Use defined resource to populate template file

I am working on making our puppet infrastructure more dynamic where I can add any number of nodes and these will automatically be applied in another nodes configuration.

An example: I have X number of hosts running a Java application, which are defined by our javaapp module with the following define in it (simplified):

define javaapp::site (
  $site_name,
  $site_port ) {
  # Doing some stuff regarding deployment, service management etc here
}

These are defined in the site.pp file like this:

javaapp::site { 'app01':
  play_site_name    => 'app01',
  play_site_port    => 9100,
}

We have a nginx reverse proxy in front of these application servers where I need to forward requests to a range of servers. Currently my nginx template looks like this:

upstream app-hosts {
  server <%= @host_app_01 %>:9100;
  server <%= @host_app_02 %>:9100;
}

Where @host_app_01 and @host_app_02 have been defined globally in the site.pp file

What I would like to do is to collect an array of the nodes with a specific javaapp site name and use it in the nginx template. Something like this pseudo-code:

upstream app-hosts {
  <% @host_app.each do |host| %>
    server <%= @host.ip %>:<%= @host.appPort %>;
  <% end %>
}

I have read a little about exported resources, but I can't figure out whether it is the right thing for this situation?

Is it even possible to do with puppet?

We are running puppet version 3.7.5.