Chef
Emerging in 2009, Chef (now known as Progress Chef) is one of the earliest configuration management tools to gain popularity. Chef “Recipes” are written in Ruby, in a primarily declarative style.
Chef requires that a client is installed on a server being managed. This client polls a Chef-Server regularly, to determine what its configuration should be. Chef-Solo is also available, a version of Chef that allows provisioning of a single node by running chef locally.
A key tenet of Chef recipe design is the concept of idempotence. All Chef recipes should be runnable multiple times and produce the same result - this is especially necessary in cases where the client/server model listed above is in use. This pattern of configuration management is highly influential for future declarative tools like Terraform and Cloud Formation.
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