Apache 2 Load Balancing

Pros:
* Protocol independent: supports web, ftp, etc.

Cons:
* Session not sticky
* Fail over only. No load sharing

* Set primary web server as primary name server and TTL to long (e.g. 30 min) so it can be cached.
* Set secondary web server as secondary name server.
* Works only when whole machine goes down so both DNS and Web servers go down.

* Need BIND 4.9 and above.
* Setup round robin configuration:

www.my.com.   300   IN  A       192.168.100.1
www.my.com.   300   IN  A       192.168.100.2
www.my.com.   300   IN  A       192.168.100.3

* Set TTL short so it won’t be cached (e.g. 5 min or 300 sec).

* Each server has its own IP. A floating or virtual IP is assigned to primary server normally and backup web during failover.
* No need for DNS servers.

Linux Virtual Server

Ultra Monkey

* Servers involved must all be in the same local network with no intermediate routers involved.

Super Sparrow

* Allows distributed clustering.

Apache Load Balancing

* Set up one Apache as dedicated proxy server

# Apache Server Configuration for Clustering Proxy
#
### Basic Server Setup

# The proxy takes the identity of the web site...
ServerName               www.alpha-complex.com

# Basic configuration.
# Note that the DocumentRoot is a 'safe' irrelevant value as we don't serve
# anything directly
ServerAdmin              webmaster@alpha-complex.com
ServerRoot               /usr/local/apache
DocumentRoot             /usr/local/apache/proxysite
ErrorLog                 /usr/local/apache/proxy_error
TransferLog              /usr/local/apache/proxy_log

# Choose a non-privileged user and group
User httpd
Group httpd

# Dynamic servers load their modules here...
# Note that if this proxy is only a proxy, we don't need most standard
# modules loaded.

# Don't waste time on things we don't need
HostnameLookups off

# This server is only for proxying so switch off everything else

  Options None
  AllowOverride None


# Allow a local client to access the server status

  order allow,deny
  deny from all
  allow from 127.0.0.1
  SetHandler server-status


### Part 1 - Rewrite

# switch on URL rewriting
RewriteEngine on

# Define a log for debugging but set the log level to zero to disable it for #
performance
RewriteLog logs/proxy_rewrite
RewriteLogLevel 0

# define the cluster servers map
RewriteMap cluster rnd:/usr/local/apache/rewritemaps/cluster.txt

# rewrite the URL if it matches the web server host
RewriteRule ^http://www\.(.*)$ http://{cluster:www}.$2 [P,L]

# forbid any URL that doesn't match
RewriteRule .* - [F]

### Part 2 - Proxy

ProxyPassReverse  /  http://wwwl.alpha-complex.com/
ProxyPassReverse  /  http://www2.alpha-complex.com/
ProxyPassReverse  /  http://www3.alpha-complex.com/
ProxyPassReverse  /  http://www4.alpha-complex.com/
ProxyPassReverse  /  http://www5.alpha-complex.com/
ProxyPassReverse  /  http://www6.alpha-complex.com/

# We don't want caching, preferring to let the back end servers take the
# load, but if we did:
#
# Apache 2.0 only:
CacheOn on
CacheEnable disk /
# Apache 2.0 and 1.3:
#CacheRoot /usr/local/apache/proxy
#CacheSize 102400

Hardware Based Load Clustering

F5 Big-IP

Cisco MNLB

ServerIron

Nortel Alteon

References

Professional Apache 2.0 by Peter Wainwright
The Linux Enterprise Cluster by Karl Kopper