Continuous Availability
Encyclopedia
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
.
In early days of computing, availability was not considered business critical. With the increasing use of mobile computing
, global access to online business transactions and business-to-business communication, continuous availability is increasingly important based on the need to support customer access to information systems.
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 companies reputation.
, continuous operation, and continuous availability are generally used to express how available a system is. 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 an unplanned outage by masking the outage from the end users.
This can be accomplished by providing redundancy or quickly restarting failed components.
Availability is usually expressed as a percentage of uptime in a given year:
When defining such a percentage it needs to be specified if it applies to the hardware, the IT infrastructure or the business application on top.
Continuous operation refers to the ability to avoid planned outages.
For continuous operation there must be ways to perform necessary administrative work, like hardware and software maintenance while the business application remains available to the end users. This is accomplished by providing multiple servers and switching end users to an available server at times when one server is made unavailable.
Note that a system running in continuous operation is not necessarily operating with high availability because an excessive number of unplanned outages could compromise this.
Continuous availability combines the characteristics of high availability and continuous operation to provide the ability to keep the business application running without any noticeable downtime.
- Hardware installation or maintenance
- Software maintenance or upgrades of the operating system, the middleware, the database server or the business application
- Database administration such as offline backup, or offline reorganization
Unplanned outages are unexpected outages that are caused by the failure of any system component.
They include hardware failures, software issues, or people and process issues.
Business continuity
Business continuity is the activity performed by an organization to ensure that critical business functions will be available to customers, suppliers, regulators, and other entities that must have access to those functions. These activities include many daily chores such as project management,...
.
In early days of computing, availability was not considered business critical. With the increasing use of mobile computing
Mobile computing
Mobile computing is a form of human–computer interaction by which a computer is expected to be transported during normal usage. Mobile computing has three aspects: mobile communication, mobile hardware, and mobile software...
, global access to online business transactions and business-to-business communication, continuous availability is increasingly important based on the need to support customer access to information systems.
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 companies reputation.
Degrees of Availability
The terms high availabilityHigh availability
High availability is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period....
, continuous operation, and continuous availability are generally used to express how available a system is. 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 an unplanned outage by masking the outage from the end users.
This can be accomplished by providing redundancy or quickly restarting failed components.
Availability is usually expressed as a percentage of uptime in a given year:
| Availability | Downtime per year |
|---|---|
| 99.9% | 8.76 hours |
| 99.99% | 1 hour |
| 99.999% | 5 minutes |
When defining such a percentage it needs to be specified if it applies to the hardware, the IT infrastructure or the business application on top.
Continuous operation refers to the ability to avoid planned outages.
For continuous operation there must be ways to perform necessary administrative work, like hardware and software maintenance while the business application remains available to the end users. This is accomplished by providing multiple servers and switching end users to an available server at times when one server is made unavailable.
Note that a system running in continuous operation is not necessarily operating with high availability because an excessive number of unplanned outages could compromise this.
Continuous availability combines the characteristics of high availability and continuous operation to provide the ability to keep the business application running without any noticeable downtime.
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 administration such as offline backup, or offline reorganization
Unplanned outages are unexpected outages that are caused by the failure of any system component.
They include hardware failures, software issues, or people and process issues.
History
Various commercially viable examples exist for hardware/software implementations. These include:- Tandem NonStop Computers
- StratusStratus VOSVOS is a proprietary operating system running on Stratus Technologies fault-tolerant computer systems. VOS is available on Stratus's ftServer and Continuum platforms...
- IBM Parallel SysplexIBM Parallel SysplexIn computing, a Parallel Sysplex is a cluster of IBM mainframes acting together as a single system image with z/OS. Used for disaster recovery, Parallel Sysplex combines data sharing and parallel computing to allow a cluster of up to 32 systems to share a workload for high performance and high...
See also
- Business continuity planningBusiness continuity planningBusiness continuity planning “identifies [an] organization's exposure to internal and external threats and synthesizes hard and soft assets to provide effective prevention and recovery for the organization, whilst maintaining competitive advantage and value system integrity”. It is also called...
- Disaster recoveryDisaster recoveryDisaster recovery is the process, policies and procedures related to preparing for recovery or continuation of technology infrastructure critical to an organization after a natural or human-induced disaster. Disaster recovery is a subset of business continuity...
- High-availability clusterHigh-availability clusterHigh-availability clusters are groups of computers that support server applications that can be reliably utilized with a minimum of down-time. They operate by harnessing redundant computers in groups or clusters that provide continued service when system components fail...
- Fault-tolerant systemFault-tolerant systemFault-tolerance or graceful degradation is the property that enables a system to continue operating properly in the event of the failure of some of its components. A newer approach is progressive enhancement...
- Service Availability ForumService Availability ForumThe Service Availability Forum is a consortium that develops, publishes, educates on and promotes open specifications for carrier-grade and mission-critical systems. SA Forum specifications enable faster development and deployment of commercial off-the-shelf ecosystems for highly available...
External links
- Continuity Central
- Plugging Continuous Availability
- Continuous Availability Blog
- Continuous Availability for SharePoint
- IBM Redbook on Continuous Availability
- Business Continuity for SAP on IBM System z
- Reliability and Availability Basics
- TechRepublic: IT should establish realistic availability requirements
- US Patent 5027269, "Method and apparatus for providing continuous availability of applications in a computer network", 1991; IBMIBMInternational Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...