Availability vs Reliability: Difference and Comparison

Key Takeaways

  1. Availability: Refers to the ability of a system, service, or resource to be accessible and operational when needed. It measures the percentage of time that a system is accessible and usable by users. High availability implies the system is consistently accessible, minimizing downtime and ensuring users can access the resource whenever required.
  2. Reliability: Refers to the ability of a system or component to consistently perform its intended function without failure over a specified period. It measures the probability that a system will function without errors or breakdowns. High reliability indicates that the system operates consistently and delivers accurate results, minimizing failures or disruptions.
  3. Difference: Availability ensures that a system is consistently accessible and available to users, while reliability emphasizes the consistent performance and absence of failures in delivering the intended function. Availability is concerned with uptime and accessibility, while reliability is concerned with consistent and accurate operation. While availability emphasizes accessibility, reliability emphasizes performance and consistency.

What is Availability?

Availability is a unique parameter that determines the ability of a system to perform tasks regularly according to the requirements. It is an operational parameter. It is also defined as the percentage of time the system or solution can be operated under normal conditions. In simple words, it is a chance that the system is ready to use. It is so known as the Uptime of a service. Availability ensures that the system is not going through any maintenance or repair.


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Availability can be monitored by continuously operating the service and examining the results. With high availability, all the tasks can be completed quickly, so high availability is preferred during operating hours. It is necessary to stay up-to-date about new operational methods to improve availability. Improving the schedule practices and implementing effective preventative maintenance also helps increase availability.

The mathematical formula that is used to calculate the percentage of availability is:

Percentage of availability = Uptime/(Uptime+downtime) ×100

Uptime is defined as the time equipment is running, and it is measured using the Mean Time Between Failure (MTBF) metric, and downtime is defined as the time equipment is being repaired, and it can be measured using the Mean Time to Repair (MTTR).

What is Reliability?                                                                             

Reliability is the probability that the system or product will operate continuously without failure in a desired time under stated conditions. It measures the frequency of failures and determines the uptime pattern. Reliability can help maintain the system or equipment and keep them in good working conditions. It helps to increase productivity and profits.

High Reliability minimizes the frequency of random downtime. Reliability can be improved by performing beta testing, optimizing repair, or planning corrective maintenance. These types of care help lessen the chance of failure of equipment.

The Reliability of a system is quite challenging to measure. Reliability can be measured by Mean Time Between Failure (MTBF) or Failure Rate. The MTBF estimates how long the system can run before experiencing any malfunction, and it can be determined by dividing the total Runtime by the number of failures. Failure Rate is the frequency at which a system fails and can be determined by dividing the number of failures by the Total Runtime.

Difference Between Availability and Reliability

  1. Availability is the percentage of time the system can operate under normal conditions, whereas Reliability is the probability of the system working without any failure.
  2. Availability refers to a failure-free operation at a specific time, whereas Reliability refers to a failure-free operation during an interval.
  3. Availability is not a factor of Reliability, whereas Reliability is a part of availability.
  4. Availability is harder to solve, whereas Reliability can be fixed by testing.
  5. Availability is calculated by the division of Uptime by the Sum of Uptime and downtime, whereas the division of Total Runtime calculates Reliability by the number of failures.

Comparison Between Availability and Reliability

Parameters of ComparisonAvailabilityReliability
DefinitionIt is the percentage of time the system can operate under normal conditionsIt is the probability of the system operating without any failure
ReferenceIt refers to a failure-free operation at a specific timeIt refers to a failure-free operation during an interval of time
VariationIt is not a factor of ReliabilityIt  is a part of the availability
RepairIt is harder to fixIt can be fixed by testing
Mathematical FormulaDivision of Uptime by the Sum of Uptime and downtimeDivision of Total Runtime by the number of failures
  1. https://ieeexplore.ieee.org/abstract/document/963122
  2. https://www.sciencedirect.com/science/article/abs/pii/S0065245808601540
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