Reliability vs Availability: What’s The Difference?

When it comes to technology and systems, you might hear the terms “reliability” and “availability” used a lot. But what do they mean, and how are they different? Think of reliability as how often something works correctly and on the other hand, availability is about how often something is ready to be used. In this article, we’ll break down the differences between reliability and availability in a way that’s easy to understand.

What is Reliability?

Reliability refers to the ability of a system, component, or process to perform its required functions under stated conditions for a specified period. In simpler terms, it’s about how well something works and how likely it is to keep working without breaking down or failing. Reliability is crucial in ensuring that systems perform consistently and meet the expectations of users or stakeholders.

For example:

A reliable car starts every morning without any issues, a reliable website loads quickly and without errors, and a reliable machine operates smoothly without frequent breakdowns.

What is Availability?

Availability is a measure of the ability of a system or service to be operational and accessible when required for use. It is typically expressed as a percentage and represents the ratio of the time a system is available for use to the total time it should be available, considering both planned and unplanned downtime.

For example:

If a website has an availability of 99%, it means that the website is operational and accessible 99% of the time and unavailable 1% of the time due to maintenance, updates, or unexpected issues.

Reliability vs. Availability

Below are the differences between Reliability and Availability:

Feature	Reliability	Availability
Definition	It is the ability of a system to deliver services correctly under given conditions for a given period of time.	It is the probability that a system, at a given point in time, would remain operational under normal circumstances.
Measurement	Reliability can be measured using metrics such as Mean Time Between Failures (MTBF) or Mean Time to Repair(MTTR).	Availability is usually measured as a percentage and is calculated as the ratio of the system’s uptime to the total time (uptime + downtime) within a given time frame.
Focus	It refers to a failure-free operation during an interval.	It refers to failure-free operation under normal circumstances at a specific instant of time.
Time Frame	It is a long-term measure that looks at the overall performance of a system over its operational lifespan	It is a short-term measure that assesses the system’s current state and its ability to be available and operational at any given moment.
Factors	Maintenance, design quality, and component lifespan.	Downtime, maintenance schedules, and fault tolerance strategies.