本文共 6979 字，大约阅读时间需要 23 分钟。
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 cmhtestswitched to db cmhtestcmh0:PRIMARY> db.cmh.insert({"name":"ChenMinghui"})WriteResult({ "nInserted" : 1 })cmh0:PRIMARY> rs.printReplicationInfo()configured oplog size: 990MBlog 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=00cmh0:PRIMARY> cfg.members[1].slaveDelay=3030cmh0: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。
转载地址：http://pezjl.baihongyu.com/