Configuration management is about versioning the configuration of your application and the components it relies on, along with your application itself. Configuration is kept in source control and takes the form of, for example, JSON or YAML files that describe the desired configuration of your application. These files are the input for tools such as Ansible, Puppet, or PowerShell DSC that configure your environment and application. These tools are often invoked from a continuous deployment pipeline.

The desired state can also be reapplied at an interval, even if there are no changes made to the intended configuration. This way, it is ensured that the actual configuration stays correct and that manual changes are automatically revoked. We call this the prevention of configuration drift. Configuration drift occurs over time due to servers being added or removed over time, or manual, ad hoc interventions by administrators. Of course, this implies that intended updates to the configuration are done in source control and only applied using tools.

Configuration management or configuration as code is highly related to infrastructure as code. The two are often intertwined and on some platforms, the difference between the two might even feel artificial. Configuration as code will be discussed in detail in Chapter 6, Infrastructure and Configuration as Code.