Server Agent Probes; Agent Probe Process; Enabling And Disabling Server Agents - Coyote Point Systems E350GX Installation And Administration Manual

Server Agent Probes

A server agent is a custom written application that runs on a server and listens on a specific port (default: 1510).
When a connection request is received on that port, the server agent returns an integer value between -1 and 100 that
indicates the relative load on the server (-1 meaning the server should be considered unavailable, 0 meaning very
lightly loaded, and 100 meaning heavily loaded). Server agents can be used with any cluster type, and have an effect
on all load balancing policies except
By default, server agents are disabled on all new clusters. To enable server agents for a cluster, you need to write the
agent, install and run it on each server in the cluster, and then enable server agents for the cluster on Equalizer.

Agent Probe Process

When Equalizer connects to the port on which the server agent is running, it uses the number returned by the agent
in its load balancing calculations, with the
value over other factors.
The number returned by the agent to Equalizer is intended to indicate the current load on the server. The agent
application that runs on the server can be written in any available scripting or programming language and can use
any appropriate method to determine server load. The result must be an integer between -1 and 100 returned on the
.
server agent port
When enabled, server agents should be running on all servers in the cluster; however, by default, a server is not
marked down when an agent value is not returned. Equalizer continues load balancing without the server agent
return value unless the cluster parameter
response or the server is marked down.
Note that server agent probing does not use any of the timeout values defined in the previous sections for High
Level Probes. For example:
• The period of time between server agent probes to a server can be as short as one second. To introduce a
timed delay, introduce a delay into the server agent code (for example, sleeping for 20 seconds). This does
have the disadvantage of leaving the server agent port connection open for at least the length of the delay,
but does reduce the frequency of agent probes.
• The period of time that Equalizer will wait for an agent response before marking it down is determined
internally by Equalizer and cannot be adjusted by the administrator.

Enabling and Disabling Server Agents

Server agents are enabled for a cluster by turning on the
parameter to the default value of port
specified up to every second -- depending on the cluster configuration, system load, and whether or not the server
agent itself introduces a delay.
The cluster parameter specifies an optional string that is sent to the
agent probe
when it open a connection. This is not used by default, but is provided for those agents (such as agents written in
Java) that require input before they reply to the probe. Agents written in C or perl, for example, usually don't require
input in order to return the agent value.
Server agent probing is disabled by setting the
automatically sets the port to 0.
Equalizer Installation and Administration Guide
, which ignores server agent return values.
round robin
policy giving highest preference to the server agent's return
server agent
require agent response
server agent
. A connection to the server agent is opened on the
1510
server agent port
Server Health Check Probes and Timeouts
is enabled; if it is, Equalizer must receive an agent
cluster flag, which sets the
server agent port
parameter to . Disabling the
0
server agent port
server agent port
by Equalizer
flag
server agent
269