Coyote Point Systems E350GX, Installation And Administration Manual

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Equalizer Installation and Administration Guide 255
Appendix A:
Server Agent Probes
Enabling Agents ..............................................................................................................................255
Server Agents and Load Balancing Policies ...................................................................................256
Server Agents and Server ‘Down’ Conditions .................................................................................256
Sample Server Agent in Perl ...........................................................................................................256
A server agent is a custom written program that runs on a server and provides direct feedback to Equalizer that is
used by the load balancing algorithms. This feedback is ontained by the agent by any means available on the target
server; the only requirement from Equalizer’s point of view is that the agent response is in the form of an integer
between -2 and 100 that represents the status of the server and/or the application that is running on it.
After returning a value to Equalizer, the agent should close the port and wait for another connection.
You configure server agents on a cluster-wide basis—all the servers in a virtual cluster must be running agents for
server agents to be used for adaptive load balancing. When you have enabled server agents, 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.
Enabling Agents
Agents are enabled for a cluster by turning on the
server agent
cluster flag. The default
agent port
is
1510
. Make
sure that any agent you deploy is listening and able to respond to TCP connections on the same port number on all
the servers in the cluster.
The time between server agent probes is determined by the
agent delay
global parameter (default is 10 seconds).
Equalizer will open up a connection to the server agent’s IP/port, and wait for a response. If no response is received,
then the Equalizer performs load balancing without the server agent value for that server.
Some agents, particularly those written in Java, may require that a string be sent to the agent before a response is
sent back to Equalizer. The
agent probe
field is provided for this purpose. If a string appears in this field, it is sent
to the agent when an agent probe occurs.
100 to 0
100 indicates that the server and/or application is lightly loaded.
0 indicates that the server and/or application is heavily loaded.
-1 The application, or a required resource (such as a database), is unavailable.
-2
The server agent cannot determine the status of the application. This is the
default return value used by Equalizer when an agent does not respond.
Note
: Server agent code written prior to Version 8.1.0a must be adjusted to reflect the server
agent return values and interpretations shown above. In particular, in previous releases the
meanings of the 0 to 100 range of values were documented as the reverse of the meanings
shown above.