This tutorial shows how to set up a two-node Apache web server cluster that provides high-availability. In front of the Apache cluster we create a load balancer that splits up incoming requests between the two Apache nodes. Because we do not want the load balancer to become another "Single Point Of Failure", we must provide high-availability for the load balancer, too. Therefore our load balancer will in fact consist out of two load balancer nodes that monitor each other using heartbeat, and if one load balancer fails, the other takes over silently.The advantage of using a load balancer compared to using round robin DNS is that it takes care of the load on the web server nodes and tries to direct requests to the node with less load, and it also takes care of connections/sessions. Many web applications (e.g. forum software, shopping carts, etc.) make use of sessions, and if you are in a session on Apache node 1, you would lose that session if suddenly node 2 served your requests. In addition to that, if one of the Apache nodes goes down, the load balancer realizes that and directs all incoming requests to the remaining node which would not be possible with round robin DNS.
For this setup, we need four nodes (two Apache nodes and two load balancer nodes) andfive IP addresses: one for each node and one virtual IP address that will be shared by the load balancer nodes and used for incoming HTTP requests.
I will use the following setup here:
- Apache node 1: webserver1.example.com (webserver1) - IP address:192.168.0.101; Apache document root: /var/www
- Apache node 2: webserver2.example.com (webserver2) - IP address:192.168.0.102; Apache document root: /var/www
- Load Balancer node 1: loadb1.example.com (loadb1) - IP address:192.168.0.103
- Load Balancer node 2: loadb2.example.com (loadb2) - IP address:192.168.0.104
- Virtual IP Address: 192.168.0.105 (used for incoming requests)
In this tutorial I will use Debian Sarge for all four nodes. I assume that you have installed a basic Debian installation on all four nodes, and that you have installed Apache onwebserver1 and webserver2, with /var/www being the document root of the main web site.
I want to say first that this is not the only way of setting up such a system. There are many ways of achieving this goal but this is the way I take. I do not issue any guarantee that this will work for you!
1 Enable IPVS On The Load BalancersFirst we must enable IPVS on our load balancers. IPVS (IP Virtual Server) implements transport-layer load balancing inside the Linux kernel, so called Layer-4 switching.
echo ip_vs_dh >> /etc/modules
echo ip_vs_ftp >> /etc/modules
echo ip_vs >> /etc/modules
echo ip_vs_lblc >> /etc/modules
echo ip_vs_lblcr >> /etc/modules
echo ip_vs_lc >> /etc/modules
echo ip_vs_nq >> /etc/modules
echo ip_vs_rr >> /etc/modules
echo ip_vs_sed >> /etc/modules
echo ip_vs_sh >> /etc/modules
echo ip_vs_wlc >> /etc/modules
echo ip_vs_wrr >> /etc/modules
Then we do this:
If you get errors, then most probably your kernel wasn't compiled with IPVS support, and you need to compile a new kernel with IPVS support (or install a kernel image with IPVS support) now.
2 Install Ultra Monkey On The Load BalancersUltra Monkey is a project to create load balanced and highly available services on a local area network using Open Source components on the Linux operating system; the Ultra Monkey package provides heartbeat (used by the two load balancers to monitor each other and check if the other node is still alive) and ldirectord, the actual load balancer.
To install Ultra Monkey, we must edit /etc/apt/sources.list now and add these two lines (don't remove the other repositories):
and install Ultra Monkey:
apt-get install ultramonkey
If you see this warning:
¦ libsensors3 not functional ¦ ¦ ¦ ¦ It appears that your kernel is not compiled with sensors support. As a ¦ ¦ result, libsensors3 will not be functional on your system. ¦ ¦ ¦ ¦ If you want to enable it, have a look at "I2C Hardware Sensors Chip ¦ ¦ support" in your kernel configuration. ¦
During the Ultra Monkey installation you will be asked a few question. Answer as follows:
Do you want to automatically load IPVS rules on boot?
Select a daemon method.
3 Enable Packet Forwarding On The Load BalancersThe load balancers must be able to route traffic to the Apache nodes. Therefore we must enable packet forwarding on the load balancers. Add the following lines to /etc/sysctl.conf:
# Enables packet forwarding net.ipv4.ip_forward = 1
4 Configure heartbeat And ldirectordNow we have to create three configuration files for heartbeat. They must be identical onloadb1 and loadb2!
logfacility local0 bcast eth0 # Linux mcast eth0 188.8.131.52 694 1 0 auto_failback off node loadb1 node loadb2 respawn hacluster /usr/lib/heartbeat/ipfail apiauth ipfail gid=haclient uid=hacluster
on loadb1 and loadb2.
loadb1 \ ldirectord::ldirectord.cf \ LVSSyncDaemonSwap::master \ IPaddr2::192.168.0.105/24/
on loadb1, no matter if you create the file on loadb1 or loadb2! After IPaddr2 we put our virtual IP address 192.168.0.105.
auth 3 3 md5 somerandomstring
/etc/ha.d/authkeys should be readable by root only, therefore we do this:
chmod 600 /etc/ha.d/authkeys
ldirectord is the actual load balancer. We are going to configure our two load balancers (loadb1.example.com and loadb2.example.com) in an active/passive setup, which means we have one active load balancer, and the other one is a hot-standby and becomes active if the active one fails. To make it work, we must create the ldirectord configuration file/etc/ha.d/ldirectord.cf which again must be identical on loadb1 and loadb2.
checktimeout=10 checkinterval=2 autoreload=no logfile="local0" quiescent=yes virtual=192.168.0.105:80 real=192.168.0.101:80 gate real=192.168.0.102:80 gate fallback=127.0.0.1:80 gate service=http request="ldirector.html" receive="Test Page" scheduler=rr protocol=tcp checktype=negotiate
Afterwards we create the system startup links for heartbeat and remove those of ldirectordbecause ldirectord will be started by the heartbeat daemon:
update-rc.d heartbeat start 75 2 3 4 5 . stop 05 0 1 6 .
update-rc.d -f ldirectord remove
Finally we start heartbeat (and with it ldirectord):
5 Test The Load BalancersLet's check if both load balancers work as expected:
ip addr sh eth0
The active load balancer should list the virtual IP address (192.168.0.105):
ldirectord ldirectord.cf status
Output on the active load balancer:
ldirectord for /etc/ha.d/ldirectord.cf is running with pid: 1455
ldirectord is stopped for /etc/ha.d/ldirectord.cf
ipvsadm -L -n
Output on the active load balancer:
IP Virtual Server version 1.2.1 (size=4096) Prot LocalAddress:Port Scheduler Flags -> RemoteAddress:Port Forward Weight ActiveConn InActConn TCP 192.168.0.105:80 rr -> 192.168.0.101:80 Route 0 0 0 -> 192.168.0.102:80 Route 0 0 0 -> 127.0.0.1:80 Local 1 0 0
IP Virtual Server version 1.2.1 (size=4096) Prot LocalAddress:Port Scheduler Flags -> RemoteAddress:Port Forward Weight ActiveConn InActConn
Output on the active load balancer:
master running (ipvs_syncmaster pid: 1591)
6 Configure The Two Apache NodesFinally we must configure our Apache cluster nodes webserver1.example.com andwebserver2.example.com to accept requests on the virtual IP address 192.168.0.105.
apt-get install iproute
Add the following to /etc/sysctl.conf:
# Enable configuration of arp_ignore option net.ipv4.conf.all.arp_ignore = 1 # When an arp request is received on eth0, only respond if that address is # configured on eth0. In particular, do not respond if the address is # configured on lo net.ipv4.conf.eth0.arp_ignore = 1 # Ditto for eth1, add for all ARPing interfaces #net.ipv4.conf.eth1.arp_ignore = 1 # Enable configuration of arp_announce option net.ipv4.conf.all.arp_announce = 2 # When making an ARP request sent through eth0 Always use an address that # is configured on eth0 as the source address of the ARP request. If this # is not set, and packets are being sent out eth0 for an address that is on # lo, and an arp request is required, then the address on lo will be used. # As the source IP address of arp requests is entered into the ARP cache on # the destination, it has the effect of announcing this address. This is # not desirable in this case as adresses on lo on the real-servers should # be announced only by the linux-director. net.ipv4.conf.eth0.arp_
Add this section for the virtual IP address to /etc/network/interfaces:
auto lo:0 iface lo:0 inet static address 192.168.0.105 netmask 255.255.255.255 pre-up sysctl -p > /dev/null
Finally we must create the file ldirector.html. This file is requested by the two load balancer nodes repeatedly so that they can see if the two Apache nodes are still running. I assume that the document root of the main apache web site on webserver1 andwebserver2 is /var/www, therefore we create the file /var/www/ldirector.html:
7 Further TestingYou can now access the web site that is hosted by the two Apache nodes by typinghttp://192.168.0.105 in your browser.
Now stop the Apache on either webserver1 or webserver2. You should then still see the web site on http://192.168.0.105 because the load balancer directs requests to the working Apache node. Of course, if you stop both Apaches, then your request will fail.
Now let's assume that loadb1 is our active load balancer, and loadb2 is the hot-standby. Now stop heartbeat on loadb1:
Wait a few seconds, and then try http://192.168.0.105 again in your browser. You should still see your web site because loadb2 has taken the active role now.
Now start heartbeat again on loadb1:
loadb2 should still have the active role. Do the tests from chapter 5 again on loadb1 andloadb2, and you should see the inverse results as before.
If you have also passed these tests, then your loadbalanced Apache cluster is working as expected. Have fun!
8 Further ReadingThis tutorial shows how to loadbalance two Apache nodes. It does not show how to keep the files in the Apache document root in sync or how to create a storage solution like anNFS server that both Apache nodes can use, nor does it provide a solution how to manage your MySQL database(s). You can find solutions for these issues here:
- Mirror Your Web Site With rsync
- Setting Up A Highly Available NFS Server
- How To Set Up A Load-Balanced MySQL Cluster
- How To Set Up Database Replication In MySQL