Now, hold on, stop to read, and think again about what we've seen in this chapter and particularly in the last section. Variables that define our infrastructure can be dynamically populated according to the number of hosts that have specific classes or resources. If we add hosts with these services, they can be automatically used by the other hosts.
This is what we need to configure with Puppet dynamic and elastic environments, where new services are made available to other nodes that are consequently configured.
For example, we can set a variable that returns all the IP addresses of the nodes that have Apache installed:
$web_servers_ip = query_nodes('Class[apache]', ipaddress)
This variable can be used in ...