What is high availability?
The general meaning of the word "availability" is a characteristic of a resource—either a person or an object that can be accessed or used. Resource availability can be measured, and therefore, a ratio of the time a resource is accessible or usable to the time the resource is inaccessible or unusable can be calculated. Adding an adjective "high" to the word "availability" suggests that the resource should be accessible and usable most of the time during a given time interval. The term "high availability" is commonly used in information technology and it describes IT systems with a high level of availability.
High availability in IT refers to a system that is continuously operational and available for the delivery of services it provides for end users. The key point when talking about IT systems is the availability to deliver services to end users, since a system can be up-and-running from the IT administrator's perspective but can fail to provide services for end users, which makes it useless. There are a number of factors that can lead to service downtime, mainly because there are so many different layers that must work together to provide service availability.
An IT system usually consists of many different components. All of these components must be continuously available for a desirable length of time. It is needless to say that it is very important for these highly available systems to be properly designed, well thought through, and thoroughly tested with the goal of eliminating any possibility of potential failure. That being said, high availability is a system design approach, and a service implementation in a way, to provide the highest achievable level of performance and availability by eliminating all system-wide single points of failure.
Not every system can be marked highly available. It is common practice in IT to measure and calculate the availability of a system. Monitoring tools such as Nagios, Zenoss, or Zabbix can be used to provide reports on system availability and also alerts in the case of system unavailability. The measurements taken must reflect the actual availability of the system to the end user. By measuring and calculating the availability of a system, we can split them into systems that are classified as highly available and systems that are not. System availability is commonly expressed as a percentage of system uptime in a given year.
