If you need to enable NewRelic monitoring on certain machines within the same Rails environment, a simple solution is to utilize the respective hostnames of you machines.

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For example, if you have 8 application servers (e.g. app1.example.com, app2.example.com, ...) and want to enable NewRelic on app1 and app2 only, utilize those steps to do so:

1. Put the attached file into your config directory (config/custom_new_relic_configuration.rb).
2. Specify on which hosts NewRelic should be enabled (see NEWRELIC_HOSTS constant and list the full hostnames (FQDN) of the machines).
3. Put this on top of your NewRelic configuration (which is config/newrelic.yml usually): require 'custom_new_relic_configuration'
4. Finally, enable your custom configuration: In the respective environment section in newrelic.yml (production environment in the example below), change your settings like this:

production:
<<: *default_settings
# NewRelic should be enabled on some specific hosts on production only
# See config/new_relic_configuration.rb how to enable on a specific host.
monitor_mode: <%= CustomNewRelicConfiguration.enable_on_current_host? %>

FYI: The newrelic.yml config file is ERB parsed when the Rails environment is booted, CustomNewRelicConfiguration.enable_on_current_host? gets evaluated and NewRelic is enabled on the hosts specified in your custom configuration class only.