Coyote Point Systems E350GX, Руководство по установке и администрированию

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Server Health Check Probes and Timeouts
Equalizer Installation and Administration Guide 269
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
round robin
, which ignores server agent return values.
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
server agent
policy giving highest preference to the server agent’s return
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
require agent response
is enabled; if it is, Equalizer must receive an agent
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
server agent
cluster flag, which sets the
server agent port
parameter to the default value of port
1510
. A connection to the server agent is opened on the
server agent 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
agent probe
cluster parameter specifies an optional string that is sent to the
server agent port
by Equalizer
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
server agent port
parameter to
0
. Disabling the
server agent
flag
automatically sets the port to 0.