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复制集是一个带有故障转移的主从集群。是从现有的主从模式演变而来，增加了自动故障转移和节点成员自动恢复。
复制集模式中没有固定的主结点，在启动后，多个服务节点间将自动选举 产生一个主结点。该主结点被称为primary，一个或多个从结点被称为secondaries。primary结点基本上就是master结点，不同之 处在于primary结点在不同时间可能是不同的服务器。如果当前的主结点失效了，复制集中的其余结点将会试图选出一个新的主结点。
复制集模式的好处是，一切自动化。首先，复制集模式本身做了大量的管理工作，自动管理从节点，确保数据不会不一致。其次，主节点挂掉后，会自动判断集群中的服务器并进行故障转移，推举新的主节点。
一个复制集集群支持1-7台服务器，在一个复制集中各个服务器数据保持完全一致。
在一个复制集集群中，各个服务器有以下几种状态：
复制集的从节点读请求，也是在各个Driver层设置slaveOk的值来实现的。
/usr/local/mongodb/mongod -dbpath=/data/mongodbreplset/19 --fork --port 27019 --logpath=/usr/local/mongodb/log/mongodbreplset/m19.log --logappend --replSet rsdemo --smallfiles /usr/local/mongodb/mongod -dbpath=/data/mongodbreplset/18 --fork --port 27018 --logpath=/usr/local/mongodb/log/mongodbreplset/m18.log --logappend --replSet rsdemo --smallfiles /usr/local/mongodb/mongod -dbpath=/data/mongodbreplset/17 --fork --port 27017 --logpath=/usr/local/mongodb/log/mongodbreplset/m17.log --logappend --replSet rsdemo --smallfiles
rsdemo:PRIMARY> var rsconf={ ... _id:‘rsdemo‘, ... members:[ ... {_id:0,host:‘192.168.0.108:27017‘}, ... {_id:1,host:‘192.168.0.108:27018‘}, ... {_id:2,host:‘192.168.0.108:27019‘} ... ]} rsdemo:PRIMARY> rs.reconfig(rs) rs( rsconf rsdemo:PRIMARY> rs.reconfig(rsconf) //rs.initiate(rsconf) { "ok" : 1 }
rsdemo:PRIMARY> rs.status() { "set" : "rsdemo", "date" : ISODate("2015-11-16T14:19:12Z"), "myState" : 1, "members" : [ { "_id" : 0, "name" : "192.168.0.108:27017", "health" : 1, "state" : 2, "stateStr" : "SECONDARY", "uptime" : 880, "optime" : Timestamp(1447683546, 1), "optimeDate" : ISODate("2015-11-16T14:19:06Z"), "lastHeartbeat" : ISODate("2015-11-16T14:19:10Z"), "lastHeartbeatRecv" : ISODate("2015-11-16T14:19:11Z"), "pingMs" : 0, "syncingTo" : "192.168.0.108:27019" }, { "_id" : 1, "name" : "192.168.0.108:27018", "health" : 1, "state" : 2, "stateStr" : "SECONDARY", "uptime" : 916, "optime" : Timestamp(1447683546, 1), "optimeDate" : ISODate("2015-11-16T14:19:06Z"), "lastHeartbeat" : ISODate("2015-11-16T14:19:11Z"), "lastHeartbeatRecv" : ISODate("2015-11-16T14:19:11Z"), "pingMs" : 0, "syncingTo" : "192.168.0.108:27019" }, { "_id" : 2, "name" : "192.168.0.108:27019", "health" : 1, "state" : 1, "stateStr" : "PRIMARY", "uptime" : 940, "optime" : Timestamp(1447683546, 1), "optimeDate" : ISODate("2015-11-16T14:19:06Z"), "electionTime" : Timestamp(1447682643, 1), "electionDate" : ISODate("2015-11-16T14:04:03Z"), "self" : true } ], "ok" : 1 }
rsdemo:PRIMARY> rs.conf() { "_id" : "rsdemo", "version" : 3, "members" : [ { "_id" : 0, "host" : "192.168.0.108:27017" }, { "_id" : 1, "host" : "192.168.0.108:27018" }, { "_id" : 2, "host" : "192.168.0.108:27019" } ] } rsdemo:PRIMARY>
rsdemo:PRIMARY> rs.isMaster( ... ) { "setName" : "rsdemo", "setVersion" : 3, "ismaster" : true, "secondary" : false, "hosts" : [ "192.168.0.108:27019", "192.168.0.108:27018", "192.168.0.108:27017" ], "primary" : "192.168.0.108:27019", "me" : "192.168.0.108:27019", "maxBsonObjectSize" : 16777216, "maxMessageSizeBytes" : 48000000, "maxWriteBatchSize" : 1000, "localTime" : ISODate("2015-11-16T14:51:16.795Z"), "maxWireVersion" : 2, "minWireVersion" : 0, "ok" : 1 }
rsdemo:PRIMARY> for(var i=0;i<1000;i++){db.demo.insert({count:i})} WriteResult({ "nInserted" : 1 }) rsdemo:PRIMARY> show dbs admin (empty) demo 0.031GB local 0.094GB rsdemo:PRIMARY> use demo switched to db demo rsdemo:PRIMARY> db.demo.find() { "_id" : ObjectId("563e24b2a731ac9621c37650"), "age" : 18, "name" : "赵兴壮" } { "_id" : ObjectId("5649e70fff8e3af9408ff52f"), "count" : 0 } { "_id" : ObjectId("5649e70fff8e3af9408ff530"), "count" : 1 } { "_id" : ObjectId("5649e70fff8e3af9408ff531"), "count" : 2 } { "_id" : ObjectId("5649e70fff8e3af9408ff532"), "count" : 3 } { "_id" : ObjectId("5649e70fff8e3af9408ff533"), "count" : 4 } { "_id" : ObjectId("5649e70fff8e3af9408ff534"), "count" : 5 } { "_id" : ObjectId("5649e70fff8e3af9408ff535"), "count" : 6 } { "_id" : ObjectId("5649e70fff8e3af9408ff536"), "count" : 7 } { "_id" : ObjectId("5649e70fff8e3af9408ff537"), "count" : 8 } { "_id" : ObjectId("5649e70fff8e3af9408ff538"), "count" : 9 } { "_id" : ObjectId("5649e70fff8e3af9408ff539"), "count" : 10 } { "_id" : ObjectId("5649e70fff8e3af9408ff53a"), "count" : 11 } { "_id" : ObjectId("5649e70fff8e3af9408ff53b"), "count" : 12 } { "_id" : ObjectId("5649e70fff8e3af9408ff53c"), "count" : 13 } { "_id" : ObjectId("5649e70fff8e3af9408ff53d"), "count" : 14 } { "_id" : ObjectId("5649e70fff8e3af9408ff53e"), "count" : 15 } { "_id" : ObjectId("5649e70fff8e3af9408ff53f"), "count" : 16 } { "_id" : ObjectId("5649e70fff8e3af9408ff540"), "count" : 17 } { "_id" : ObjectId("5649e70fff8e3af9408ff541"), "count" : 18 } Type "it" for more
rsdemo:SECONDARY> rs.slaveOk()// 注意 rsdemo:SECONDARY> db.demo.find() { "_id" : ObjectId("563e24b2a731ac9621c37650"), "age" : 18, "name" : "赵兴壮" } { "_id" : ObjectId("5649e70fff8e3af9408ff52f"), "count" : 0 } { "_id" : ObjectId("5649e70fff8e3af9408ff530"), "count" : 1 } { "_id" : ObjectId("5649e70fff8e3af9408ff531"), "count" : 2 } { "_id" : ObjectId("5649e70fff8e3af9408ff532"), "count" : 3 } { "_id" : ObjectId("5649e70fff8e3af9408ff533"), "count" : 4 } { "_id" : ObjectId("5649e70fff8e3af9408ff534"), "count" : 5 } { "_id" : ObjectId("5649e70fff8e3af9408ff535"), "count" : 6 } { "_id" : ObjectId("5649e70fff8e3af9408ff536"), "count" : 7 } { "_id" : ObjectId("5649e70fff8e3af9408ff537"), "count" : 8 } { "_id" : ObjectId("5649e70fff8e3af9408ff538"), "count" : 9 } { "_id" : ObjectId("5649e70fff8e3af9408ff539"), "count" : 10 } { "_id" : ObjectId("5649e70fff8e3af9408ff53a"), "count" : 11 } { "_id" : ObjectId("5649e70fff8e3af9408ff53b"), "count" : 12 } { "_id" : ObjectId("5649e70fff8e3af9408ff53c"), "count" : 13 } { "_id" : ObjectId("5649e70fff8e3af9408ff53d"), "count" : 14 } { "_id" : ObjectId("5649e70fff8e3af9408ff53e"), "count" : 15 } { "_id" : ObjectId("5649e70fff8e3af9408ff53f"), "count" : 16 } { "_id" : ObjectId("5649e70fff8e3af9408ff540"), "count" : 17 } { "_id" : ObjectId("5649e70fff8e3af9408ff541"), "count" : 18 } Type "it" for more rsdemo:SECONDARY>
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原文地址：http://www.cnblogs.com/timelesszhuang/p/4970369.html
