Coyote Point Systems Equalizer, Installation And Administration Manual

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Layer 7 Load Balancing
Equalizer Installation and Administration Guide 5
IP-Address Based Persistence
For generic TCP and UDP clusters that support Layer 4 load balancing, Equalizer supports
IP-Address based persistent sessions. When Equalizer’s sticky connections feature is
enabled, Equalizer identifies clients by their IP addresses when they connect to a cluster.
Requests received from a particular client during a specified period of time are routed to
the same server in the cluster.
A sticky timer measures the amount of time that has passed since there was a connection
from a particular IP address to a specific cluster. The sticky time period begins to expire as
soon as their are no longer any active connections between the client and the selected
cluster. The timer is reset to zero whenever a new connection occurs. If the client does not
establish any new connections to the same cluster, the timer continues to run until the
sticky time period expires. Once the sticky time period expires, a new connection from that
client will be handled like any other incoming connection and routed to an available server
based on the selected load-balancing criteria.
To correctly handle sticky connections from ISPs who use multiple proxy servers to direct
user connections, Equalizer supports sticky network aggregation. When sticky network
aggregation is enabled, only the network portion of a client's IP address is used to
maintain a sticky connection. This way, the user is directed to the same server no matter
which proxy they connect through.
Equalizer can also be configured to ensure that requests from a particular client are
directed to the same server even if the connection is to a different virtual cluster. When
inter-cluster stickiness is enabled for a cluster, Equalizer checks the cluster for a sticky
record as each connection request is received, just like it does for ordinary sticky
connections. If no sticky record is found, Equalizer proceeds to check all of the other
clusters that have the same IP address. If a sticky record is found, the user is connected to
the server that it references.
Layer 7 Load Balancing
Traditional Layer 4 load balancing is an efficient method of distributing traffic across a
cluster of servers when any of the servers in the cluster are equally capable of processing
the requests. Layer 4 refers to the fourth tier in the OSI network model: the transport layer.
When Layer 4 load balancing is used, requests are routed to the servers in a cluster based
on information held in the TCP or UDP layers of the Internet protocol stack.
Layer 7 load balancing is an advanced technology that enables requests to be routed based
on information from the application layer. This provides access to the actual data payloads
of the TCP/UDP packets exchanged between a client and server. For example, by
examining the payloads, load balancing decisions for HTTP requests can be based on
information in client request headers and methods, server response headers, and page
data.
Equalizer’s Layer 7 load balancing support allows administrators to define rules for
routing HTTP and HTTPS requests according to the request content. For example, Layer 7
rules can be used to specify routing preferences such as, “Send all requests for graphics
files to servers A, B and E; send all requests for Perl scripts to servers C and D; and send all
other requests to server Z.” This enables administrators to create extremely flexible cluster
configurations. Layer 7 technology can also be used to implement client-server persistence
based on HTTP cookies.