Backends

A backend section defines a pool of servers to which the load balancer will route requests.

The following video is an overview of backends in an HAProxy configuration file and showcases several load balancing algorithms.

You can add as many backend sections as needed. Each backend keyword is followed by a label, such as web_servers, to differentiate it from others.

haproxy
backend web_servers
  mode http
  balance roundrobin
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check
  server s3 192.168.1.27:80 check

haproxy
backend web_servers
  mode http
  balance roundrobin
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check
  server s3 192.168.1.27:80 check

The label is mostly for readability, but it does come into play when referencing stick tables and categorizing traffic metrics. It should consist of only upper or lowercase letters, digits, dashes, underscores, dots, and colons.

Backend configuration example Jump to heading #

The following example shows a pool of servers that are defined within the backend named web_servers. The server keyword identifies each server’s IP address and port, which the load balancer will use to send traffic to that server. The basic syntax of a server line includes a unique name, an IP address, and a port.

haproxy
frontend myfrontend
  mode http
  bind :80
  default_backend web_servers
backend web_servers
  mode http
  balance roundrobin
  server myfirstserver  192.168.1.25:80 check
  server mysecondserver 192.168.1.26:80 check
  server mythirdserver  192.168.1.27:80 check

haproxy
frontend myfrontend
  mode http
  bind :80
  default_backend web_servers
backend web_servers
  mode http
  balance roundrobin
  server myfirstserver  192.168.1.25:80 check
  server mysecondserver 192.168.1.26:80 check
  server mythirdserver  192.168.1.27:80 check

Define multiple backends Jump to heading #

You can add multiple backend sections to service traffic for multiple websites or applications. In the configuration sample below, frontend foo_and_bar listens for all incoming HTTP requests and uses the use_backend directive to route traffic to either foo_servers or bar_servers, depending on the host HTTP header.

haproxy
frontend foo_and_bar
  mode http
  bind :80
  use_backend foo_servers if { req.hdr(host) -i foo.com }
  use_backend bar_servers if { req.hdr(host) -i bar.com  }
backend foo_servers
  mode http
  balance roundrobin
  server foo1 192.168.1.25:80 check
  server foo2 192.168.1.26:80 check
  server foo3 192.168.1.27:80 check
backend bar_servers
  mode http
  balance roundrobin
  server bar1 192.168.1.35:80 check
  server bar2 192.168.1.36:80 check
  server bar3 192.168.1.37:80 check

haproxy
frontend foo_and_bar
  mode http
  bind :80
  use_backend foo_servers if { req.hdr(host) -i foo.com }
  use_backend bar_servers if { req.hdr(host) -i bar.com  }
backend foo_servers
  mode http
  balance roundrobin
  server foo1 192.168.1.25:80 check
  server foo2 192.168.1.26:80 check
  server foo3 192.168.1.27:80 check
backend bar_servers
  mode http
  balance roundrobin
  server bar1 192.168.1.35:80 check
  server bar2 192.168.1.36:80 check
  server bar3 192.168.1.37:80 check

Change the load balancing algorithm Jump to heading #

By default, servers in the backend share the work of handling requests by using round-robin load-balancing. In the next example, we have set the balance directive to leastconn, which will send traffic to the server with the fewest number of connections.

haproxy
backend web_servers
  mode http
  balance leastconn
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check
  server s3 192.168.1.27:80 check

haproxy
backend web_servers
  mode http
  balance leastconn
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check
  server s3 192.168.1.27:80 check

Other load balancing algorithms include hash, source, first, random, and more. See all the available load-balancing algorithms in HAProxy Configuration Manual’s balance.

Enable health checks Jump to heading #

Adding the check argument to each server line is a good practice; this enables health checking to remove unhealthy servers that don’t respond to a TCP connection from the load balancing rotation.

haproxy
backend web_servers
  mode http
  balance roundrobin
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check
  server s3 192.168.1.27:80 check

haproxy
backend web_servers
  mode http
  balance roundrobin
  server s1 192.168.1.25:80 check
  server s2 192.168.1.26:80 check
  server s3 192.168.1.27:80 check

See also Jump to heading #

For complete information on these directives that can be used in backends, see the HAProxy Configuration Manual:

• To specify the type of traffic being processed, see mode.
• For complete information on server directive syntax and options, see server.
• To direct traffic to a specific backend, see use_backend.