{
  "$schema": "http://json-schema.org/schema#", 
  "type": "object", 
  "description": "Preconditions must be fulfilled before an operation (update, delete, etc.) is carried out.", 
  "properties": {
    "uid": {
      "type": [
        "string", 
        "null"
      ], 
      "description": "Specifies the target UID."
    }
  }
}