Coyote Point Systems Equalizer, Installation And Administration Manual

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Chapter 1:Overview
4 Equalizer Installation and Administration Guide
Request Information
In addition to cluster and server characteristics, Equalizer can take into account
characteristics of the request itself when determining how to route a request. Equalizer’s
support for Layer 7 content-sensitive load balancing enables administrators to define rules
for routing HTTP and HTTPS requests according to the content of the request. The Envoy
add-on to Equalizer supports geographic load balancing, which enables requests to be
automatically distributed across servers in different physical locations. In addition to other
load-balancing criteria, geographic load balancing can take into account which server
location is closest to the client (in network terms) and offers the least network latency.
For more information about Layer 7 load balancing with Equalizer, see “Layer 7 Load
Balancing” on page 5. For more information about geographic load balancing with Envoy,
see “Geographic Load Balancing” on page 6.
Load Balancing UDP Services
You can configure Equalizer Virtual Clusters to provide load balancing and server failure
detection for many UDP (User Datagram Protocol) based services.
UDP load balancing is ideal for stateless protocols such as DNS and RADIUS. It can be
used to load balance WAP (Wireless Application Protocol) gateways, and can even be used
for certain types of NFS server cluster that provide a single-system image.
Active Content Verification is not supported for UDP clusters.
Maintaining Persistent Sessions
Maintaining persistent sessions is useful when state information is shared between the
client and server. For example, when using the SSL protocol, the client and server perform
an expensive session establishment procedure that must be repeated whenever the client
connects to a new server. Maintaining persistent sessions minimizes the number of times
that a client must reconnect to a new server during a session. Equalizer supports two
mechanisms for maintaining persistent sessions: Cookie-based persistence, and IP-address
based persistence.
Cookie-Based Persistence
Cookie-based persistence can be used for HTTP and HTTPS clusters that support Layer 7
load balancing. When cookie-based persistence is used, Equalizer “stuffs” a cookie into the
server’s response header on its way back to the client. This cookie uniquely identifies the
server to which the client was connected and is included in subsequent requests from the
client to the same cluster. Equalizer can use the information in the cookie to route the
requests back to the same server.
Requests from a particular client can be directed to the same server even if the connection
is to a different virtual cluster. For example, if a user switches from an HTTP cluster to an
HTTPS cluster, the persistence cookie will still be valid if the HTTPS cluster contains a
server with the same IP address.
If the server with which a client has a persistent sessions is unavailable, a different server
is selected automatically. The client will have to establish a new session and a new cookie
will be stuffed in the next response.