Reliability testing
Reliability testing checks that your software performs its required functions under stated conditions for a stated period of time. It measures how often the system fails, how fast it recovers, and whether it meets its uptime target.
What a reliability test measures
- MTBFMean time between failures
- MTTRMean time to repair
- AvailabilityMTBF / (MTBF + MTTR)
- RecoveryAfter a crash, outage or failover
What is reliability testing?
Reliability testing measures the software's ability to perform its required functions under specified conditions for a specified period of time. It is a non-functional testing type: it looks at stability, failures and uptime rather than at features.
It answers three questions:
- Does the system stay up under expected load, for hours or days at a time?
- When something fails, does it recover gracefully, and how long does that take?
- Is the data still consistent after recovery?
The answers give you confidence in long-term system behaviour, not just in how it behaves during a ten-minute test run.
Key reliability metrics
Three numbers describe reliability. Agree targets for them before testing starts, so a result is a pass or a fail rather than an opinion.
Mean time between failures (MTBF)
Average time the system operates without failure. Higher MTBF means better reliability.
Calculated as total operational time divided by the number of failures in that period.
Mean time to repair (MTTR)
Average time needed to repair a failed system and restore it to operation.
Lower MTTR means faster recovery and a more maintainable system.
System availability
Percentage of time the system is operational and reachable: MTBF / (MTBF + MTTR).
Five nines (99.999%) allows about 5.26 minutes of downtime per year.
Reliability testing types
Most reliability work combines these. Load on its own is covered in more depth on our performance testing page.
| Type | What it checks |
|---|---|
| Stress testing under load | How the system behaves under extreme conditions, beyond normal capacity. Finds breaking points and checks that it degrades gracefully instead of falling over. |
| Recovery testing | Whether the system recovers from crashes, hardware failures or unexpected interruptions. Covers backup systems, data integrity after recovery, and restoration procedures. |
| Failover testing | Automatic switching to redundant systems or backup components when a primary fails, with minimal disruption and no data loss. |
| Long-duration testing | Runs the system for hours, days or weeks to catch memory leaks, rising resource use and slow performance decline. |
Reliability testing process
Define reliability requirements
Set target MTBF, MTTR and availability from your business needs and SLA commitments. Write down acceptable failure rates and recovery time objectives.
Design test scenarios
Build realistic usage patterns that mirror production, with stress scenarios, failure injection points and recovery procedures.
Execute long-duration tests
Run for hours, days or weeks. Watch resource use, memory leaks and performance decline over time.
Inject failures and measure recovery
Introduce network outages, component crashes and resource exhaustion on purpose. Measure response, recovery time and data consistency.
Analyze results and optimize
Calculate the metrics, find failure patterns, fix, and repeat until the targets are met.
Benefits of reliability testing
System stability
Consistent performance under real-world conditions, with less unexpected downtime.
- Find and remove stability issues before production
- Check behaviour under sustained load
- Prevent cascading failures
- Keep customer trust with a service that stays up
Uptime requirements
Meet SLA commitments by showing that the system reaches its target availability.
- Verify five nines or whatever target you have agreed
- Reduce MTTR with recovery procedures that have been tested
- Raise MTBF by spotting failure patterns early
- Limit the financial impact of outages
Do you have an uptime target but no test for it?
We can help you set realistic MTBF and MTTR targets first. Talk to us
Based on the ISTQB Certified Tester Foundation Level syllabus.
How BetterQA does reliability testing
We agree the targets with you, run the long tests and failure drills in an environment close to production, and report the metrics against those targets.
Engineers who test for a living
50+ QA engineers, working since 2018. People plan and run the reliability testing and report what the results mean for your release.
Bugs you can act on
Every defect we find is written up in BugBoard, our own test management tool, or in your tracker, with steps to reproduce, the expected and actual result, and the environment.
A certified process
ISO 9001 for quality management and ISO 27001:2022 for information security. See our certifications.
How an engagement works
- €20-45 per hour
- First month is a trial
- First two weeks complimentary if you are not happy
- 30 days notice after that
Reliability testing: common questions
What is reliability testing in software testing?
A non-functional test type that checks whether software performs its required functions under stated conditions for a stated period of time. It measures failures, recovery time and availability.
What are the types of reliability testing?
The main ones are stress testing under load, recovery testing, failover testing and long-duration testing. Most projects combine several of them.
How do you calculate availability?
Availability is MTBF divided by the sum of MTBF and MTTR. Five nines (99.999%) leaves about 5.26 minutes of downtime per year.
How long does a reliability test run?
Long enough to see behaviour change over time: hours for a short soak, days or weeks when you are looking for memory leaks or slow performance decline.
What does reliability testing cost with BetterQA?
Our rate is €20-45 per hour, depending on seniority and how long you commit for. The first month is a trial, and the first two weeks are complimentary if you are not happy.
What are the sub-characteristics of reliability?
ISO/IEC 25010 splits reliability into faultlessness (called maturity in the 2011 edition), availability, fault tolerance and recoverability, and ISTQB advanced syllabi use this model (CTAL-TTA 4.4, CTAL-TA Appendix F). In practice that means measuring failure rate over a long run, tracking uptime, injecting faults to see the system cope, and rehearsing restores after a crash.
Need reliability testing?
Tell us your uptime target and how your system fails today. We will design reliability tests that show whether it meets that target and recovers when it does not.