Availability

Availability indicates that data and services are available when needed. For some organizations, this simply means that the data and services must be available between 8:00 a.m. and 5:00 p.m., Monday through Friday. For other organizations, this means they must be available 24 hours a day, 7 days a week, 365 days a year.

Organizations commonly implement redundancy and fault-tolerant methods to ensure high levels of availability for key systems. Additionally, organizations ensure systems stay up to date with current patches to ensure that software bugs don’t affect their availability.

Redundancy and Fault Tolerance

Redundancy adds duplication to critical systems and provides fault tolerance. If a critical component has a fault, the duplication provided by the redundancy allows the service to continue without interruption. In other words, a system with fault tolerance can suffer a fault, but it can tolerate it and continue to operate.

A common goal of fault tolerance and redundancy techniques is to remove each single point of failure (SPOF). If an SPOF fails, the entire system can fail. As an example, if a server has a single drive, the drive is an SPOF because its failure takes down the server.

Here are some common fault-tolerance and redundancy techniques:

Disk redundancies. Fault-tolerant disks, such as RAID-1 (mirroring), RAID-5 (striping with parity), and RAID-10 (striping with a mirror), allow a system to continue to operate even if a disk fails.
Server redundancies. Failover clusters include redundant servers and ensure a service will continue to operate, even if a server fails. In a failover cluster, the service switches from the failed server in a cluster to an operational server in the same cluster. Virtualization can also increase availability of servers by reducing unplanned downtime.
Load balancing. Load balancing uses multiple servers to support a single service, such as a high-volume web site. It can increase the availability of web sites and web-based applications.
Site redundancies. If a site can no longer function due to a disaster, such as a fire, flood, hurricane, or earthquake, the organization can move critical systems to an alternate site. The alternate site can be a hot site (ready and available 24/7), a cold site (a location where equipment, data, and personnel can be moved to when needed), or a warm site (a compromise between a hot site and cold site).
Backups. If personnel back up important data, they can restore it if the original data is lost. Data can be lost due to corruption, deletion, application errors, human error, and even hungry gremlins that just randomly decide to eat your data. Without data backups, data is lost forever after any one of these incidents.
Alternate power. Uninterruptible power supplies (UPSs) and power generators can provide power to key systems even if commercial power fails.
Cooling systems. Heating, ventilation, and air conditioning (HVAC) systems improve the availability of systems by reducing outages from overheating.

Patching

Another method of ensuring systems stay available is with patching. Software bugs cause a wide range of problems, including security issues and even random crashes. When software vendors discover the bugs, they develop and release code that patches or resolves these problems. Organizations commonly implement patch management programs to ensure that systems stay up to date with current patches.