Coyote Point Systems Equalizer Installation And Administration Manual Download

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Chapter 5: Administering Virtual Clusters

Equalizer Installation and Administration Guide

Using Active Content Verification

Active Content Verification (ACV) is a mechanism for checking the validity of a server.
When ACV is enabled for a cluster, Equalizer requests data from each server in the cluster
and verifies that the returned data contains a character string that indicates that the data is
valid. ACV can be used with most network services that support a text-based request/
response protocol, such as HTTP. However, ACV cannot be used with UDP-based services.

You control what information is used to verify the servers by specifying an ACV Probe
String and an ACV Response string. Equalizer uses the probe string to request data from
each server. To verify the server’s content, Equalizer searches the returned data for the
response string. By default, Equalizer expects to receive a response within .2 seconds when
performing active content verification. If no response is received or the response string is
not found in the first 1024 characters of the response, the verification fails and Equalizer
will stop routing new requests to that server. However, if cookie-based persistence is being
used for a HTTP or HTTPS cluster, requests from cookie holders will continue to be routed
to the server until it’s weight goes to zero.

For example, the HTTP protocol enables you to establish a connection to a server, request a
file, and read the result. Figure 51 illustrates the connection process when telnet is used to
connect to an HTTP server and request an HTML page.

Figure 51 Retrieving content from a server via telnet.

Equalizer can perform the same exchange automatically and verify the server’s response
by checking the returned data against an expected result.

You control what information is used to perform the verification by specifying an ACV
Probe String and an ACV Response string. Equalizer uses the probe string to request data
from each server. To verify the server’s content, Equalizer searches the returned data for
the response string.

For example, “GET /index.html” could be used as the ACV probe string and the response
string set to some text that appears on the home page, such as “Welcome”. The response
string should be a string that only appears in a valid response. For example, most web
servers automatically generate error pages that contain valid HTML, so using a response
string of “HTML” wouldn’t be a good verification of the content.

Enabling Active Content Verification

To enable active content verification:

Log into the Equalizer Administration interface with edit access.

In the left frame, click the name of the cluster you want to configure. The Cluster
Parameters are displayed in the right frame.

Select Change Cluster Parameters from the local menu in the Cluster Parameters
frame. The Change Cluster dialog is displayed in the right frame.

> telnet www.myserver.com 80

Connected to www.myserver.com

> GET /index.html

<HTML>

<TITLE>Welcome to our Home Page</Title>

</HTML>

Connection closed by foreign host

User requests telnet connection to server.
Telnet program indicates connection is established.
User sends request for HTML page to server.
Server responds with requested page.

Telnet program indicates server closed connection.