Puppet: How to gather selected node names in manifest? - puppet

Question

I am trying to set up a set of nodes running various parts of the ELK stack. In particular, I've got a set of systems running Elasticsearch and I'd like to fill out my logstash config file so that it knows which systems have ES on them.
I can see something like this in the logstash config (obviously untested):
output {
elasticsearch {
hosts => [
<%
#es_hosts.each do |host|
"#{host}",
end
-%>
]
}
}
But what I can't figure out is how to collect the hostnames for systems which are running elasticsearch. I've got a modules which apply RabbitMQ and ES, and it already exports some resources, but this one looks like it just needs nodenames for merging into a list.
--------EDIT BELOW--------
I stumbled across datacat after examing some of the PF modules I use, and thought it might be a candidate. Here's what I've done, posted here because it's not working the way I would have expected.
On my elasticsearch nodes (there are several):
##datacat_fragment { "${::hostname} in hosts":
tag => ['elasticsearch_cluster'],
target => '/etc/logstash/conf.d/master.conf',
data => {
host => ["${::hostname}" ],
}
}
Then, on the logstash node that needs to output to these ES nodes:
Datacat_fragment<| tag == 'elasticsearch_cluster' |>
datacat { '/etc/lostash/conf.d/master.conf':
template => "${module_name}/logstash-master.conf.erb",
}
Finally, the template itself:
input { [...snip...] }
filter {}
output {
elasticsearch {
<% #data.keys.sort.each do |host| %>
hosts = [
<%= #data[host.sort.join(',') %>
]
}
}
Sadly, the result of all this is
input { [...snip...] }
filter {}
output {
elasticsearch {
}
}
So at present, it looks like the exported resources aren't being instantiated as expected and I can't see why. If I add a datacat_fragment defined the same way but local to the logstash manifest, the data gets inserted into the .conf file just fine. It's just the ones from the ES nodes that are being ignored.
To further complicate matters, the input section needs to have a value inserted into it that's based on the system receiving the file. So there's one part that needs to behave like a traditional template, and another section that needs to have data inserted from multiple sources. Datacat looks promising, but is there another way to do this? Concat with an inline template somehow?