背景：

  我们一般配置的Mongodb主从，或者Mongodb复制集，数据同步都是实时的。但如果在主节点上进行了错误的数据操作，这时候就会导致整个集群的数据都出错。因此，我们可以在一个集群中，挑选一个mongodb实例，用作复制延迟。当在主节点上误操作的时候，集群中有一个实例是不受影响的。这时候就可以利用这台不受影响的实例进行数据恢复。

  以上就是mongodb的延迟复制节点的功能，当主节点进行一次数据操作后，延迟复制节不立马进行数据同步操作，而是在一段时间后，才同步数据。

配置：

  以我的实验环境为例，以下为我的mongodb复制集：

cmh0:PRIMARY> rs.status()
{
        "set" : "cmh0",
        "date" : ISODate("2016-08-22T02:43:16.240Z"),
        "myState" : 1,
        "members" : [
                {
                        "_id" : 1,
                        "name" : "192.168.52.128:27017",
                        "health" : 1,
                        "state" : 1,
                        "stateStr" : "PRIMARY",
                        "uptime" : 82,
                        "optime" : Timestamp(1470581983, 1),
                        "optimeDate" : ISODate("2016-08-07T14:59:43Z"),
                        "electionTime" : Timestamp(1471833721, 1),
                        "electionDate" : ISODate("2016-08-22T02:42:01Z"),
                        "configVersion" : 1,
                        "self" : true
                },
                {
                        "_id" : 2,
                        "name" : "192.168.52.135:27017",
                        "health" : 1,
                        "state" : 2,
                        "stateStr" : "SECONDARY",
                        "uptime" : 71,
                        "optime" : Timestamp(1470581983, 1),
                        "optimeDate" : ISODate("2016-08-07T14:59:43Z"),
                        "lastHeartbeat" : ISODate("2016-08-22T02:43:15.138Z"),
                        "lastHeartbeatRecv" : ISODate("2016-08-22T02:43:14.978Z"),
                        "pingMs" : 0,
                        "lastHeartbeatMessage" : "could not find member to sync from",
                        "configVersion" : 1
                },
                {
                        "_id" : 3,
                        "name" : "192.168.52.135:27019",
                        "health" : 1,
                        "state" : 2,
                        "stateStr" : "SECONDARY",
                        "uptime" : 75,
                        "optime" : Timestamp(1470581983, 1),
                        "optimeDate" : ISODate("2016-08-07T14:59:43Z"),
                        "lastHeartbeat" : ISODate("2016-08-22T02:43:15.138Z"),
                        "lastHeartbeatRecv" : ISODate("2016-08-22T02:43:15.138Z"),
                        "pingMs" : 0,
                        "configVersion" : 1
                }
        ],
        "ok" : 1
}

  这时还未配置延迟复制节点，所以数据是实时同步的：

cmh0:PRIMARY> use cmhtest
switched to db cmhtest
cmh0:PRIMARY> db.cmh.insert({"name":"ChenMinghui"})
WriteResult({ "nInserted" : 1 })
cmh0:PRIMARY> rs.printReplicationInfo()
configured oplog size:   990MB
log length start to end: 195secs (0.05hrs)
oplog first event time:  Mon Aug 22 2016 10:51:22 GMT+0800 (CST)
oplog last event time:   Mon Aug 22 2016 10:54:37 GMT+0800 (CST)
now:                     Mon Aug 22 2016 10:55:00 GMT+0800 (CST)
cmh0:PRIMARY> rs.printSlaveReplicationInfo()
source: 192.168.52.135:27017
        syncedTo: Mon Aug 22 2016 10:54:37 GMT+0800 (CST)
        0 secs (0 hrs) behind the primary 
source: 192.168.52.135:27019
        syncedTo: Mon Aug 22 2016 10:54:37 GMT+0800 (CST)
        0 secs (0 hrs) behind the primary

  可以看到两个Secondary节点都在同一时间实时同步了数据。

  配置192.168.52.135:27017为延迟复制节点：

cmh0:PRIMARY> cfg=rs.conf();
{
        "_id" : "cmh0",
        "version" : 1,
        "members" : [
                {
                        "_id" : 1,
                        "host" : "192.168.52.128:27017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                        },
                        "slaveDelay" : 0,
                        "votes" : 1
                },
                {
                        "_id" : 2,
                        "host" : "192.168.52.135:27017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                        },
                        "slaveDelay" : 0,
                        "votes" : 1
                },
                {
                        "_id" : 3,
                        "host" : "192.168.52.135:27019",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                        },
                        "slaveDelay" : 0,
                        "votes" : 1
                }
        ],
        "settings" : {
                "chainingAllowed" : true,
                "heartbeatTimeoutSecs" : 10,
                "getLastErrorModes" : {
                },
                "getLastErrorDefaults" : {
                        "w" : 1,
                        "wtimeout" : 0
                }
        }
}
cmh0:PRIMARY> cfg.members[1].priority=0
0
cmh0:PRIMARY> cfg.members[1].slaveDelay=30
30
cmh0:PRIMARY> rs.reconfig(cfg);
{ "ok" : 1 }
cmh0:PRIMARY> rs.conf()
{
        "_id" : "cmh0",
        "version" : 2,
        "members" : [
                {
                        "_id" : 1,
                        "host" : "192.168.52.128:27017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                        },
                        "slaveDelay" : 0,
                        "votes" : 1
                },
                {
                        "_id" : 2,
                        "host" : "192.168.52.135:27017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 0,
                        "tags" : {
                        },
                        "slaveDelay" : 30,
                        "votes" : 1
                },
                {
                        "_id" : 3,
                        "host" : "192.168.52.135:27019",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                        },
                        "slaveDelay" : 0,
                        "votes" : 1
                }
        ],
        "settings" : {
                "chainingAllowed" : true,
                "heartbeatTimeoutSecs" : 10,
                "getLastErrorModes" : {
                },
                "getLastErrorDefaults" : {
                        "w" : 1,
                        "wtimeout" : 0
                }
        }
}

  可以看到192.168.52.135:27017节点出现了"slaveDelay":30的值，说明该节点的同步时间向后推迟了30秒。

  具体大家可以测试一下，延迟复制时间大概会在30秒左右。有一点要注意，mongodb的系统时间必须一致，否则会造成延迟复制异常，导致在规定同步时间到了之后不进行同步操作。