is there a way to run a command before puppet implements a change?

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http://serverfault.com – I want to have puppet run a specific command before performing any type of change. I am aware of the prerun_command option in the main puppet.conf, but this is not what I'm looking for. I want the command to only run if something is about to change, not on every puppet run. Here's the scenario. Let's say I have a bunch of web servers behind a load balancer. I then want puppet to update the web site files. But in order to prevent issues where some files have been updated, but other files haven't, and the mixed versions causing problems, I want to take the server out of the load balancer pool. (HowTos)