Coyote Point Systems Equalizer, Installation And Administration Manual

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Appendix A:Using Server Agents
100 Equalizer Installation and Administration Guide
Introducing Server Agents
Equalizer's load balancing algorithms can be configured to accept direct feedback from
servers that describes the current server load or availability of critical resources. To use
server agents, agent software must be installed on each of the servers in a cluster. These
agents must be able to respond to TCP connections on a well-known port. This response is
in the form of an ASCII string that represents the current load on the server or indicates
that the service is unavailable. If an agent indicates that the service is unavailable,
Equalizer will automatically stop sending requests to that server.
Server Agents are configured on a cluster-wide basis—all of the servers in a virtual cluster
must be running agents for server agents to be used for adaptive load balancing. When
server agents are enabled, Equalizer periodically probes the agent at each server's IP
address through the configured agent port. Equalizer uses the collected server agent
values when performing adaptive load balancing calculations. You configure Equalizer to
use server agents through the Change Cluster dialog. (For more information, see
“Configuring a Cluster to Use Server Agents” on page 68.)
Custom Server Agents
Custom agents can be written as shell scripts, or in Java, C, or other languages. Upon
request, Coyote Point can provide sample agents written in C that you can modify for your
specific needs.
The agent protocol is extremely simple: when Equalizer connects to the agent’s port, the
agent must respond with an ASCII string and close the port. The ASCII string is a number
that represents the current condition of the server:
-1 Service is unavailable. Might also be used to indicate that a required resource, such as a database, is
unavailable.
0 to 100 0 indicates that the server is very lightly loaded; 100 indicates that the server is very overloaded.