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Availability, in the context of information systems, refers to the ability of a system or a resource to be accessible and usable upon demand by an authorized entity. It is one of the key principles in the domain of information security, often encapsulated in the CIA triad – Confidentiality, Integrity, and Availability.

Availability ensures that authorized users have reliable and timely access to the resources they require. This includes everything from data and network resources to hardware and software components. An ‘available’ system is one that is up, functioning, and able to serve requests, virtually without any unexpected or unscheduled downtime.

Multiple factors can affect availability, including hardware failures, software bugs, network connectivity issues, malicious attacks (such as Distributed Denial-of-Service, or DDoS attacks), and even natural disasters. To mitigate these risks, systems and networks are often designed with redundancies and fail-safes, such as backup servers, redundant data centers, load balancing, and disaster recovery plans.

Availability is often measured as a percentage, known as uptime, over a specific period of time. For instance, a system with 99.999% availability – sometimes referred to as “five nines” – is expected to have no more than about five minutes of downtime per year.

It’s important to note that maintaining high availability can come with significant costs, both in terms of the infrastructure required and the ongoing maintenance efforts. Therefore, decisions about the required level of availability for a system usually involve balancing the need for access against these costs.

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