Sometimes your Puppet manifest doesn't do exactly what you expected, or perhaps someone else has checked in changes you didn't know about. Either way, it's good to know exactly what Puppet is going to do before it does it.

When you are retrofitting Puppet into an existing infrastructure you might not know whether Puppet is going to update a config file or restart a production service. Any such change could result in unplanned downtime. Also, sometimes manual configuration changes are made on a server that Puppet would overwrite.

To avoid these problems, you can use Puppet's noop mode, which means no operation or do nothing. When run with the noop option, Puppet only reports what it would do but doesn't actually do anything. One caveat here is that even during a noop run, pluginsync still runs and any lib directories in modules will be synced to nodes. This will update external fact definitions and possibly Puppet's types and providers.