Continuous availability is an approach to
computer system
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
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
Business continuity may be defined as "the capability of an organization to continue the delivery of products or services at pre-defined acceptable levels following a disruptive incident", and business continuity planning (or business continuity a ...
.
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 company's reputation.
Degrees of availability
The terms
high availability
High availability (HA) is a characteristic of a system which aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period.
Modernization has resulted in an increased reliance on these systems. F ...
, 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
Information technology infrastructure is defined broadly as a set of information technology (IT) components that are the foundation of an IT service; typically physical components (computer and networking hardware and facilities), but also vario ...
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, upgrades, and platform refreshes 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:
*
BIND
BIND () is a suite of software for interacting with the Domain Name System (DNS). Its most prominent component, named (pronounced ''name-dee'': , short for ''name daemon''), performs both of the main DNS server roles, acting as an authoritative ...
*
BitTorrent
*
Ceph
*
CockroachDB
CockroachDB is a commercial distributed SQL database management system, developed by Cockroach Labs.
History
Cockroach Labs was founded in 2015 by ex-Google employees Spencer Kimball, Peter Mattis, and Ben Darnell. Cockroach Labs founders Kim ...
*
Dovecot
*
IBM Parallel Sysplex
In 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 ...
*
Stratus
Stratus may refer to:
Weather
*Stratus cloud, a cloud type
**Nimbostratus cloud, a cloud type
**Stratocumulus cloud, a cloud type
**Altostratus cloud, a cloud type
**Altostratus undulatus cloud, a cloud type
**Cirrostratus cloud, a cloud type
Mus ...
*
Tandem NonStop Computers
*
YugabyteDB
YugabyteDB is a high-performance transactional distributed SQL database for cloud-native applications, developed by Yugabyte.
History
Yugabyte was founded by ex-Facebook engineers Kannan Muthukkaruppan, Karthik Ranganathan, and Mikhail Baut ...
See also
*
Business continuity planning
Business continuity may be defined as "the capability of an organization to continue the delivery of products or services at pre-defined acceptable levels following a disruptive incident", and business continuity planning (or business continuity a ...
*
Disaster recovery
*
High-availability cluster
High-availability clusters (also known as HA clusters, fail-over clusters) are groups of computers that support server applications that can be reliably utilized with a minimum amount of down-time. They operate by using high availability softwa ...
*
Fault-tolerant system
Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of one or more faults within some of its components. If its operating quality decreases at all, the decrease is proportional to the ...
*
Service Availability Forum
The Service Availability Forum (SAF or SA Forum) is a consortium that develops, publishes, educates on and promotes open specifications for carrier-grade and mission-critical systems. Formed in 2001, it promotes development and deployment of com ...
References
{{Reflist
External links
Continuity CentralContinuous Availability BlogBusiness Continuity for SAP on IBM System zTechRepublic: IT should establish realistic availability requirements* US Patent 5027269, "Method and apparatus for providing continuous availability of applications in a computer network", 1991;
IBM
Computer systems