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。
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