scaling drupal step three - using heartbeat to implement a redundant load balancer
if you've setup a clustered drupal deployment (see scaling drupal step two - sticky load balancing with apache mod_proxy), a good next-step, is to cluster your load balancer.
one way to do this is to use heartbeat to provide instant failover to a redundant load balancer should your primary fail. while the method suggested below doesn't increase the loadbalancer scalability, which shouldn't be an issue for a reasonably sized deployment, it does increase your the redundancy. as usual, my examples are for apache2, mysql5 and drupal5 on debian etch. see the scalability overview for related articles.
deployment overview
this table summaries the characteristics of this deployment choice| scalability: | fair |
| redundancy: | good |
| ease of setup: | fair |
servers
in this example, i use:
| web server | drupal-lb1.mydomain.com | 192.168.1.24 |
| web server | drupal-lb2.mydomain.com | 192.168.1.25 |
| data server | drupal-data-server1.mydomain.com | 192.168.1.26
|
| load balancer | apache-balance-1.mydomain.com | 192.168.1.34 |
| load balancer | apache-balance-2.mydomain.com | 192.168.1.35 |
| load balancer cluster | apache-balance-cluster.mydomain.com | 192.168.1.51 |
network diagram
install and setup heartbeat on load balancers
setup two load balancers,apache-balance-1 and apache-balance-2 as described in my previous blog.
install heartbeat and it's dependencies on both:
# apt-get install heartbeat-2/etc/ha.d/ha.cf (see http://www.linux-ha.org/GettingStarted for more info) identically on each of the load balancers as follows:
logfile /var/log/ha-log
bcast eth0
keepalive 2
warntime 10
deadtime 30
initdead 120
udpport 694
auto_failback yes
node apache-balance-1
node apache-balance-2
uuidfrom nodename
respawn hacluster /usr/lib/heartbeat/ipfail- the nodenames must be the output of
uname -n - i had problems with my nodenames flapping, using
uuidfromfixed this.
/etc/ha.d/haresources. this must be identical on apache-balance-1 and apache-balance-2. really! . this file should look like:
apache-balance-1 192.168.1.51 apache2apache-balance-1here refers to the "preferred" host for the service192.168.1.51is the vip that your load balancer will appear to be onapache2here refers specifically to the name of the script in the directory/etc/init.d
/etc/ha.d/authkeys on both load balancers. if you're paranoid, see more secure options, in "configuring authkeys" here. this fiile should look like:
auth 2
2 crc# chmod 600 /etc/ha.d/authkeys/etc/apache2/ports.conf on both load balancers:
Listen 192.168.1.51:80theoretically you should configure each load balancer to stop apache2 starting on boot. this allows the ha daemon to take full control of starting and stopping apache. in practice i didn't need to. you might want to.
restart the ha daemons and test
restart the ha daemon on both load balancers and test:# etc/init.d/heartbeat restart# tail -f /var/log/apache2/access.log
# tail -f /var/log/ha-logfinal word
this is a fairly simplistic configuration. there is more you can do on detecting abormal situations and failing over. for more information, visit http://www.linux-ha.orgtech blog
if you found this article useful, and you are interested in other articles on linux, drupal, scaling, performance and LAMP applications, consider subscribing to my technical blog.
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what would you need to
what would you need to change to load balance https?
actually, i'd like to rewrite http requests to https automatically like this:
RewriteEngine On
RewriteCond %{HTTPS} !on
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
and then have the virtual host balance https-- but what should change in httpd.conf on the load balancers? should the load balancers and the web servers all have the same ssl cert on them?
John, great summary. If
John, great summary.
If you're a little hardware constrained and only have two machines, could you install heardbeat, drupal/apache/php on each, mysql_proxy and a db on each and then have two machines that both scale and are fully (and automatically) redundant for each other?
John Why did you choose for
John
Why did you choose for Heartbeat + Apache Mod proxy ?
Did you look at LVS ? Ultramonkey etc ?
The setup you described is a good one , but there are alternatives .. and I`m interested to know why you opted for this solution.
kris, my main motivation for
kris, my main motivation for mod_proxy_balancer was to use a tool that people were likely to be already familiar with (apache2). i was trying to use as few technologies as possible to scale. i also got a sense that ultramonkey wasn't a very active project. lvs is a great tool, but a bit trickier to configure.
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