You should have a working Nagios Core 3.0 or better server running, with web interface.

You should also have at least two services defined that form a meaningful group; perhaps they're similar kinds of services, such as mail services, or are monitored by the same team, or all on the same set of servers at a physical location.

In this example, we have three servers performing mail functions: smtp.naginet, pop3.naginet, and imap.naginet, running an SMTP, POP3, and IMAP daemon, respectively. All three of the hosts are set up in Nagios Core, and so are their services. We're going to add them into a new servicegroup called mailservices.

Here are the definitions of the hosts and services used in this example, so you can see how everything fits together:

define host {
    use                 linux-server
    host_name           smtp.naginet
    alias               smtp
    address             10.128.0.31
    hostgroups          webservers
}
    define service {
        use                  generic-service
        host_name            smtp.naginet
        service_description  SMTP
        check_command        check_smtp
    }
define host {
    use                 linux-server
    host_name           pop3.naginet
    alias               pop3
    address             10.128.0.32
    hostgroups          webservers
}
    define service {
        use                  generic-service
        host_name            pop3.naginet
        service_description  POP3
        check_command        check_pop
    }
define host {
    use                 linux-server
    host_name           imap.naginet
    alias               imap
    address             10.128.0.33
    hostgroups          webservers
}
    define service {
        use                  generic-service
        host_name            imap.naginet
        service_description  IMAP
        check_command        check_imap
    }

We can add our new servicegroup with the following steps:

We should now be able to visit the Service Groups section of the web interface, and see a new servicegroup with three members:

The configuration we added includes a new file with a new servicegroup into the Nagios Core configuration, and inserts the appropriate services into the group. This creates a separate section in the web interface for us to get a quick overview of only the services in that particular group.

The way we've added services to the groups is actually not the only way to do it. If we prefer, we can name the services (and their applicable hosts) for the group inside the group definition, using the members directive, so that we could have a code snippet similar to the following:

define servicegroup {
    servicegroup_name  mailservices
    alias              Mail services
    members            smtp.naginet,SMTP,pop3.naginet,POP3
}

Note that we need to specify both the host that the service is on, and then the services to monitor on it, comma-separated. The hostname comes first, and then the service.

This extends to allowing us to make a servicegroup that always includes every single service, if we find that useful:

define servicegroup {
    servicegroup_name  all
    alias              All services
    members            *
}

If we're going to be using servicegroup definitions extensively in our Nagios Core configuration, we should use whichever of the two methods to add services to groups that we think is going to be easiest for us to maintain.

It's worth noting that a service can be in more than one group, and there is no limit on the number of groups we can declare, so we can afford to be quite liberal with how we group our services into categories. Examples could be organising services by the appropriate contact for their notifications, for internal functions, or customer facing functions.