Syntax

DSC provides a DSL to help you create MOF files. We call the file that holds the DSL syntax the DSC configuration file. Even though it is a PowerShell script file (a text file with a .ps1 extension), it can't do anything on its own. You can try to execute a configuration file all you want; it won't do anything to the system by itself. A DSC configuration file holds the information for the desired state, not the execution code to bring the node to a desired state. We talked about this separation of configuration information and execution logic earlier, and we are going to keep seeing this repeatedly throughout our use of DSC.

The DSC DSL allows both imperative and declarative commands. What this means is that configuration files can both describe what has to be done (declarative) as well as have a PowerShell code that is executed inline (imperative).

Declarative code will typically be DSC functions and resource declarations and will make up the majority of code inside your DSC configuration file. Remember, the purpose of DSC is to express the expected state of the system, which you do by declaring it in these files in the human-readable language.

Imperative code will typically make decisions based on the metadata provided inside the configuration file, for example, choosing whether to apply a configuration to a target node inside the $AllNodes variable or deciding which files or modules to apply based on some algorithm. You will find that putting a lot of imperative code inside your configuration files will cause maintenance and troubleshooting problems in the future. Generally, a lot of imperative code indicates that you are performing actions or deciding on logic that should be in a DSC resource, which is the best place to put imperative code. We will cover what indicators for this are and other important aspects of DSC resources in Chapter 4, DSC Resources.