Network Availability: An Example

Availability has always been an important characteristic of systems but becomes an even more critical and complex issue on networks. By their nature, network services are commonly distributed across several computers and can depend on various other auxiliary devices as well.

Take the Domain Name System (DNS), for example -- used on the Internet and many private networks to maintain a list of computer names based on their network addresses. DNS keeps its index of names and addresses on a server called the primary DNS server. When only a single DNS server is configured, a server "crash" takes down all DNS capability on that network.

DNS, however, offers support for distributed servers. Besides the primary server, an administrator can also install secondary and tertiary DNS servers on the network. Now, a failure in any one of the three systems is much less likely to cause a complete loss of DNS service.

Server crashes aside, other types of network outages also affect DNS availability. Link failures, for example, can effectively take down DNS by making it impossible for clients to communicate with a DNS server. It's not uncommon in these scenarios for some people (depending on their physical location on the network) to lose DNS access but others to remain unaffected. Configuring multiple DNS servers also helps to deal with these indirect failures that can impact availability.

Perceived and High Availability

Outages are not all created equal. The timing of failures also plays a big role in the perceived availability of a network. A business system that suffers frequent weekend outages, for example, may show relatively low availability numbers, but this downtime may not even be noticed by the regular workforce.

The networking industry uses the term "high availability" to refer to systems and technologies specially-engineered for reliability, availability, and serviceability. Such systems typically include redundant hardware (e.g. disks and power supplies) and intelligent software (e.g., load balancing and fail-over functionality). The difficulty in achieving high availability increases dramatically at the four- and five-nines levels, so vendors can charge a cost premium for these features.