Web OS 10.0 Application Guide246 n Chapter 10: Health Checking 212777-A, February 2002Failure TypesService FailureIf a certain number of connection requests for a particular service fail, the session switchplaces the service into the service failed state. While in this state, no new connection requestsare sent to the server for this service; however, if graceful real server failure is enabled(/cfg/slb/adv/grace ena), state information about existing sessions is maintained andtraffic associated with existing sessions continues to be sent to the server. Connection requeststo, and traffic associated with, other load-balanced services continue to be processed by theserver.Example: A real server is configured to support HTTP and FTP within two real server groups.If a session switch detects an HTTP service failure on the real server, it removes that realserver group from the load-balancing algorithm for HTTP but keeps the real server in the mixfor FTP. Removing only the failed service from load balancing allows users access to allhealthy servers supporting a given service.When a service on a server is in the service failed state, the session switch sends Layer 4connection requests for the failed service to the server. When the session switch hassuccessfully established a connection to the failed service, the service is restored to the load-balancing algorithm.Server FailureIf all load-balanced services supported on a server fail to respond to switch connection requestswithin the specified number of attempts, then the server is placed in the server failed state.While in this state, no new connection requests are sent to the server; however, if graceful realserver failure is enabled (/cfg/slb/adv/grace ena), state information about existingsessions is maintained and traffic associated with existing sessions continues to be sent to theserver.N OTE – All load-balanced services on a server must fail before the switch places the server inthe server failed state.The server is brought back into service as soon as the first service is proven to be healthy.Additional services are brought online as they are subsequently proven to be healthy.
