static long getXid(InstanceConfig instanceConfig) {
    String xid = instanceConfig.getRecord().getSimpleField(XID_STR);
    return xid == null ? DEFAULT_XID : Long.parseLong(xid);
  }
Get the xid associated with this instance. The xid is like a timestamp or a change number, so if it is absent,
a value representing the earliest point in time is returned.
@param instanceConfig the {@link InstanceConfig} associated with the interested instance.
@return the xid associated with the given instance.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/ClusterMapUtils.java