5. Optional - Configure Cluster Alerting Using Nagios

For each of the newly added slave node, complete the following instructions in order to use Nagios for cluster alerting:

On the Nagios server, add the host definition to the /etc/nagios/objects/hadoop-hosts.cfg file:
On the Nagios server, edit the /etc/nagios/objects/hadoop-hostsgroups.cfg file.
Add hostnames of the newly added slave nodes to members list under the slaves host group.
Restart the Nagios server:
```
service nagios start
```