What is Puppet?

Puppet is two things: a language for expressing the desired state (how your nodes should be configured), and an engine which interprets code written in the Puppet language and applies it to nodes to bring about the desired state.

What does this language look like? It's not exactly a series of instructions, like a shell script or a Ruby program. It's more like a set of declarations about the way things should be. Consider the following example:

package { 'curl':
  ensure => installed,
}

In English, this code says—The curl package should be installed. When you apply this manifest (Puppet programs are called manifests), the tool will do the following:

Another example:

user { 'bridget':
  ensure => present,
}

This is Puppet language for the declaration The bridget user should be present (the keyword ensure means the desired state of the resource is....). Again, this results in Puppet checking for the existence of the bridget user on the node, and creating it if necessary. This is also a kind of documentation which expresses human-readable statements about the system in a formal way. The code expresses the author's desire that Bridget should always be present.

So you can see that the Puppet program—the Puppet manifest—for your configuration is a set of declarations about what things should exist, and how they should be configured.

You don't give commands such as Do this, then do that. Rather, you describe how things should be, and let Puppet take care of making it happen. These are two quite different kinds of programming. One kind (so-called procedural style) is the traditional model used by languages like C, Python, shell, and so on. Puppet's is called declarative style, because you declare what the end result should be, rather than specify the steps to get there.

This means that you can apply the same Puppet manifest repeatedly to a node and the end result will be the same, no matter how many times you apply the manifest. It's better to think of Puppet manifests as a kind of specification, or declaration, rather than as a program in the traditional sense.