Providing Open Architecture High Availability Solutions

In order to provide this type of availability, a system must be designed in a reliable manner, have

processes in place to ensure rapid recovery from faults and must be used only within its design

parameters. Additionally, the system must be well managed and secure from unauthorized use and

activities.

3.2 Terminology

There are many related terms in the context of reliability engineering principles. In order to

effectively present and use these terms some definitions are required. Here, we’ll apply these terms

in the following narration to be able to use each term in a context that is consistent with the goals

and positioning of this paper.

A system is composed of a collection of interacting components. A component may itself be a

system, or it may be just a singular component. Components are the result of system decomposition

chiefly motivated to aid in the partitioning of complex systems for either technical, or very often,

for organizational or business reasons. Decomposition of systems into components is a recursive

exercise. A component that is not decomposed further is called an atomic component. Components

are typically delineated by the careful specification of their inputs and outputs. A system provides

one or more services to its consumers. A service is the output of a system that meets the

specification for which the system was devised, or which agrees with what system users have

perceived the correct values to be [Lyu96].

A failure in a system occurs when the consumer (human or non-human) of a service is affected by

the fact that the system has not delivered the expected service. Failures are incorrect results with

respect to a specification or unexpected behavior perceived by the consumer or user of a service.

The cause of a failure is said to be a fault. Faults are identified or detected in some manner either

by the system or by its users. Finally, an error is a discrepancy between the computed, measured,

or observed value or condition and the correct, or specified value or condition. Errors are often the

result of exceptional conditions or unexpected interference. If an error occurs, then a failure of

some form has occurred. Similarly, if a failure has occurred, then an error of some form has

occurred. Since the difference between an error and a failure is very subtle, the remainder of this

document will treat the terms synonymously.

Note: Faults are active when they produce an error. Faults may also be latent. A latent fault is one that

has not yet been detected.

Since faults, when activated, cause errors, it is important to detect not only active faults, but also

the latent faults. Finding and removing these faults before they become active leads to less

downtime and higher availability.

Table 2. Service Expectations

Service Type Service Expectation

Telephone System

Dial tone within seconds of lifting receiver

Call completion (ring or busy) within a second of dialing the last digit

Call does not get dropped

Web Site First visual frame within a few seconds

Financial Backend No lost transactions