add server

Add a dynamic server to a backend.

Description Jump to heading #

Instantiate a new server attached to an existing backend. Such a server may also be called a dynamic server. Dynamic servers refer to servers that do not have an explicit entry within your HAProxy configuration file.

The specified server name must not already be in use in the backend. The backend must be configured to use a dynamic load balancing algorithm for the balance directive: roundrobin, leastconn, first, or random.

A dynamic server is not restored after a load balancer reload operation. If you need the server definition to persist, add it to the load balancer’s configuration file.

Dynamic servers are statically initialized with the none init-addr method. In this method, the server starts in the down state without a valid IP address. After you add a dynamic server, you can enable it with the enable server command.

It may be possible to reach the fd (file descriptor) limit when using a large number of dynamic servers. See also fd-hard-limit and ulimit-n in the global keyword documentation.

The following keywords are supported for configuring the new server:

Settings from the default-server statement, if any, in the backend configuration are not applied to the new server.

Servers added with this command can be deleted using the del server command.

Examples Jump to heading #

To add a server to an existing backend configured with a dynamic load balancing algorithm:

1. Add the server using the add server command. Below, we add the server web2 to backend webservers:

   nix
   echo "add server webservers/web2 172.16.0.12:8080 check port 8080" | \
     sudo socat stdio tcp4-connect:127.0.0.1:9999

   nix
   echo "add server webservers/web2 172.16.0.12:8080 check port 8080" | \
     sudo socat stdio tcp4-connect:127.0.0.1:9999

   output
   text
   New server registered.

   output
   text
   New server registered.

2. Enable the server using the enable server command. The server is in maintenance mode by default.

   nix
   echo "enable server webservers/web2" | sudo socat stdio tcp4-connect:127.0.0.1:9999

   nix
   echo "enable server webservers/web2" | sudo socat stdio tcp4-connect:127.0.0.1:9999

   There is no output for this command.

3. Optional: enable health checks using the enable health command. Health checks are disabled by default. In the example above, we added check to our server definition which allows us to enable health checks using enable health:

   echo "enable health webservers/web2" | sudo socat stdio tcp4-connect:127.0.0.1:9999

   echo "enable health webservers/web2" | sudo socat stdio tcp4-connect:127.0.0.1:9999

   There is no output for this command.

You can use static cookies for session persistence for dynamic servers. To add a dynamic server to your backend with a static cookie, issue the add server command, specifying the name of your cookie:

nix
echo "add server webservers/web2 172.16.0.12:8080 cookie app2" | \
  socat stdio tcp4-connect:127.0.0.1:9999

nix
echo "add server webservers/web2 172.16.0.12:8080 cookie app2" | \
  socat stdio tcp4-connect:127.0.0.1:9999

output
text
New server registered.

output
text
New server registered.

See also Jump to heading #

• enable server
• enable health
• del server
• disable server
• set server