docs: def _HostPrefix(client_id):
    """Build a host prefix for a notification message based on a client id."""
    if not client_id:
      return ""
  
    hostname = None
    if data_store.RelationalDBEnabled():
      client_snapshot = data_store.REL_DB.ReadClientSnapshot(client_id)
      if client_snapshot:
        hostname = client_snapshot.knowledge_base.fqdn
    else:
      client_fd = aff4.FACTORY.Open(client_id, mode="rw")
      hostname = client_fd.Get(client_fd.Schema.FQDN) or ""
  
    if hostname:
      return "%s: " % hostname
    else:
      return ""