.
  • Have a standard template for the reviews to ensure the required material is included in the presentation.
  • Presenters are to present the architecture and the rationale for their architecture choices.

The results of the software architecture review, including findings, concerns, best practices, etc. are captured in a report and presented by the review board to management and others who can improve future processes and ensure that identified concerns are addressed.  Problems found during the software architecture review should be addressed in the project’s closed-loop problem tracking system or corrective action system to ensure they are addressed.

Expanded criteria definitions

Category 1 projects are defined in NPR 7120.5E, NASA Space Flight Program and Project Management Requirements, as human space flight projects, projects with a life-cycle cost exceeding $1B, or projects with significant radioactive material.

Category 2 projects are defined in NPR 7120.5E as projects that have life-cycle costs greater than $250M and less than $1B or have life-cycle costs less than $250M with a high priority level based on “the importance of the activity to NASA, the extent of international participation (or a joint effort with other government agencies), the degree of uncertainty surrounding the application of new or untested technologies” and a Class A or Class B payload risk classification.

Class A payload risk classifications are defined in NPR 8705.4, Risk Classification for NASA Payloads, as payloads with high priority, very low (minimized) risk, very high national significance, very high to high complexity, greater than 5 year mission lifetime, high cost, critical launch constraints, no alternative or re-flight opportunities, and/or payloads where “all practical measures are taken to achieve a minimum risk to mission success. The highest assurance standards are used.”

Class B payload risk classifications are defined in NPR 8705.4 as payloads with high priority, low risk, high national significance, high to medium complexity, two- to five-year mission lifetime, high to medium cost, medium launch constraints, infeasible or difficult in-flight maintenance, few or no alternative or re-flight opportunities, and/or payloads where “stringent assurance standards [are applied] with only minor compromises in application to maintain a low risk to mission success.” 

Per NPR 8705.4, “The importance weighting assigned to each consideration is at the discretion of the responsible Mission Directorate.”

The Software Architecture Review Board sub-community of practice, accessible to NASA users on NASA Engineering Network (NEN), provides additional information, including guidance, checklists, and examples, for conducting software architecture reviews.

Additional guidance related to software architecture may be found in the following related requirements in this Handbook:

4. Small Projects

No additional guidance is available for small projects. 

5. Resources

5.1 References

5.2 Tools

6. Lessons Learned

6.1 NASA Lessons Learned

The NASA Lesson Learned database contains the following lessons learned related to software architecture reviews:

• MER Spirit Flash Memory Anomaly (2004). Lesson Number 1483

  Swerefn
  refnum 557

  : "Shortly after the commencement of science activities on Mars, an MER rover lost the ability to execute any task that requested memory from the flight computer. The cause was incorrect configuration parameters in two operating system software modules that control the storage of files in system memory and flash memory. Seven recommendations cover enforcing design guidelines for COTS software, verifying assumptions about software behavior, maintaining a list of lower priority action items, testing flight software internal functions, creating a comprehensive suite of tests and automated analysis tools, providing downlinked data on system resources, and avoiding the problematic file system and complex directory structure.."
• NASA Study of Flight Software Complexity.  Lesson Learned 2050

  Swerefn
  refnum 571

  : “Flight software development problems led NASA to study the factors that have led to the accelerating growth in-flight software size and complexity. The March 2009 report on the NASA Study on Flight Software Complexity contains recommendations in the areas of systems engineering, software architecture, testing, and project management."

6.2 Other Lessons Learned

No other Lessons Learned have currently been identified for this requirement.

7. Software Assurance

7.1 Tasking for Software Assurance

1. Assess the results of or participate in software architecture review activities held by the project.

7.2 Software Assurance Products

• Software assurance assessment of the architecture review.
• Software Assurance Status Reports
• Assessment of Software Reviews results
• Issues and risks identified relating to the software architecture review; 

  
  

  Note
  title Objective Evidence
  Software design analysis results. Software Architecture Review Board report and findings (if applicable).

  Expand
  title Definition of objective evidence
  Include Page SITE:Definition of Objective Evidence SITE:Definition of Objective Evidence

7.3 Metrics

• # of architectural issues identified vs. number closed
• # of Non-Conformances from reviews (Open vs. Closed; # of days Open)

7.4 Guidance

Software assurance and software safety participate in all software architecture review activities held by the project. Software architecture reviews are held during the software architecture formulation phase before the software architecture is finalized.  Generally, a software architecture review should be held before the software preliminary design review (PDR). The earlier the review board is involved in the software architecture development process, the more effective the inputs will be to the architecture development.

A checklist provides the following advice for software architecture reviews (see sample below).  Confirm that these questions and topics are discussed at the software architecture review. Some of the basic questions to be answered during the review include: 

• Is it clear what the architecture needs to do? 
• Is the architecture capable of supporting what the system must do? 
• How does the architecture compare with previously implemented architectures, both successful and unsuccessful?  In particular, does the architecture display superior qualities that should be incorporated on other missions and/or at other Centers?
• Have all the requirements been defined?
• Have all the stakeholders been identified to provide the essential design input specifying how the software is to be operated, how it is to execute its functions, and how it is to provide its products?
• Have all the relevant quality attributes been identified and prioritized, and have they been characterized relative to the nature of the mission?
• Has the source of all software elements been identified? Is it clear which portions of the system are custom-built, which are heritage/reused, and which are commercial off-the-shelf (COTS)?  Does the rationale exist explaining the project’s respective choices? 
• Is it clear whether the architecture will be compatible with the other system components? 
• Has the architecture been properly documented for future reference?
• Does the architecture induce unnecessary complexity when addressing what must be done?
• Is the proposed fault management approach appropriate for the software and system? 
• Is the architecture flexible enough to support the maturation of the architects’ understanding of what the software must do?
• Does the architecture support efficient code development and testing?
• Have safety-critical elements of the architecture been identified properly?
• Will the architecture support future systems that are natural extensions or expansions of the current system?
• Does the architecture support performance requirements, including processing rates, data volumes, downlink constraints, etc.? 
• To enable effective reuse, does the proposed architecture support some degree of scalability? 
• Have safety-critical requirements been identified properly?
• Have the appropriate hazards been traced to the software architecture components?
• Are the software architecture rules defined and documented?

  • Patterns, abstractions, algorithms

  • Monitoring and control

  • Data representation and data management

  • Concurrent threads, processes, memory management

  • Real-time execution, throughput

  • Synchronization

  • Inter-process communication

  • Languages, libraries, operating systems

  • Verification and validation

Confirm that the software architecture review includes the following activities:

Software Architect Essential Activities

• Understand what a system must do
• Define a system concept that will accomplish this
• Render that concept in a form that allows the work to be shared
• Communicate the resulting architecture to others
• Ensure throughout development, implementation, and testing that the design follows the concepts and comes together as envisioned
• Refine ideas and carry them forward to the next generation of systems

Confirm that the documentation of the architectural concept includes at least the minimum information listed below:

The actual format for recording and describing the architectural concept is left to the software project team.  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.
• The documented rationale for architectural changes(if made).
• Evaluation and impact of proposed changes.

Check to see that: the format is acceptable and the content is clear and understandable 

Confirm that the following attributes have been considered: quality attributes, principles of design, verifiability, and operability.

• Give team members a clearer understanding of the project

Confirm that the following attributes and characteristics of the architecture have been considered:

Software architecture is not just high-level design, it includes quality attributes, rationale, and principles.

Software architecture is not a one-time effort

• Make software architecture a driving force throughout the lifecycle
• Good architectures don’t step aside once development starts

Embrace well-architected software as a response to system complexity

• Weak architecture …
  • Can’t be analyzed or validated for correct behavior, except case by case
  • Can’t be changed with confidence, even to correct errors
  • Can’t be operated with confidence, other than the way it was tested
  • Can’t be reused easily or inherited from
  • Conduct software architecture reviews

Document any issues or risks that arise during the architecture review and confirm that the project is addressing these.