Availability refers to the proportion of time a system or service is operational and accessible for use by its users. It is typically expressed as a percentage and represents the reliability of a system in being consistently available when needed. Uptime, on the other hand, specifically refers to the duration during which a system or service is up and running without experiencing any outages or interruptions.
To understand the nuances and differences consider server monitoring.
Current uptime of a server is measured as the amount of time that the server has been up and running since the last time it rebooted. Uptime is expressed in years, months, days, hours, minutes, and seconds. Every time the server reboots, up time is reset to 0 (zero) and starts increasing as the server remains up and operational.
On Unix systems, uptime is measured using the “uptime” command. On Windows, uptime is reported on the Performance tab of the Task Manager. Check the “CPU” setting.
Sometimes, uptime is indicated as a percentage. Uptime, as a percentage, is computed as the time that the server has been operational, divided by the total measurement period. For high reliability, up time should be close to 100%. A value below 100% indicates that the server has been rebooted during the measurement period.
Uptime and availability are often used interchangeably but they are not the same:
Availability is usually an external check, unlike uptime, which is an internal check. Availability is often measured using Synthetic Monitoring tools.
Uptime measures the duration a system remains operational without interruptions, irrespective of its ability to fulfill requests. In contrast, availability considers not only uptime but also the system's ability to handle incoming requests. For instance, a web server operating at full capacity might have 100% uptime since it remains functional, but its availability would be nearly zero because it cannot process new requests effectively. Thus, while uptime reflects continuous operation, availability encompasses functionality and accessibility, making it an essential metric for assessing the reliability of a system under varying conditions.
Uptime specifically focuses on the periods when the system is operational. Essentially, availability encompasses uptime but also includes factors such as maintenance and planned downtime. When choosing monitoring tools and platforms it is essential that they include features to handle maintenance and scheduled downtime if you are to use them to track and quantify meaningful measures of either availability or uptime. See: Managing Monitoring and Alerting during IT Maintenance.
The UK government publishes advice for those providing online web sites regarding availability: Uptime and availability: keeping your service online - Service Manual - GOV.UK. This advice is aimed at those mandated to provide service availability 24x7 but covers provisions to define availability as being the user gets some sort of satisfactory alternative if the primary service is available, for example if a web form is unavailable an acceptable measure may be presenting a web page informing the user that the primary service is down and a predicted time when the service will be operational again.
Neither availability nor uptime quantify or measure performance. Slow application or service performance can manifest to the end user as a lack of availability if the service is too slow to be effectively used. Effective Application Performance Monitoring (APM) tools measure application and user experience beyond simple availability.
