Alteon Application Switch Operating System Application Guide
Health Checking
504
Document
ID:
RDWR-ALOS-V2900_AG1302
Example
A real server is configured to support HTTP and FTP within two real server groups. If a session
device detects an HTTP service failure on the real server, it removes that real server group from the
load-balancing algorithm for HTTP, but keeps the real server in the mix for FTP. Removing only the
failed service from load balancing allows users access to all healthy servers supporting a given
service.
When a service on a server is in the service failed state, the Alteon sends Layer 4 connection
requests for the failed service to the server. When Alteon has successfully established a connection
to the failed service, the service is restored to the load-balancing algorithm.
Server Failure
If all load-balanced services supported on a server fail to respond to connection requests within 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 real server failure is
enabled (using
/cfg/slb/adv/grace ena
), state information about existing sessions is
maintained and traffic associated with existing sessions continues to be sent to the server.
All load-balanced services on a server must fail before Alteon places the server in the 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.
Preventing a Flood of Server Connections
Alteon performs a slow start on the real server which comes back after the health check fails to
prevent flood of connections. This occurs for groups configured with leastconns as the metric. When
the real server comes up, it contains zero connections and all the new connections are directed to it.
This heavy connection flow brings down the real server.
To prevent this flood, Alteon temporarily changes the group metric to round-robin for one minute
and reverts back to leastconns metric, assuming the next real server which comes up is ready to
accept new connections.
By default the slow start feature is disabled.
To check the slow start mode of a real server in a group
Enter the following command:
The following information displays:
>> Main# /info/slb
>> Server Load Balancing Information# virt 1
1: 10.10.7.1, 00:01:81:2e:a0:8e
virtual ports:
http: rport http, group 1, backup none, slowstart real servers:
1: 192.168.2.11, backup none, 0 ms, group ena, up, dynamic weight 20
2: 192.168.2.12, backup none, 0 ms, group ena, up, dynamic weight 40
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