configure an application by looking up stuff from a .json file and writing XMLs from it

configure_app.ps1

#<#
#.Synopsis
#   Rewrite configuration files based on hiera data
#.Description
#   ..
#   
#.Parameter DataFile
#   Exported data from hiera (JSON)
#.Parameter AppData
#   File to read configurable settings from
#.Parameter DryRun
#   In dry run mode, we exit with status 100 to indicate changes needed but do not save the file
#
#.Example
#   # Reconfigure all files
#         configure_app
#
##>
param(
    [string] $DataFile = "C:\vagrant\mock.json",
    [string] $AppData  = "C:\vagrant\appdata.txt",
    [switch] $DryRun = $false
)

function ChildKeyFromXpath {
    param(
        [string] $XPath,
        [switch] $IgnoreMissing = $false
    )
    $childKeyCapture = [regex]::Match($xPath, '@[^=]+=''([^'']+?)'']/[^/]+$').captures.groups
    if ($childKeyCapture.length -ne 2) {
        if (-not $IgnoreMissing) {
            write-error "unable to find childKey in $($xPath) - xPath needs to map to an element in hiera eg //foo[@key='bar']/@value to lookup bar"
            exit 1
        }
        $childKey = $null
    } else {
        $childKey = $childKeyCapture[1]
    }

    return $childKey
}

function ReadXml {
    param(
        [string] $FilePath
    )
    # Read all the xml and do the edit at the requested xPath
    if(-not [System.IO.File]::Exists($FilePath)){
        write-error "File not found reading $($FilePath)"
        exit 1
    }

    [xml] $xml = Get-Content($FilePath)
    if ($xml -eq $null -or $xml.ChildNodes.Count -eq 0 ) {
        write-error "Could not parse XML from $($FilePath) - invalid file content"
        exit 1
    }

    return $xml
}

# Ensure XML fragment exists at correct location
function Ensure-XmlFragment {
    param(
        [string]$FilePath,
        [string]$XPath,
        [string]$ParentKey
    )

    # remove the `ENSURE:` from the start of the XPath
    $XPath = $XPath -replace "ENSURE:", ""

    $xml = ReadXml -FilePath $FilePath

    # XPath should look like this:
    #  //configuration/system.serviceModel/client/endpoint[@name='SystemEventsService']/identity
    # which means we need to get a reference to:
    #  1. //configuration/system.serviceModel/client/endpoint[@name='SystemEventsService']
    #  2. the child `identity`
    $captures = [regex]::Match($xPath, '^(.*?)/([^/]+)$').captures.groups
    $xPathBase = $captures[1]
    $targetElement = $captures[2]
    

    $nodes = (Select-Xml -Xml $xml -XPath $xPathBase)
    if ($nodes.node.count -eq 0) {
        write-error "XPath expression $($xPathBase) matches no nodes"
        exit 1
    }
    $childKey = ChildKeyFromXpath -xPath $XPath -IgnoreMissing
    $data = DataLookup -parentKey $ParentKey -childKey $childKey

    $target = $nodes.node.$targetElement
    if ($target -eq $null) {
        # child (`identity` in above example) doesn't exist yet - create it
        $target = $xml.CreateElement($targetElement)
        $nodes.node.AppendChild($target) | Out-Null
    }

    # update the xml string inside the target (`identity`) to reflect the data from hiera
    if ($target.InnerXml -ne $data) {
        $target.InnerXml = $data

        if ($DryRun) {
            write-host "needs updating"
            exit 100
        } else {
            $xml.save($FilePath)
        }
    }
}

# Set an XML attribute to a particular values
function Set-AttributeValue {
  Param(
    [string]$FilePath,
    [string]$XPath,
    [string]$ParentKey
  )
    $xPathSplit = $XPath -split("/@")
    if ($xPathSplit.Length -ne 2) {
        write-error "xpath input is not correct - needs to capture an element, eg //foo[@key='bar']/@value but you sent '$($XPath)'"
        exit 1
    }
    $xPathBase = $xPathSplit[0]
    $xPathAttrib = $xPathSplit[1]

    $childKey = ChildKeyFromXpath -xPath $xPath

    # lookup the value to use for this config item
    $value = DataLookup -parentKey $parentKey -childKey $childKey

    $xml = ReadXml -FilePath $FilePath
    $nodes = (Select-Xml -Xml $xml -XPath $xPathBase)

    if ($nodes -eq $null) {
        # There must be a "slot" for us to insert values in the XMLs - we are not in the business
        # of creating parent nodes any more
        write-error "missing parent - create $($xPathBase) first and be aware of namespaces in xpath expressions"
        exit 1
    } else {
        if ($nodes.Node.Attributes[$xPathAttrib] -ne $null) {
            if ($nodes.Node.Attributes[$xPathAttrib].Value -eq $value) {
                $changesNeeded = $false
            } else {
                $nodes.Node.Attributes[$xPathAttrib].Value = $value
                $changesNeeded = $true
            }
        } else {
            $nodes.Node.SetAttribute($xPathAttrib,$value)
            $changesNeeded = $true
        }

        if ($changesNeeded -and $DryRun) {
            write-host "Changes are required"
            exit 100
        }
    }

    if (-not $DryRun) {
        $xml.Save($FilePath)
    }
}


#.Synopsys
#   Lookup data (from JSON file simulating hiera lookup)
#.Parameter parentKey
#   The key to ask hiera for, eg `profile::foo::bar`
#.Parameter childKey
#   Assuming the data returned from looking up `parentKey` is a hash, return the contents of this child element. If omitted,
#   Return all data looked up from parentKey
function DataLookup {
    param(
        [String] $parentKey,
        [String] $childKey
    )
    $json = Get-Content $DataFile | ConvertFrom-Json

    if ([string]::IsNullOrEmpty($childKey)) {  
        $data = $json.$parentKey
    } else {
        $data = $json.$parentKey.$childKey
    }

    if ($data -eq $null) {
        write-error "No data in hiera for '$($parentKey)' element '$($childKey)' - fix this!"
        exit 25
    }

    return $data
}


$workingFile = $null

# appdata.txt is a simple textfile listing each app setting that must be managed
# Format:
#   [c:\file\to\edit.xml]
#   XPATH==PARENT KEY IN HIERA
#   ENSURE:XPATH==PARENT KEY IN HIERA
# Real example:
#   [C:\vagrant\web_config.xml]
#   //configuration/appSettings/add[@key='smtpHostName']/@value==boards::ipadserver::settings::appSettings
#   ENSURE://configuration/system.serviceModel/client/endpoint[@name='SystemEventsService']/identity==boards::ipadserver::spn
#
# The XPATH *must* take the form above, since this tells the script:
#   1. How to find existing settings (attribute predicate: `key=SmtpHostName`, `name=SystemEventService`)
#   2. Which attribute or element to write to (at the end of the XPATH: `/@value`, `/identity`)
#   3. The element under the parent key to find the data in hiera (the value the predicate searches for: `smtpHostName`, `SystemEventService`)
#   4. How to perform the update (line starts `ENSURE` we are adding a blob of xmltext otherwise we are setting an attribute)
foreach($line in Get-Content $AppData) {
    # skip blank lines and comments
    $line = $line.trim()
    if ((-not ($line -match '^\s*[\n\r]*$')) -and (-not ($line -match '^\s*#'))) {
        if ($line.StartsWith('[')) {
            # Specify the current working file
            $workingFile = $line -replace "[][]", ""
            write-host "Working file set to $($workingFile)"
        } else { 
            # must be something to lookup
            $lineSplit = $line -split('==')
            $xPath = $lineSplit[0]
            $parentKey = $lineSplit[1]

            if ($lineSplit.Length -eq 2) {
                if ($workingFile -eq $null) {
                    write-error "There is no working file! specify it in square brackets in appdata.txt before items to configure"
                    exit 1
                }
                
                if($line.StartsWith("ENSURE:")) {
                    Ensure-XmlFragment -FilePath $workingFile -XPath $xPath -ParentKey $parentKey
                } else {
                    Set-AttributeValue -FilePath $workingFile -XPath $xPath -ParentKey $parentKey
                }
            } else {
                write-error "item to lookup is not in correct format - should be XPATH==HIERA KEY but got: \n $($line)"
                exit 1
            }
        }
    }
}