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5.5. Backend Settings

The backend settings specify the real server IP addresses as well as the load balancer scheduling algorithm. The following example shows a typical backend section:

backend app
   balance     roundrobin
   server  app1 check
   server  app2 check
   server  app3 check inter 2s rise 4 fall 3
   server  app4 backup
The back-end server is named app. The balance specifies the load balancer scheduling algorithm to be used, which in this case is Round Robin (roundrobin), but can be any scheduler supported by HAProxy. For more information configuring schedulers in HAProxy, see Section 5.1, “HAProxy Scheduling Algorithms”.
The server lines specify the servers available in the back end. app1 to app4 are the names assigned internally to each real server. Log files will specify server messages by name. The address is the assigned IP address. The value after the colon in the IP address is the port number to which the connection occurs on the particular server. The check option flags a server for periodic healthchecks to ensure that it is available and able receive and send data and take session requests. Server app3 also configures the healthcheck interval to two seconds (inter 2s), the amount of checks app3 has to pass to determine if the server is considered healthy (rise 4), and the number of times a server consecutively fails a check before it is considered failed (fall 3).