In order to provide better understanding of the availability concept, it is necessary to define and review the terms that shape a framework for information systems availability. This section introduces the concept of availability and the three terms that are most associated with the concept of availability, namely: dependability, reliability and maintainability. A short introduction to availability modeling is also presented in this section by explaining three most widely used methods: Reliability Block Diagrams, Fault Trees Diagrams, and Markov Chains.
TopIn today's world, most business activities are associated with the use of information technology. Information technology enables and facilitates business processes. In addition, business success is becoming more dependent on the adequate use of information technologies, and risks associated with this dependence. Definition of the “Information technology” term is adopted from ITIL glossary where IT refers to “the use of technology for the storage, communication or processing of information. The technology typically includes computers, telecommunications, applications and other software”.
Modern businesses employ several information technologies such as servers, desktop computers, portable/mobile computing devices, operating systems, application software, data communication (networking) technologies by implementing them in several forms of business (enterprise) information systems. These information systems are in most cases built on client-server model of information architecture model which consists of server(s) and clients (desktop computers and mobile devices) with applications installed on server computers. A client/server-based information architecture divides processing into two major categories: clients and servers. A client is a computer such as a PC or a workstation attached to a computer network consisting of several dozens (hundreds or thousands) clients and one or more servers. Server’s side of such an information architecture is called “Business Server”, “Enterprise Server” or “Server Operating Environment”. It consists of standard server-based and additional continuous computing technologies that are used to enhance key server platform features such as reliability, availability, and scalability. Servers can be installed “on premises” (standard “client-server” model) or in the “cloud” (cloud computing provider’ premises, “client-cloud” model). In the digital age, server configurations and server operating systems that run them are expected to provide such an operating environment that must meet rigorous requirements with regard to system uptime and application availability.
As the client-server model of information architecture is most widely used for all enterprise information systems (ERP, SCM, CRM, BI), in both on-premises and cloud-based infrastructures, the server side of such an architecture determines the availability of applications installed on application servers. EIS applications are installed on enterprise servers that are run by server operating systems. Therefore, availability, reliability and scalability of these systems is of extreme importance for modern organizations. Such environment created a particular business pressure of being “up-and-running” for many businesses. Server operating systems (SOS) play crucial role in keeping business “in-business” as their crashes are one of the main reasons of “going out of business.”
When it comes to modern e-business applications, system downtime is simply not an option. Several industries such as banks, financial institutions, airline reservation systems, point-of-sale systems, dispatching systems, online shops must be running on “always-on” basis.
Today's enterprises are exposed to several types of threats coming from outside and inside the system such as:
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Physical threats result from any kind of physical damage that may occur on IT centres, servers, hardware components, communication devices.
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Natural-catastrophic events such as fire, lightning, flood, earthquake, hurricane, tornado, snow.
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Logical threats such as deleted system files, corrupted files, broken processes or programs, corrupted file system, crashed operating system.
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Technical glitches related to hardware component failures on memory chips, fans, mainboards, hard disks, disk controllers, tapes or tape drivers, network cards, switches, routers, communication lines, power supply units).
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System software/operating system crashes such as BSOD (“Blue Screen of Death”) on Windows Server systems or similar crashes on other server OS platforms.
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Application software defects, failures and crashes, badly integrated applications, file corruptions.
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LAN/WAN/Internet problems, in addition to possible hardware glitches on data communication devices, include the problems such as those with Domain Controllers, Active Directory, DNS configuration files, DNS servers, network configuration files.