DSC

Composing DSC Roles

Another common challenge with DSC, is how to compose DSC configurations.

People have seen the trick of having a Configuration, and the following code within:


Configuration MyDSCConfig {
Node $ConfigurationData.AllNodes.nodename {
if($Node.roles -contains 'MyRole') {
# do stuff here, like calling DSC Resources
MyCompositeResource StuffForMyRole {
# …
}
}
if($Node.roles -contains 'MyOtherRole') {
MyOtherResource OtherStuff {
# …
}
}
}
}

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MyDscConfig.ps1

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This is a good way to get started and works well for small-ish configurations, but it gets out of hand pretty quickly, as it’s hard to read all the if statements and their content. Some variant of this are using Where clauses around the Node expression.

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The DSC Configuration Data Problem

I’m writing and using a PowerShell module called “Datum” to manage DSC Configuration Data, and raise cattle at work. This is a write up to explain the problem, and why I’m writing it.

I assume you already know a bit about Desired State Configuration, but if it’s new to you or vague, go grab yourself The DSC Book by Don Jones & Missy Januszko, which packs all you need to know about out-of-the-box DSC, and more.

This post tries to explain the DSC Configuration Data composition challenge that I attempt to solve with Datum. It does not include the solution and how to use Datum to solve those problems: you can look on the GitHub repository and see an example of a Control Repository, or come to the PowerShell Summit 2018, where I’ll be presenting Datum used in a DSC Infrastructure.

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Pseudo-Splatting DSC Resources

Here’s a quick function to allow you something close to splatting with DSC resources:


function Get-DscSplattedResource {
[CmdletBinding()]
Param(
[String]
$ResourceName,
[String]
$ExecutionName,
[hashtable]
$Properties
)
$stringBuilder = [System.Text.StringBuilder]::new()
$null = $stringBuilder.AppendLine("Param([hashtable]`$Parameters)")
$null = $stringBuilder.AppendLine(" $ResourceName $ExecutionName { ")
foreach($PropertyName in $Properties.keys) {
$null = $stringBuilder.AppendLine("$PropertyName = `$(`$Parameters['$PropertyName'])")
}
$null = $stringBuilder.AppendLine("}")
Write-Debug ("Generated Resource Block = {0}" -f $stringBuilder.ToString())
[scriptblock]::Create($stringBuilder.ToString()).Invoke($Properties)
}
Set-Alias –Name x –Value Get-DscSplattedResource

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Get-DscSplattedResource.ps1

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Here’s why you may need it.

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