What is software stability?
In systems as well as in software, stability is a clear quality feature. With increased stability, software can also run under higher loads and crashes occur less frequently.
As one of the quality features of software, stability is a measurable metric that, together with conformity, analyzability, testability and modifiability, make up a sub-part of the software standard ISO / IEC 9126. The stability is considered to be part of the maintainability of a software.
Apart from (of course non-binding) models for evaluating software quality, stability is above all a user-centered metric. Thanks to high stability (low performance fluctuations under load, no abnormal behavior, no crashes), users can use software as intended and do not have to fear that they will have to work with an unstable version even if the code changes.
This not only requires extensive tests, but also updates and improvements in order to ensure operability with new functions and in new environments.
Stable releases – the ambiguity of stability
In English, the term “stable software” has two connotations. On the one hand, we are talking about a stable version, i.e. a software version in which no more changes are made (with the exception of minor fixes and updates). In order to acquire new functionality for this stable software, users have to upgrade to a newer version.
Such stable software versions are particularly relevant for software that operates with a large number of plug-ins and networked software and that could endanger the functionality of all operating add-ons through a major update. Ultimately, it is also up to the users’ preference whether they prefer to use stable, but possibly stagnant software, or whether they want to use the latest features (and also bugs) in newer beta releases .
The other meaning of the term “stable software”, which is more closely related to stability for software, is that of the stable running software version. We are talking about reliably functioning software without crashes or major bugs – the most pronounced are jumps to the stable version, often on the way to the first stable beta. In many development cycles, however, it is only the release version that is actually stable.
Especially in beta and functionality tests, users like to describe software as stable. To make this clear again: Stably functioning software can run more stably than a previous version, while a stable software version describes an absolute status.
How is stability measurably tested?
As part of performance tests, stability tests are also carried out during the development of software. These stability tests are intended to ensure that the software does not slow down under load, that there are no complete failures or “abnormal” behavior.
In Quality Assurance , large numbers of virtual users are usually used for this type of test, who put a system under high load and measure deviations from normal operation. Stability tests are one of the tests that must be carried out regularly in order to eliminate bugs, check the stability of new versions and monitor changes to functions during operation.
Such stability tests are not only important for sophisticated software and server farms, but must also be used for small apps or smartphones. For example, a check is made to determine whether the smartphone is still working as quickly when the memory is almost fully utilized as when the storage device is empty.
For example, websites have to check whether forms can still be processed just as efficiently when several hundred inquiries are received or buttons are clicked several hundred times. Often this form of stability is also a protection against bot attacks.
Comparison of “stability” and “reliability”
Stability guarantees the performance of a system under (near) maximum load, the efficiency of the system and a runtime without occurring memory leaks outside of the development environment.
The term stability must be separated from reliability, even if we are talking about similar terms here: Stability is a factor in the broader sense of reliability. Reliability tests loads, failover tests (relocating to another server in the event of a server failure), recovery tests (recovering the system after a failure) and finally the overall stability.
Even with faulty stability, fault-free handling can still be guaranteed through complimenting reliability. For example, if an app crashes, it is less stable. However, if users can resume their work with a seamless restart without data loss, reliability is still guaranteed.
A good example of this is the entire internet in our surfing habits. Connections are constantly breaking off, links lead to dead pages or individual browser tabs no longer react – but overall reliability is only marginally affected.
Combine stability and complexity
Following this chain, stability is an important factor, as it has a significant share in the usability and subjectively assessed quality of a software / system for users – however, stability must not be viewed in isolation.
As complexity increases, it is not uncommon for stability to decrease. Simpler programs leave less room for error and are easier to modify without unpredictable problems.
After all, stability tests are also a cost and time factor, but a lack of stability for users and, above all, software in a corporate context can also be expensive and tedious.