Coyote Point Systems E350GX, Installation And Administration Manual

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Chapter 9: Administering GeoClusters
238 Equalizer Installation and Administration Guide
Overview of Geographic Load Balancing with Envoy
In non-Envoy Equalizer configurations, there is a one-to-one correspondence between a cluster and a website: when
a client makes a request for a website (say,
www.example.com
), the client uses the Domain Name Service (DNS) to
resolve the website name to an IP address. For a website that is load balanced by an Equalizer, the IP address
returned is the IP address of an Equalizer cluster. After resolving the name, the client sends the request to the cluster
IP. When Equalizer receives the client request, it load balances the request across the servers in the cluster, based on
the current load balancing policy and parameters.
In an Envoy conversation, you have two or more Equalizers located in separate locations. Each Equalizer and its set
of clusters and servers forms a
site (or Envoy site) . With Envoy, the website name in the client request is resolved to
a GeoCluster IP . A GeoCluster is analogous to a cluster, but one level above it: in other words, a GeoCluster actually
points to two or more clusters that are defined on separate Equalizers.
In the same way that Equalizer balances requests for a cluster IP across the servers in the cluster, Equalizer load
balances a request for a GeoCluster IP across the clusters in the GeoCluster configuration. Once a site is chosen and
the client request arrives at that site, the request is load balanced across the servers in the appropriate cluster. In this
way, you can set up geographically distant Equalizers to cooperatively load balance client requests.
Overview of Configuration Process
Follow this general procedure when setting up Envoy for the first time on two or more Equalizers running Version 8:
1. Configure appropriate clusters (and servers) on all of the Equalizers to be included as Envoy sites in the
GeoCluster.
2. Configure the GeoCluster on each Equalizer; the parameters used should be the same on all sites.
3. Configure the authoritative DNS server for your website’s domain with DNS records for all Equalizers in the
GeoCluster. The DNS server returns these records to clients in response to DNS requests to resolve the website
(GeoCluster) name.
Overview of Envoy Site Selection
When a client uses DNS to resolve the address of a website name, it first contacts its local DNS server to resolve the
name. The local DNS server then begins the process of resolving the website name by contacting other DNS servers
to locate the authoritative DNS server for the website’s domain. The authoritative DNS server for the website’s
domain returns a list of Envoy sites to the client’s DNS server. Once it has this list, the client’s local DNS server
sends requests, one at a time, to each of the Envoy sites until it reaches an active site. (An overview of this process is
given in the section “Distributing the Geographic Load” on page 26.) The local DNS server then waits for Envoy to
resolve the website name into an IP address and return it.
Once an active Envoy site is reached, Envoy performs the following steps to determine the virtual cluster IP to
return to the client’s DNS:
Note – While it is possible to mix Version 8 and Version 7 Equalizers in the same GeoCluster, we recommend
that you run the same version of Equalizer software on all Equalizers in your GeoCluster. If you must run Version
8 and Version 7 Equalizers in an Envoy configuration, or if you are upgrading an existing Version 7 Envoy
configuration to Version 8, see the section “Upgrading a Version 7 GeoCluster to Version 8” on page 244 for
additional notes.