HTTP-ECV Monitor Probes sent by the NetScaler Appliance to the Server are Failing

HTTP-ECV monitor probes sent by the NetScaler appliance to the server are failing, even though you can access the Web page successfully on the Web server.

Background

A monitor of type HTTP-ECV might be configured to check that the response to a specific HTTP request from the NetScaler appliance to the Web server contains a response containing particular data. When the NetScaler appliance receives response containing the particular data, it deduces that the site is UP. This might be required in scenarios where simpler monitoring does not provide an adequate guarantee that the server is actually functioning sufficiently to serve the requests. For example, a monitor might be defined to probe a Web server that is a front end to a database, and the HTTP response should be checked to ensure that the Web server is querying the database correctly.

The HTTP-ECV monitor uses the parameters

send [""] - HTTP data is sent to the service
recv [""] - the expected HTTP response data from the service

As an example, you can define a custom HTTP-ECV monitor as follows:

> add monitor <name> http-ecv –send “GET /siteup.html” –recv “site_is_up”

This monitor causes a NetScaler appliance to probe the backend HTTP service that is bound to a monitor with a HTTP GET request for the file %wwwroot%/siteup.html. The appliance expects to receive a response containing the string “site_is_up”.

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1. Find the string expected by the monitor (as specified in the recv parameter) within the first 24 KB of the body of the HTTP response, otherwise the probe fails.

2. To determine if the required string exists within the first 24 KB of the response, use an Ethereal or Wireshark network trace. If it is not found in the first 24 KB of the body of the HTTP response from the server, then the HTTP-ECV monitor fails, but you can access the web page successfully.

This behavior is by design. Unnecessary processing overhead would be introduced by expecting the NetScaler appliance to search an arbitrary length HTTP response from the server.