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High-Availability Load Balancer (With Failover and Session Support) With HAProxy/Wackamole/Spread On Debian Etch

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This article explains how to set up a two-node load balancer in an active/passive configuration with HAProxy, Wackamole, and Spread on Debian Etch. The load balancer sits between the user and two (or more) backend Apache web servers that hold the same content. Not only does the load balancer distribute the requests to the two backend Apache servers, it also checks the health of the backend servers. If one of them is down, all requests will automatically be redirected to the remaining backend server. In addition to that, the two load balancer nodes monitor each other using Wackamole and Spread, and if the master fails, the slave becomes the master, which means the users will not notice any disruption of the service. HAProxy is session-aware, which means you can use it with any web application that makes use of sessions (such as forums, shopping carts, etc.).
From the HAProxy web site: "HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for web sites crawling under very high loads while needing persistence or Layer7 processing. Supporting tens of thousands of connections is clearly realistic with todays hardware. Its mode of operation makes its integration into existing architectures very easy and riskless, while still offering the possibility not to expose fragile web servers to the Net."
I do not issue any guarantee that this will work for you!

1 Preliminary Note

In this tutorial I will use the following hosts:
  • Load Balancer 1: lb1.example.com, IP address: 192.168.0.100
  • Load Balancer 2: lb2.example.com, IP address: 192.168.0.101
  • Web Server 1: http1.example.com, IP address: 192.168.0.102
  • Web Server 2: http2.example.com, IP address: 192.168.0.103
  • We also need a virtual IP address that floats between lb1 and lb2: 192.168.0.99
Here's a little diagram that shows our setup:
    shared IP=192.168.0.99
 192.168.0.100  192.168.0.101 192.168.0.102 192.168.0.103
 -------+------------+--------------+-----------+----------
        |            |              |           |
     +--+--+      +--+--+      +----+----+ +----+----+
     | lb1 |      | lb2 |      |  http1  | |  http2  |
     +-----+      +-----+      +---------+ +---------+
     haproxy      haproxy      2 web servers (Apache)
     Wackamole    Wackamole
     Spread       Spread
The shared (virtual) IP address is no problem as long as you're in your own LAN where you can assign IP addresses as you like. However, if you want to use this setup with public IP addresses, you need to find a hoster where you can rent two servers (the load balancer nodes) in the same subnet; you can then use a free IP address in this subnet for the virtual IP address.
http1 and http2 are standard Debian Etch Apache setups with the document root /var/www (the configuration of this default vhost is stored in /etc/apache2/sites-available/default). If your document root differs, you might have to adjust this guide a bit.
To make HAProxy session-aware, I'm assuming that the web application that is installed on http1 and http2 uses the session idJSESSIONID.

2 Preparing The Backend Web Servers

We will configure HAProxy as a transparent proxy, i.e., it will pass on the original user's IP address in a field called X-Forwarded-For to the backend web servers. Of course, the backend web servers should log the original user's IP address in their access logs instead of the IP addresses of our load balancers. Therefore we must modify the LogFormat line in /etc/apache2/apache2.conf and replace %h with %{X-Forwarded-For}i:
http1/http2:
vi /etc/apache2/apache2.conf
[...]
#LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
LogFormat "%{X-Forwarded-For}i %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
[...]
Also, we will configure HAProxy to check the backend servers' health by continuously requesting the file check.txt (translates to/var/www/check.txt if /var/www is your document root) from the backend servers. Of course, these requests would totally bloat the access logs and mess up your page view statistics (if you use a tool like Webalizer or AWstats that generates statistics based on the access logs).
Therefore we open our vhost configuration (in this example it's in /etc/apache2/sites-available/default) and put these two lines into it (comment out all other CustomLog directives in your vhost configuration):
vi /etc/apache2/sites-available/default
[...]
SetEnvIf Request_URI "^/check\.txt$" dontlog
CustomLog /var/log/apache2/access.log combined env=!dontlog
[...]
This configuration prevents that requests to check.txt get logged in Apache's access log.
Afterwards we restart Apache:
/etc/init.d/apache2 restart
... and create the file check.txt (this can be an empty file):
touch /var/www/check.txt
We are finished already with the backend servers; the rest of the configuration happens on the two load balancer nodes.

3 Installing HAProxy

Unfortunately HAProxy is available as a Debian package for Debian Lenny (testing) and Sid (unstable), but not for Etch. Therefore we will install the HAProxy package from Lenny. To do this, open /etc/apt/sources.list and add the line deb http://ftp2.de.debian.org/debian/ lenny main; your /etc/apt/sources.list could then look like this:
lb1/lb2:
vi /etc/apt/sources.list
deb http://ftp2.de.debian.org/debian/ etch main
deb-src http://ftp2.de.debian.org/debian/ etch main

deb http://ftp2.de.debian.org/debian/ lenny main

deb http://security.debian.org/ etch/updates main contrib
deb-src http://security.debian.org/ etch/updates main contrib
Of course (in order not to mess up our system), we want to install packages from Lenny only if there's no appropriate package from Etch - if there are packages from Etch and Lenny, we want to install the one from Etch. To do this, we give packages from Etch a higher priority in/etc/apt/preferences:
vi /etc/apt/preferences
Package: *
Pin: release a=etch
Pin-Priority: 700

Package: *
Pin: release a=lenny
Pin-Priority: 650
(The terms etch and lenny refer to the appropriate terms in /etc/apt/sources.list; if you're using stable and testing there, you must use stable and testing instead of etch and lenny in /etc/apt/preferences as well.)
Afterwards, we update our packages database:
apt-get update
(If you get an error like this one:
E: Dynamic MMap ran out of room
then open /etc/apt/apt.conf...
vi /etc/apt/apt.conf
... and add a line for APT::Cache-Limit with a very high value, e.g. like this:
APT::Cache-Limit "100000000";
Then run
apt-get update
again.)
Upgrade the installed packages:
apt-get upgrade
... and install HAProxy:
apt-get install haproxy

4 Configuring The Load Balancers

The HAProxy configuration is stored in /etc/haproxy/haproxy.cfg and is pretty straight-forward. I won't explain all the directives here; to learn more about all options, please read http://haproxy.1wt.eu/download/1.3/doc/haproxy-en.txt andhttp://haproxy.1wt.eu/download/1.2/doc/architecture.txt.
We back up the original /etc/haproxy/haproxy.cfg and create a new one like this:
lb1/lb2:
cp /etc/haproxy/haproxy.cfg /etc/haproxy/haproxy.cfg_orig
cat /dev/null > /etc/haproxy/haproxy.cfg
vi /etc/haproxy/haproxy.cfg
global
        log 127.0.0.1   local0
        log 127.0.0.1   local1 notice
        #log loghost    local0 info
        maxconn 4096
        #debug
        #quiet
        user haproxy
        group haproxy

defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull
        retries 3
        redispatch
        maxconn 2000
        contimeout      5000
        clitimeout      50000
        srvtimeout      50000

listen webfarm 192.168.0.99:80
       mode http
       stats enable
       stats auth someuser:somepassword
       balance roundrobin
       cookie JSESSIONID prefix
       option httpclose
       option forwardfor
       option httpchk HEAD /check.txt HTTP/1.0
       server webA 192.168.0.102:80 cookie A check
       server webB 192.168.0.103:80 cookie B check
Afterwards, we set ENABLED to 1 in /etc/default/haproxy:
vi /etc/default/haproxy
# Set ENABLED to 1 if you want the init script to start haproxy.
ENABLED=1
# Add extra flags here.
#EXTRAOPTS="-de -m 16"

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