Is there a difference between stability and reliability (at least in software engineering context) or can they be used interchangeably? If not, what would be some examples of reliable but not necessarily stable systems, and vice versa?
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3Someone might refer to a system as stable but unreliable if it is always up and running but intermittently produces incorrect results or occasionally loses data. It's a matter of opinion what these words mean.– joshpCommented Jul 24, 2012 at 19:28
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In my mind they boil down to the same thing, consistency...– DarknightCommented Jul 25, 2012 at 10:16
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Why are you asking?– JᴀʏMᴇᴇCommented Jul 18, 2016 at 12:09
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7 Answers
Let's say for instance we have an app, it works perfectly, aside from it crashing every 5 minutes, but it's back up instantly without data loss.
That in my mind is reliable, but not stable.
I can rely on it not losing data and working correctly, despite it not being stable.
In fact, the internet is basically that. It's far from stable—connections drop and reappear, packets collide and are lost, and all kinds of other unstable things happen. However, it's pretty amazing how reliable it is given all the instability inherent in it.
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3Ian Somerville defines reliability as "The probability of failure-free operation over a specified time, in a given environment, for a specific purpose." in the book Software Engineering. So your system crashing every 5 minutes is not so reliable (depends on your metrics though).– Random42Commented Jul 24, 2012 at 19:31
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2@m3th0dman, depends how you define a failure then. No data loss, and 100% data accuracy would not be a failure in my books, even if the app kept crashing and restarting itself. I'd want to fix the issue, but it wouldn't be a high priority if it kept 'working'– CaffGeekCommented Jul 24, 2012 at 19:37
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If your system crashes, then there obviously was a failure somewhere and thus your operation was not failure-free (as suggested by the definition); thus your system is not so reliable. The discussion can be detailed into talking about availability, response and restart time.– Random42Commented Jul 24, 2012 at 19:45
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I seriously don't think this example app is reliable. Reliability and stability are not necessarily two separate things.– ZippoCommented Jul 24, 2012 at 20:24
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@m3th0dman: Erlang programs are considered reliable, not because they are infallible, but because they consist of many concurrent processes (not OS processes) which can fail without compromising the program as a whole, which is designed to handle those failures. Commented Jul 25, 2012 at 9:53
These definitions come from the ISO 9126 Standard, which divides in characteristics and sub characteristics : this table , this pdf or wikipedia or article
Stability is : Characterizes the sensitivity to change of a given system that is the negative impact that may be caused by system changes.
Reliability is a main characteristic that contains:
- maturity : This sub characteristic concerns frequency of failure of the software.
- fault tolerance : The ability of software to withstand (and recover) from component, or environmental, failure.
- recoverability : Ability to bring back a failed system to full operation, including data and network connections.
Goal: write a program to add two numbers
Reliable but unstable:
add(a,b):
if randomInt mod 5 == 0:
throw exception
else
print a+b
Stable but unreliable:
add(a,b):
if randomInt mod 5 == 0:
print a+a
else
print a+b
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This definition is incorrect. It is not how the terms are really used in the industry.– FedericoCommented Sep 12, 2019 at 17:54
In the context of evaluating libraries, the terms mean completely different things.
A reliable library is one that does its job without intermittent failures.
A stable library is one that doesn't change much.
answered May 17, 2019 at 11:58
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I am surprised to see so many answers that interpret stable as "does not crash". To me the first meaning is indeed "has not changed much for some time", it is no longer moving. The thing not crashing anymore may be something that comes along with it but stability is not primarily about software behavior. Commented May 17, 2019 at 21:08
Stability and Reliability are different things in software development, however they both used as the sister-terms :)
I agree with some mentioned previous comments and would like to add my 2 cents.
Reliability is the extent to which an experiment, test, or any measuring procedure yields the same result on repeated trials.
Stability reliability (sometimes called test, re-test reliability) is the agreement of measuring instruments over time. To determine stability, a measure or test is repeated on the same subjects at a future date. Results are compared and correlated with the initial test to give a measure of stability.
More references on this topic are provided :
To my mind, "reliability" means you have a grasp on the bound of the system. You can, with confidence, say that "we provide response time X at the Xth percentile" (the higher the X, the better, obviously).
Stability, on the other hand, is merely an availability measure. "If you try to connect tio our service, it will be there at least X% of the times".
Reliable but not Stable:
add(a,b):
if a == nil || b == nil:
throw exception
else
return (a+b)
Stable but not Reliable:
add(a,b):
if a == nil || b == nil:
return 0
else
return (a+b)
answered May 17, 2019 at 8:01
