Software Failures and Faults

Does software fail? We tend to believe that well written, well tested, safety critical software never fails. Experience proves otherwise with software making headlines when it actually does fail, sometimes critically.

Software does not exhibit the random or wear out related failure behavior we see in hardware. Software will always function in the same way. Software faults are introduced in a variety of ways during the design and development period. They do not change over time. However, software faults will only cause failures if the fault is encountered during usage. Therefore faults existing in often used code will cause failures more often than faults residing in rarely used code.

What are "software failure modes"? Software, especially in critical systems, tends to fail where least expected. We are usually extremely good at setting up test plans for the main line code of the program, and these sections usually do run flawlessly. Software does not "break" but it must be able to deal with "broken" input and conditions, which are often causes for "software failures". The task of dealing with abnormal/anomalous conditions and inputs is handled by the exception code dispersed throughout the program. Setting up a test plan and exhaustive test cases for the exception code is by definition difficult and somewhat subjective.  

Anomalous inputs can be due to failed hardware, timing problems, harsh/unexpected environmental conditions, multiple changes in conditions and inputs that are beyond what the hardware is able to deal with. Bad user input may also be a source for such exception conditions. Often the conditions most difficult to predict are multiple coinciding irregular inputs and conditions.

How do we overcome these failures? Much in the same way we treat hardware:

Software Reliability
Software Safety
Software FMEA
Software FTA
Requirements Analysis

For more information about out software reliability and safety services please contact:

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(323) 410-0992