System Center Service Manager 2012 R2 CSV sync by using Import-SCSMClassInstance

Problem: import-SCSMinstance imports objects that are in a specified CSV file, but... it does not remove "obsolete" objects. - I want to really sync - load what is missing, and remove what is obsolete

 if you drop all objects and then import them again, objects get a new GUID, so you lose history for the objects that are imported again, so this is not a preferred solution.

here is how you truly sync (mirror) the objects - this example is a simple class only with displaynames of groups:

Create your CSV file, this should contain all the objects that should be in your class
in my example, I load them from a SQL database:

$query = "select last_name from ca_contact where contact_type=2308"
$connection = new-object system.data.sqlclient.sqlconnection($StrConn);
$adapter = new-object system.data.sqlclient.sqldataadapter ($query, $connection)
$set = new-object system.data.dataset
$adapter.Fill($set)
$table = new-object system.data.datatable
$table = $set.Tables[0]

this is collecting some group names into $table
then create the CSV file and I add the values also to array $csv (I  run a "-contains" against this array - which is not possible against the $table variable)

$csvfile = "C:\SCSMimport\USDGroups.csv"
$csv=@()
Foreach ($R in ($table))
{
     Add-Content $csvfile ($R.last_name)
     $csv += $name
}

get all your current objects loaded in SCSM:
$GrpClass = Get-SCSMClass -Name USDgroups
$all = Get-SCSMClassInstance -Class $GrpClass

and now we compare $all against the array that contains anything that should be in the group.
 
foreach ($obj in $all)
{    if ($csv -notcontains $obj.DisplayName)
    {        "obsolete group : " + $obj.DisplayName
        Remove-SCSMClassInstance -Instance $obj
     }
}  

So.. anything not in the CSV but still loaded in the class in SCSM is now removed from SCSM

and then the last step to ensure new objects are added:
Import-SCSMInstance -DataFileName $csvfile -FormatFileName 'C:\tools\SCSMimport\USDGroups.xml'

 that's it !