HAproxy configuration as an HTTP router / balancer

HAproxy is a very flexible reliable TCP proxy and balancer middleware.
It can be used as a generic TCP proxy / port mapper or as a TCP load balancer.
It also supports using HTTP protocol mode where it is able to work as an http proxy server and loadbalancer.

In this post i will focus on the http mode, it is used more for implementing web proxies providing high availability for web applications. below is a sample config for HAproxy as an http proxy and router:


#---------------------------------------------------------------------
# Example configuration for a possible web application.  See the
# full configuration options online.
#   http://haproxy.1wt.eu/download/1.3/doc/configuration.txt
#---------------------------------------------------------------------
#---------------------------------------------------------------------
# Global settings
#---------------------------------------------------------------------
global
    log         127.0.0.1 
    maxconn     400000
    user        sherif
    group       sherif
    daemon
#---------------------------------------------------------------------
# common defaults that all the 'listen' and 'backend' sections will
# use if not designated in their block
#---------------------------------------------------------------------
defaults
    mode        http
    log         global
    option      dontlognull
    option      httpclose
    option      httplog
    option      forwardfor
    option      redispatch
    timeout connect 100000 # default 10 second time out if a backend is not found
    timeout client 600000
    timeout server 600000
    maxconn     600000
    retries     3
    errorfile 503 /etc/haproxy/errors/503.http
#---------------------------------------------------------------------
# main frontend which proxys to the backends
#---------------------------------------------------------------------
frontend  main vardamire:8080
acl app1_url                     path_beg     /app1
acl test_url                    path_beg    /tst
acl app2_url                    path_dir    app2
###################################################################
use_backend app1_8080                        if   app1_url
use_backend app2_8081                        if   test_url app2_url   
use_backend default_services
###################################################################
backend app1_8080
    balance     roundrobin
    option httpchk GET /app1/alive
    cookie JSESSIONID prefix nocache
    server      app1    server1:8080 check cookie a1
    server      app2    server2:8080 check cookie a2
    server      app3    server3:8080 check cookie a3 

backend app2_8081
    balance     roundrobin
    option httpchk GET /tst/app2/alive
    server      app1     server2:8081 check
    server      app2     server3:8081 check
 
backend default_services
   server app1 web.toplevelbalancer:8080 check

listen stats *:1936
    stats enable
    stats uri /
    stats hide-version
    stats auth admin:admin

The above config is an example for doing http routing and balancing based on URL patterens.
Also it shows how HAproxy can handle session stickiness.

The keyword acl defines a url pattern using either pathbeg (path begins) or path_dir (path directory portion).
Then the backend keyword is used to define a couple of application backends to HAproxy, those will do the actual serving of the content.
the balance keyword is used to tell HAproxy to do a round robin load balancing between the defined servers, also it adds the option httpchk which will do an http check on the given URI for each defined server to determine if it is up or not.
Also cookie keyward is used to append a part to the JSESSIONID cookie and have it checked by HAproxy to be able to maintain session stickiness. HAproxy will prefix JSESSIONID with the cookie defined in each server and thus will be able to keep track which session goes to which server.

Lastly we are enabling HAproxy statistics so that we can monitor the status of our backends and also the stats of the requests coming to them.

This config was used successfully to route and balance 40+ services for a big project and it working fairly smooth even under load testing.

HAproxy is very light weight and can handle 10s of thousands of connections without issues.