Specifications

Equalizer Installation and Administration Guide 149
9 Administering Geographic Clusters
Geographic Load Balancing with Envoy
The Envoy geographic load balancer, an optional software add-on for the Equalizer product line,
supports geographic clustering and load balancing. Geographic clustering and load balancing
enables requests to be automatically distributed across servers in different physical locations or on
different networks.
Envoy Overview
Equalizer and its set of servers in a particular location forms a site (or Envoy site). A geographic
cluster contains multiple sites, and Equalizer’s geographic load balancing technology balances
incoming requests across those sites.
When a client uses DNS to resolve the address of a domain name, it performs a recursive search
with a number of name servers to resolve that address. Envoy is the last name server in this search.
The name server in the recursive chain immediately before Envoy returns a list of Envoy sites. The
client sends requests, one at a time, to each of the Envoy sites until it reaches an active site. If the
Envoy site is active, Envoy performs the following steps to determine the site in the geographic
cluster that should handle the request:
1. If, for example, Site A in Figure 67 is the first active Envoy site accessed by the client, Site A
then identifies the geographic cluster that has been configured with the requested domain
name—in this example,
www.coyotepoint.com.
Figure 67 Sending name resolution requests to an Equalizer in a geographic cluster
Envoy Site B
(West Coast USA)
Envoy Site A
(East Coast USA)
Internet
Envoy Site C
(Europe)
Client
(California, USA)
Client’s
Local DNS
DNS
Authoritative
DNS
for
coyotepoint.com
D
N
S