The best practice to set a global variable in puppet

I got it, I did everything from scratch again, but made sure before even beginning the tutorial in the article I did the following on the server and client:

/etc/hosts
192.168.2.5             puppet-db.apt15 puppet-db
192.168.2.4             puppetmaster.apt15 puppetmaster puppet
127.0.0.1               localhost.localdomain localhost
::1             localhost6.localdomain6 localhost6

/etc/sysconfig/network Set hostname to [server name].apt15

yum install ntp
ntpdate pool.ntp.org
ntpd
shutdown -r now (then login)
setenforce 0

Then start tutorial I also had to use puppetd --server myserver.domain.com --waitforcert 60 --test from the documentation instead of the lines in the guide to get the certificate signed and accepted.

The best practice depends on the number of nodes/systems you manage. Modeling data in puppet manifests is sufficient if you can keep track of the data (I've seen it done with thousands of systems, however it gets challenging if you have a large number of variables and deep inheritance especially if you start referencing by scope such as $value = $some_class::some_var). If you have a complex infrastructure it makes sense to look at separating the data from puppet manifests using extlookup, heira, or an external node classifier (ENC).

The way you have done it in the puppet manifests is using dynamic scoping and node inheritance, and dynamic scoping is being deprecated in Puppet 2.7. If this is rewritten in extlookup, it would simply be:

# configured globally
# linux_qa is more likely linux_%{env}
$extlookup_precedence = ["%{fqdn}", "linux_trunk", "linux_qa", "common"]

# in the appropriate class
$relayhost = extlookup("relayhost")