HAProxy For Load Balancing and Subdomain-Port Redirection

HAProxy is a high performance TCP/HTTP load balancer that can also be used as reverse-proxy. I used HAProxy for load balancing multiple web servers and managing different web applications serving under the same domain with different subdomains and ports. For an introduction to load balancing concept, you can check Digital Ocean's tutorial.

I will explain how to install and configure HAProxy on an Ubuntu Server. To install HAProxy, simply use apt-get command:

apt-get install haproxy

To be able to start HAProxy with an init script, simply edit the file /etc/default/haproxy with your favorite editor and add the following line

ENABLED=1

After saving the file, please check that you get the following output once you start the init script.

service haproxy
Usage: /etc/init.d/haproxy {start|stop|reload|restart|status}

Now we are good to go for configuring HAProxy. Open a config file under the path /etc/haproxy/haproxy.cfg and add the following defaults. These are the basic settings and can be changed according to different purposes.

global
    log 127.0.0.1 local0 notice
    maxconn 2000
    user haproxy
    group haproxy

defaults
    log     global
    mode    http
    option  httplog
    option  dontlognull
    retries 3
    option redispatch
    timeout connect  5000
    timeout client  10000
    timeout server  10000

Now, let's assume that we have two web applications, one called alpha and another called beta, both serving under the same domain, which is mydomain.com. And let beta have two web servers working under two different ports. For the sake of simplicity, the applications are assumed to be working on the same host which HAProxy is working.

alpha
127.0.0.1:8000

beta_1
127.0.0.1:8080

beta_2
127.0.0.1:8081

We will continue adding new blocks to the config file in /etc/haproxy/haproxy.cfg. The first block will be a frontend block which defines all the domain and port related configurations.

frontend http-in
    bind *:80
    acl sub1 hdr_sub(host) -i alpha.mydomain.com
    acl sub2 hdr_sub(host) -i beta.mydomain.com

    use_backend alpha_backend if sub1
    use_backend beta_backend if sub2

Here we binded all the connections coming through port 80. And we defined alpha subdomain as sub1 and beta subdomain as sub2 which will be directed to corresponding backends alpha_backend and beta_backend. All these names are free to be configured as desired. Let's continue adding backend blocks to the config file in /etc/haproxy/haproxy.cfg.

backend alpha_backend
    mode http
    option forwardfor
    server alpha_server 127.0.0.1:8000

backend beta_backend
    mode http
    balance roundrobin
    option httpclose
    option forwardfor
    server beta_server_1 127.0.0.1:8080 check
    server beta_server_2 127.0.0.1:8081 check

With this configuration, any request on alpha.mydomain.com on default HTTP port (80) will be redirected to alpha_backend which serves under the port 8000. Also, any request on beta.mydomain.com on port 80 will be redirected to beta_backend which serves under the ports 8080 and 8081 using the algorithm called roundrobin where each server is used in turns. To see other balance options, you can check HAProxy Configuration Manual. To see if load balancing works, you can kill one of your beta servers and check that beta.mydomain.com is still up an running.