Testing for repl_lag This script will display the difference between the optime of current node and the primary node in a replSet. login to your server `mongo` and run `rs.repl_lag()`

rs.repl_lag.js

rs.repl_lag = function () {
  var s = db._adminCommand("replSetGetStatus");

  // if this is primary, go no further.
  if (s.myState == 1) {
    return 0;
  }

  var mytime = 0;
  var primarytime = 0;

  for (var i in s.members) {
    if (s.members[i].self == true) {
      mytime = s.members[i].optime.t;
    }

    if (s.members[i].stateStr == "PRIMARY") {
      primarytime = s.members[i].optime.t;
    }

    if (mytime > 0 && primarytime > 0) {
      break;
    }
  }

  return (primarytime - mytime) / 1000;
}