Advisors
Advisors are lightweight clients that run inside the Dispatcher, providing information about the load of a given server. The product provides protocol-specific advisors for most popular protocols (HTTP, HTTPS, SSL, FTP, Ping, DB2, POP3, DNS, Telnet, and others).
Standard advisors send transactions periodically to determine the status of the servers (for example, for HTTP an HTTP HEAD request is sent, and for FTP, a SYST). If the transaction succeeds, the server is considered up.
To obtain the load value, the advisor:
- Pings and opens a connection with each server.
- Sends a protocol-specific request message.
- Listens for a response from the server.
- Calculates the load value.
After getting the response, the advisor makes an assessment of the server. To calculate this "load" value, most advisors measure the time for the server to respond, and then use this value (in milliseconds) as the load.
You may also set the connecttimeout and receivetimeout. The connecttimeout is the amount of time the advisor will wait before aborting the connection and the receivetimeout is the amount of time the advisor will wait before giving up on the data over the socket. The combined time in most cases is the advisor load.
- Reports the load value to the manager function.
If the server does not respond, the advisor returns a negative value (-1) for the load.
The manager function obtains the load value reported by the advisor, which is available in the manager report, in the Port column. The manager obtains these values from all of its sources and sets proportional weight values for the Executor function.
A down server is given a weight of zero, and packets will not be forwarded to it until the server responds to the advisor again.
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