Specifications
Introducing Equalizer
Equalizer Installation and Administration Guide 5
Geographic Load Balancing
The optional Envoy add-on supports geographic load balancing, which enables requests to be
automatically distributed across Equalizer sites in different physical locations. An Equalizer site is a
cluster of servers under a single Equalizer’s control. A geographic cluster is a collection of sites
that provide a common service, such as Web sites. The various sites in a geographic cluster can be
hundreds or even thousands of miles apart. For example, a geographic cluster might contain two
sites, one in the eastern U.S. and one on the U.S.’s west coast (Figure 1).
Geographic load balancing can dramatically improve reliability by ensuring that your service
remains available even if a site-wide failure occurs. Equalizer can also improve performance by
routing requests to the location with the least network latency.
Figure 1 Geographicclusterwithtwosites
Geographic Load Balancing Routing
Envoy routes each incoming request to the site best able to handle it. If a site is unavailable or
overloaded, Envoy routes requests to the other sites in the geographic cluster. When you enable
geographic load balancing, Envoy directs incoming client requests to one of the sites in the
geographic cluster based on the following criteria:
• Availability: If a site is unavailable due to network outage, server failure, or any other reason,
Equalizer stops directing requests to that site.
• Performance: Envoy tracks the load and performance at each site and uses this information
to determine the site that can process the request most efficiently.
• Distance: Envoy notes the site that is closest to the client (in network terms) and offers the
least network latency.
Distributing the Geographic Load
Envoy uses the Domain Name System (DNS) protocol
1
to perform its geographic load distribution.
DNS translates fully-qualified domain names such as www.coyotepoint.com into the IP
addresses that identify hosts on the Internet. For Envoy, the authoritative name server for the
domain is configured to query the Equalizers in the geographic cluster to resolve the domain name.
When Envoy receives a resolution request, it uses the load-balancing algorithms configured for the
1.FormoreinformationaboutDNS,seePaulAlbitzandCricketLiu,DNSandBIND,3rded.(OʹReilly&Associates,1998).
Internet
Envoy
Site A
Envoy
Site B