Continuous availability

Continuous Availability is an approach to computer system and application design that protects users against downtime, whatever the cause and ensures that users remain connected to their documents, data files and business applications. Continuous availability describes the information technology methods to ensure business continuity . [1] [ citation needed ]

In the early days of computing, availability was considered business critical. With the increasing use of mobile computing , global access to online business transactions and business-to-business communication, it is important to increase the availability of customer support to information systems. [2]

Solutions to continuous availability exists in different forms and implementations depending on the software and hardware manufacturer. The goal of the discipline is to reduce the user or business application downtime, which can have a severe impact on business operations. Inevitably, such downtime can lead to loss of productivity, loss of revenue, customer dissatisfaction and ultimately can damage a company’s reputation.

Degrees of Availability

The terms of high availability , continuous operation, and continuous availability. [3] [4] The following is a definition of each of these terms.

High availability refers to the ability to avoid unplanned outages by eliminating single points of failure . This is a measure of the reliability of the hardware, operating system, middleware, and database manager software. Another measure of high availability is the ability to minimize the effect of unplanned outage by masking the outage of the end users. This can be accomplished by providing redundancy or fast restarting failed components.

Availability is usually expressed as a percentage of a given year:

Availability Downtime per year
99.9% 8.76 hours
99.99% 1 hour
99.999% 5 minutes

When defining such a rate, the IT infrastructure or the business application on top. [5]

Continuous operation refers to the ability to avoid planned outages. For continuous operation there is a need to be able to work in the same way as the hardware and software. This is accomplished by providing multiple servers and switching end users. Note that a system running in uncomplicated equipment may not be sufficient.

Continuous availability combines the characteristics of high availability and continuous operation with the ability to keep the business running.

Types of outages

Planned outages are deliberate and are scheduled at a convenient time. These involve such activities as: – Hardware installation or maintenance – Software maintenance or upgrades of the operating system, the middleware, the database server or the business application – Database management as offline backup, or offline reorganization

Unplanned outages are unexpected that are caused by the failure of any system component. They include hardware failures, software issues, or people and process issues.


Various commercially viable examples exist for hardware / software implementations. These include:

  • Tandem NonStop Computers
  • Stratus
  • IBM Parallel Sysplex

See also

  • Business continuity planning
  • Disaster recovery
  • High-availability cluster
  • Fault-tolerant system
  • Service Availability Forum


  1. Jump up^ Business Continuity: Delivering Data and Applications Through Continuous Availability, A META Group White Paper, June 2003[1]
  2. Jump up^ Gartner Survey Shows IT Availability Remain Top Priorities for IT Services Buyers US, September 2010[2]
  3. Jump up^ High availability (again) versus continuous availability, WebSphere Developer Technical Journal, April 14, 2010[3]
  4. Jump up^ Bob Dickerson: Service Recovery & Availability, IEEE Computer Society 2010 Meeting[4]
  5. Jump up^ itSM Solutions Newsletter December 2006: The Paradox of the 9s

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