- [Narrator] We've already discussed some of the ways
that security professionals can ensure
the continued operation of systems.
Let's dig into this in a little more detail.
There are two key technical concepts
that improve the availability of systems.
High availability, otherwise known as HA,
uses multiple systems to protect against failures.
These are techniques like the ones we discussed
in the previous video.
Having a cluster of web servers in place
that can continue to operate,
even if a single server fails,
is one example.
Another is using a pair of firewalls
with one designated as the backup.
The core concept of high availability
is having operationally redundant systems,
sometimes at different locations.
Fault tolerance, or FT,
helps protect a single system
from failing in the first place
by making it resilient in the face of technical failures.
One quick note:
load balancing is a related but different concept.
Load balancing uses multiple systems
in an attempt to spread the burden
of providing a service across those systems,
providing a scalable computing environment.
While they use similar technologies,
load balancing and high availability are different goals.
Most implementations of clustering,
and similar technologies,
are designed to achieve
both high availability and load balancing,
but it is possible to have one without the other.
Two of the most common points of failure
within a computer system
are the devices power supply and storage media.
Fault tolerance controls can prevent a system from failing,
even if one of those components fails completely.
Power supplies contain moving parts
and, as such, are common points of failure.
If a power supply fails,
the results can be catastrophic.
For this reason,
server manufacturers often build
two independent power supplies into their servers.
When a customer installs the server,
they connect both of the power supplies to a power source.
This way, if one power supply fails,
the other power supply can continue powering
the server's uninterrupted operation.
For added redundancy,
data centers with two separate sources of power
can connect each power supply
to a different power source.
The second priority of many fault tolerance efforts
is protecting against the failure
of a single storage device.
They achieve this through the use of a technology
known as RAID: redundant arrays of inexpensive disks.
RAID comes in many different forms,
but each of them is designed to provide redundancy
by having more disks than needed to meet business needs.
Let's take a look at two RAID technologies:
mirroring and striping.
The most basic form of RAID, known as RAID level 1,
is called disk mirroring.
In this approach, the server contains two disks.
Each disk has identical data contents,
and when the system writes any data to one disk,
it automatically makes the same change to the other disk,
keeping the second disk as a synchronized copy,
or mirror, of the primary disk.
Then, if the primary disk fails,
the system can automatically switch over to the backup disk
and continue operating.
The second major RAID technology
is disk striping with parity,
known as RAID level 5.
In this approach,
the system contains three or more disks,
and writes data across all of those disks,
but also includes additional elements,
known as parity blocks,
spread across the disks.
If one of the disks fails,
the system can regenerate that disk's contents
by using the parity information.
When you take the exam, you won't need to know
detailed technical information about RAID,
but you should remember the number of disks
required to implement each level of RAID.
Disk mirroring requires two disks,
while disk striping with parity requires three or more.
One other very important exam tip:
RAID is a fault tolerance strategy
designed to protect against a single disk failure.
It is not a backup strategy.
You should still perform regular data backups
to protect your organization's information
in the event of a more catastrophic failure,
such as the physical destruction of an entire server.
Fault tolerance also extends to many other domains.
For example,
networking professionals use quality of service,
or QoS controls,
to provide a consistent level of service
to high criticality applications.
This reduces the likelihood
that network capacity will become a single point of failure
in the event of a denial of service attack
or other unanticipated burden on the network.