Failing to perform peer reviews can reasonably be expected to increase the defect rate in software for several reasons. All real-world projects have limited time and resources, so by skipping or skimping on peer reviews developers have missed an easy chance to eliminate defects. With inadequate reviews, developers are spread thin chasing down bugs found during testing. Additionally, peer reviews can find defects that are impractical to find in most types of testing, especially in cases of fault management or handling unexpected/infrequent operating scenarios.
- Every line of code must be reviewed by at least one independent, technically skilled person. That review must include actually reading the entirety of the code rather than just looking at selected portions.
- Peer reviews must be documented so that it is possible to audit the fact that they took place and the effectiveness of the reviews. At a minimum this includes recording the name of the reviewer(s), the code reviewed, the date of the review, and the number of defects found. If no auditable documentation of software quality is available for incorporated components (e.g., safety certification or peer review reports), then new peer reviews must be performed on that third-party code.
- Acceptable safety critical system software processes normally require a formal meeting-based review rather than a remote review, e-mail review, or other casual checking mechanism.
Peer reviews involve having an independent person – other than the author – look at source code and other design documents. The main purposes of the review are to ensure that code conforms to style guidelines and to find defects missed by the author of the code. Running a static analysis tool is not a substitute for a peer review, and neither is an in-person discussion that solely discusses the output of a static analysis tool. A proper peer review requires having an independent person (or, strongly preferable, a small group of independent reviewers) read the code in its entirety to ensure quality. The everyday analogy to a peer review is having someone else proof-read something you’ve written. It is nearly impossible to see all our own mistakes whether we are writing software or writing English prose.
It is well known that more formal reviews provide more efficient and effective results, with the gold standard being what is known as a Fagan Style Inspection (a “code inspection”) that involves a pre-review, a formal meeting with defined roles, a written review report, and follow up actions. Regardless of the type of review, accepted practice is to record the results of reviews and audit them to make sure every single line of code has been reviewed when written, and re-reviewed when a module has been modified.
MISRA requires a “structure program review” for SIL 2 and above. (MISRA Report 2 p. ix). MISRA specifically lists “Fagan Inspection” as a type of review (MISRA Software Guidelines p. 12), and devotes two appendices of a report on verification and validation to “walkthroughs,” listing structured walkthroughs, code inspections, Fagan inspections, and peer reviews (MISRA Report 6 pp. 132-136). MISRA points out that walkthroughs (their general term for peer reviews) “are acknowledged to be an effective process for identifying errors in programs – indeed they can be more effective than computer-based testing for certain types of error.”
MISRA also points out that fixing a bug may make things worse instead of better, and says that code reviews and analysis should be used to validate bug fixes. (MISRA Report 5 p. 135)
494. Peer reviews are somewhat labor intensive, and might account for 10% of the effort on a project. However, it is common for good peer reviews to find 50% or more of the defects in a code base, and thus finding defects via peer review is much cheaper than finding them via testing. Ineffective reviews can be diagnosed by the fact that they find far fewer defects. Acceptable peer reviews normally find defects that would be missed by testing, especially in parts of the code that are difficult to test thoroughly (for example, exception and failure management code).
McConnell devotes Chapter 24 to a discussion of reviews and inspections (McConnell 1993). Boehm & Basili summarized best practices for reducing software defects, and included the following point relevant to peer reviews: “Peer reviews catch 60 percent of the defects.” (Boehm 2001, pg. 137).
Ganssle lists four steps that should be the first steps taken to improve software quality. They are: “1. Buy and use a version control system; 2. Institute a Firmware Standards Manual; 3. Start a program of Code Inspection; 4. Create a quiet environment conducive to thinking.” #3 is his term for peer reviews, indicating his recommendation for formal code inspections. He also says that he knows companies that have made all these changes to their software process in a single day. (Ganssle 2000, p. 13). (Ganssle’s #2 item is coding style, discussed in Section 8.6. ).
MISRA Software Guidelines list the following as techniques on a one-picture overview of the software lifecycle: “Walkthrough, Fagan Inspection, Code Inspection, Peer Review, Argument, etc.” (MISRA Guidelines 1994, pg. 20) indicating the importance of formal peer reviews in a safety critical software lifecycle. Integrity Level 2 (which is only somewhat safety critical) and higher integrity levels require a “structured program review” (pg. 29). That document also gives these rules: “22.214.171.124 Before dynamic testing begins the code should be reviewed in accordance with the software verification plan to ensure that it does conform to the design specification” (pg. 56) and “126.96.36.199 Code reviews and/or walkthroughs should be used to identify any inconsistencies with the specifications” (pg. 56) and “188.8.131.52 The communication of information regarding errors to design and development personnel should be as clear as possible. For example, errors found during reviews should be fully recorded at the point of detection.”
MISRA C rule 116 states: “All libraries used in production code shall be written to comply with the provisions of this document, and shall have been subject to appropriate validation” (MISRA C pg. 55). Within the context of embedded systems, an operating system such as OSEK would be expected to count as a “library” in that it is code included in the system that is relied upon for safety, and thus should have been subject to appropriate validation, which would be expected to include peer reviews. If there is no evidence of peer review or safety certification, the system designer should perform peer reviews on the OS code (which is an excellent reason to use a safety certified OS!)
Fagan-style inspections are a formal version of a “peer review,” which involves multiple software developers looking at software and other design artifacts to find defects. Fagan-style inspections originated at IBM (Fagan 1976). A later paper presented updated techniques, concluding that “inspections increase productivity and improve final program quality. Furthermore, improvements in process control and project management are enabled by inspections.” (Fagan 1986). It is widely recognized that Fagan-style inspections are a best practice, and that some sort of effective peer review technique is an accepted practice.
Fagan-style Formal Inspections are recommended by the FAA (FAA 2000, p. J-23). IEC 61503-3 highly recommends performing some sort of design review on all software at all SILs, and recommends Fagan inspections at SIL4. (p. 91).
- Boehm, B. & Basili, V., Software Defect Reduction Top 10 List, IEEE Computer, pp. 135-137, Jan. 2001.
- FAA, System Safety Handbook, Appendix J: Software Safety, Federal Aviation Administration, Dec. 2000
- Fagan, M., "Advances in software inspections," IEEE Trans. Software Engineering, SE-12, July 1986, pp. 744-751.
- Fagan, M., "Design and code inspections to reduce errors in program development," IBM Systems Journal, 15(3), 1976, pp. 182-211.
- Ganssle, J., The Art of Designing Embedded Systems, Newnes, 2000.
- McConnell, Code Complete, Microsoft Press, 1993.
- MISRA, (MISRA C), Guideline for the use of the C Language in Vehicle Based Software, April 1998.
- MISRA, Development Guidelines for Vehicle Based Software, November 1994 (PDF version 1.1, January 2001).
- MISRA, Report 2: Integrity, February 1995.
- MISRA, Report 5: Software Metrics, February 1995.
- MISRA, Report 6: Verification and Validation, February 1995.