How to check secondary is synced now or not

Question

there is the replica with three member (primary,secondary,secondary). Suppose one of secondaries down for a day, after return secondary back to replica how can i find, i

Answer 1

Note: Be sure to check the answer provided by arcseldon for an user friendly equivalent.

You can use output of rs.status(). If secondary is synced and wasn't created with slaveDelay option then optime and optimeDate of secondary should be equal or close (if there are current operations) to those of primary. In that case stateStr should be equal to SECONDARY. So if secondary is synced you should see output similar to this (one member has been removed from output for clarity):

 {
    "set" : "rs0",
    "date" : ISODate("2013-11-08T14:58:49Z"),
    "myState" : 1,
    "members" : [
        {
            "_id" : 0,
            "name" : "hostname:27001",
            "health" : 1,
            "state" : 1,
            "stateStr" : "PRIMARY",
            "uptime" : 155,
            "optime" : Timestamp(1383915748, 1),
            "optimeDate" : ISODate("2013-11-08T13:02:28Z"),
            "self" : true
        },

        {
            "_id" : 2,
            "name" : "hostname:27003",
            "health" : 0,
            "state" : 8,
            "stateStr" : "SECONDARY",
            "uptime" : 0,
            "optime" : Timestamp(1383915748, 1),
            "optimeDate" : ISODate("2013-11-08T13:02:28Z"),
            "lastHeartbeat" : ISODate("2013-11-08T14:58:48Z"),
            "lastHeartbeatRecv" : ISODate("2013-11-08T14:58:42Z"),
            "pingMs" : 0,
            "syncingTo" : "hostname:27001"
        }
    ],
    "ok" : 1
}

Here you have output of rs.status() for the same replica set if one of secondaries is not synced. First of all you'll see that optime and optimeDate for hostname:27003 differs from primary, stateStr is set to RECOVERING and there is appropriate lastHeartbeatMessage.

{
    "set" : "rs0",
    "date" : ISODate("2013-11-08T15:01:34Z"),
    "myState" : 1,
    "members" : [
        {
            "_id" : 0,
            "name" : "hostname:27001",
            "health" : 1,
            "state" : 1,
            "stateStr" : "PRIMARY",
            "uptime" : 320,
            "optime" : Timestamp(1383922858, 767),
            "optimeDate" : ISODate("2013-11-08T15:00:58Z"),
            "self" : true
        },

        {
            "_id" : 2,
            "name" : "hostname:27003",
            "health" : 1,
            "state" : 3,
            "stateStr" : "RECOVERING",
            "uptime" : 14,
            "optime" : Timestamp(1383915748, 1),
            "optimeDate" : ISODate("2013-11-08T13:02:28Z"),
            "lastHeartbeat" : ISODate("2013-11-08T15:01:34Z"),
            "lastHeartbeatRecv" : ISODate("2013-11-08T15:01:34Z"),
            "pingMs" : 0,
            "lastHeartbeatMessage" : "still syncing, not yet to minValid optime 527cfc90:19c4",
            "syncingTo" : "hostname:27001"
        }
    ],
    "ok" : 1
}

If secondary has been created with slaveDelay then optime and optimeDate can be different but stateStr and lastHeartbeatMessage will indicate if there is some lag.