Page 1302 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Failover clusters are not the only method of fault tolerance

for servers. Some systems provide automatic fault tolerance for
servers, allowing a server to fail without losing access to the

provided service. For example, in a Microsoft domain with two or
more domain controllers, each domain controller will regularly
replicate data with the others so that all the domain controllers
have the same data. If one fails, computers within the domain can
still find the other domain controller(s) and the network can
continue to operate. Similarly, many database server products
include methods to replicate database content with other servers

so that all servers have the same content. Three of these methods—
electronic vaulting, remote journaling, and remote mirroring—are
discussed later in this chapter.

Protecting Power Sources

Fault tolerance can be added for power sources with an
uninterruptible power supply (UPS), a generator, or both. In general,
a UPS provides battery-supplied power for a short period of time

between 5 and 30 minutes, and a generator provides long-term power.
The goal of a UPS is to provide power long enough to complete a
logical shutdown of a system, or until a generator is powered on and
providing stable power.

Ideally, power is consistently clean without any fluctuations, but in
reality, commercial power suffers from a wide assortment of problems.
A spike is a quick instance of an increase in voltage whereas a sag is a

quick instance of a reduction in voltage. If power stays high for a long
period of time, it’s called a surge rather than a spike. If it remains low
for a long period of time, it’s called a brownout. Occasionally, power
lines have noise on them called transients that can come from many
different sources. All of these issues can cause problems for electrical

equipment.
A very basic UPS (also called an offline or standby UPS) provides

surge protection and battery backup. It is plugged into commercial