HP e-Commerce / XML director server appliance sa8250 - Users Guide

C H A P T E R 2 Error Detection

Error

Detection

The SA8250 is capable of recognizing and reacting to server error

conditions, detecting non-responsive (comatose) servers, and

directing traffic to alternate resources until the server is back in

operation. The SA8250 can also capture many HTTP errors before

they reach the client, and redirect the request to an alternate server.

Server Status Detection

The SA8250 uses multiple means to monitor the status of the

fulfillment servers. The Intelligent Resource Verification (IRV)

module periodically pings the servers to verify that they are alive. The

SA8250 also monitors a “dup-syn” interval to calculate packet loss

rate.

Intelligent Resource Verification

When the IRV module pings a server and receives no response, it tries

to connect to each port on which the suspect server is configured to

listen. If the SA8250 itself does not receive a response from a given

port, then that server/port combination is declared dead. If the server

maintains network connectivity and responds positively to IRV

pings, but its ports stop responding, then the dup-syn interval

threshold (described below) is used to decide if the server is declared

dead.

Dup-syn Interval

The SA8250 dynamically calculates the threshold for the acceptable

number of dropped packets within a given interval. If at any time in

this interval the number of dropped packets exceeds this threshold,

the server is considered dead. After the specified time value has

expired the lost packet (or dup-syn) count is divided by two and the

time interval starts again. In this way, some history information is

kept between time intervals.

The dup-syn interval for this threshold is established with the

dup-

syn

CLI command, and ranges in value from 1000 to 2,147,483,647

microseconds. The default time interval value is 500,000

microseconds (one half second), which is appropriate for most

environments. By lowering or raising this value, you render the

SA8250 respectively less or more sensitive to dropped packets, and

less or more likely to declare a server dead. The volume of network

traffic must be taken into account when setting the dup-syn interval.

Higher volumes of traffic require a shorter dup-syn interval to avoid

mistakenly declaring a server dead due to network congestion.