Setting Up A High-Availability Load Balancer (With Failover and Session Support) With HAProxy/Keepalived On Debian Etch
Version 1.0
Author: Falko Timme
This article explains how to set up a two-node load balancer in an active/passive configuration with HAProxy and keepalived 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 keepalived, 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)
keepalived keepalived
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. Here in Germany, Hetzner is a hoster that allows you to do this - just talk to them. Update: Hetzner's policies have changed - please read here for more details: https://www.howtoforge.com/forums/showthread.php?t=19988
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 demonstrate the session-awareness of HAProxy, I'm assuming that the web application that is installed on http1 and http2 uses the session id JSESSIONID.
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
... upgrade the installed packages:
apt-get upgrade
... and install HAProxy:
apt-get install haproxy
