To all those who needs uninterrupted webservice here is an article that describes basic settings of how it can be done using linux servers.
For this purpose we can use any Red-Hat distribution and another utility called Heartbeat.
The heartbeat service provides the basic functions required for the HA system. In a cluster environment, a high availability (HA) system is responsible for starting and stopping services, mounting and dismounting resources, monitoring the system availability in the cluster environment, and handling the ownership of the virtual IP address that’s shared between cluster nodes.
The most common cluster configuration is called standby configuration, as described here. In the standby cluster configuration, one node performs all the work while the other node is idle. Heartbeat monitors health of particular service(s) usually through a separate Ethernet interface used only for HA purposes using special ping. If a node fails for some reason, heartbeat transfers all the HA components to the healthy node. When the node recovers, it can resume its former status.
Installing and Configuring
First of all install heartbeat in a centos machine.
yum install -y heartbeat
To test High Availability Linux, you need a second Ethernet adapter devote to heartbeat on each nodes. Install the Apache Web server and the heartbeat program on both nodes.
The configuration files for heartbeat are not in place when the software is installed. You need to copy them from the documentation folder to the /etc/ha.d/ folder:
cp /usr/share/doc/heartbeat*/ha.cf /etc/ha.d/ cp /usr/share/doc/heartbeat*/haresources /etc/ha.d/ cp /usr/share/doc/heartbeat*/authkeys /etc/ha.d/
Imagine you have two servers in a famous Datacenter and owns few public IP’s. In the /etc/hosts file you must add hostnames and IP addresses to let the two nodes see each other. In my case it looks like this:
126.96.36.199 node1.sparksupport.com node1 188.8.131.52 node2.sparksupport.com node2 127.0.0.1 localhost.localdomain localhost
The two nodes will have same gateway IP and that will be set by Datacenter. For our example let node1 be primary and node2 backup or secondary. The primary will run an extra IP address (assigned using IP aliasing) and with that IP you will have to configure your webserver. Now configure these IP’s on eth0 and eth1 can be configured with private IP address for the use of hearbeat. The node1 and node2 should be connected using a crossover cable across these second Ethernet Card eth1.
Now ifconfig on node1 should provide following results.
eth0 inet addr:184.108.40.206 Bcast:220.127.116.11 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:18617 errors:0 dropped:0 overruns:0 frame:0 TX packets:14682 errors:0 dropped:0 overruns:0 carrier:0 collisions:3 txqueuelen:100 Interrupt:10 Base address:0x6800 eth0:0 Link encap:Ethernet HWaddr 00:60:97:9C:52:28 inet addr:18.104.22.168 Bcast:22.214.171.124 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:10 Base address:0x6800 eth1 Link encap:Ethernet HWaddr 00:60:08:26:B2:A4 inet addr:192.168.1.2 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:15673 errors:0 dropped:0 overruns:0 frame:0 TX packets:17550 errors:0 dropped:0 overruns:0 carrier:0 collisions:2 txqueuelen:100 Interrupt:10 Base address:0x6700 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:3924 Metric:1 RX packets:38 errors:0 dropped:0 overruns:0 frame:0 TX packets:38 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0
And ifconfig on node2 should provide the following results.
eth0 inet addr:126.96.36.199 Bcast:188.8.131.52 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:18617 errors:0 dropped:0 overruns:0 frame:0 TX packets:14682 errors:0 dropped:0 overruns:0 carrier:0 collisions:3 txqueuelen:100 Interrupt:10 Base address:0x6800 eth1 Link encap:Ethernet HWaddr 00:60:08:26:B2:A4 inet addr:192.168.1.3 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:15673 errors:0 dropped:0 overruns:0 frame:0 TX packets:17550 errors:0 dropped:0 overruns:0 carrier:0 collisions:2 txqueuelen:100 Interrupt:10 Base address:0x6700 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:3924 Metric:1 RX packets:38 errors:0 dropped:0 overruns:0 frame:0 TX packets:38 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0
We will have to edit three more files and these files should be exactly the same in both nodes.
/etc/ha.d/ha.cf /etc/ha.d/haresources /etc/ha.d/authkeys
Next, modify the configuration file /etc/ha.d/ha.cf. Edit the following entries in order to get heartbeat to work:
logfile /var/log/ha-log #where to log everything from heartbeat logfacility local0 #Facility to use for syslog/logger keepalive 2 # the time between the heartbeats deadtime 30 #how long until the host is declared dead warntime 10 #how long before issuing "late heartbeat" warning initdead 120 # Very first dead time (initdead) udpport 694 #udp port for the bcast/ucast communication bcast eth1 #on what interface to broadcast ucast eth1 192.168.1.1 #this is a 2-node cluster, so no need to use multicast here auto_failback on #we want the resources to automatically fail back to its primary node node node1.sparksupport.com #the name of the first node node node2.sparksupport.com #the name of the second node
The ucast in node2 should be ucast eth1 192.168.1.2
The next file is /etc/ha.d/haresources with the following entry
node1.sparksupport.com 184.108.40.206 httpd
The last heartbeat-related file is /etc/ha.d/authkeys with the entry.
auth 1 1 crc
The file should be readable only by root user for security reasons
chmod 600 /etc/ha.d/authkeys
Now configure httpd.conf with the public IP address.
Listen 220.127.116.11: DocumentRoot "/home/spark/www"
It’s important for the Apache service to not start automatically at boot time, since heartbeat will start and stop the service as needed. Disable the automatic start with the command (on a Red Hat-based system):
chkconfig httpd remove
Make sure you have the same Apache configuration on both nodes.
Testing Fail-over cluster
At this point we’re done with configuration. Now it’s time to start the newly created cluster. Start the heartbeat service on both nodes:
Watch the /var/log/ha-log on both nodes. If everything is configured correctly, you should see something like this in your log files:
Configuration validated. Starting heartbeat 1.2.3.cvs.20050927 heartbeat: version 1.2.3.cvs.20050927 Link node1.sparksupport.com:eth1 up. Link node2.sparksupport.com:eth1 up. Status update for node node2.sparksupport.com: status active Local status now set to: 'active' remote resource transition completed. Local Resource acquisition completed. (none) node2.example.com wants to go standby [foreign] acquire local HA resources (standby). local HA resource acquisition completed (standby). Standby resource acquisition done [foreign]. Initial resource acquisition complete (auto_failback) remote resource transition completed.
Now test the failover. Reboot the master server. The slave should take over the Apache service. If everything works well, you should see something like this:
Received shutdown notice from 'node1.sparksupport.com'. Resources being acquired from node1.sparksupport.com. acquire local HA resources (standby). local HA resource acquisition completed (standby). Standby resource acquisition done [foreign]. Running /etc/ha.d/rc.d/status status Taking over resource group 18.104.22.168 Acquiring resource group: node1.sparksupport.com 22.214.171.124 httpd mach_down takeover complete for node node1.sparksupport.com node node1.sparksupport.com: is dead Dead node node1.sparksupport.com gave up resources. Link node1.sparksupport.com:eth1 dead.
And when the master comes back online again, he should take over the Apache service:
Heartbeat restart on node node1.sparksupport.comheartbeat Link node1.sparksupport.com:eth1 up. node2.sparksupport.com wants to go standby [foreign] standby: node1.sparksupport.com can take our foreign resources give up foreign HA resources (standby). Releasing resource group: node1.sparksupport.com 126.96.36.199 httpd Local standby process completed [foreign]. remote resource transition completed. Other node completed standby takeover of foreign resources.
This is a basic setup you can increase the number of services. You should sync the data in primary and secondary nodes.