前言副本集整理 。开始逐步把mongodb博客补齐了 。
正文什么是副本集副本集是一组服务器，其中一个是用于处理写入操作的主节点，还有多个用于保存主节点的数据副本的从节点 。
如果主节点崩溃了 ， 则从节点会从其中选取出一个新的主节点 。
作用起到一个热备份 和 容灾的作用，如果出现不可预料的事故，比如主节点磁盘损坏，那么可以故障转移，其他节点将会提到主节点进行写入 。
实验现在一台机器上演示 。
步骤一创建对应的目录:
数据:
mkdir -p ~/data/rs{1,2,3}日志:
mkdir -p ~/logs/rs{1,2,3}步骤二启动3个mongod:
mongod --replSet mydb --dbpath ~/data/rs1 --logpath ~/logs/rs1/log --port 27017 --smallfiles --oplogSize=200 &mongod --replSet mydb --dbpath ~/data/rs2 --logpath ~/logs/rs2/log --port 27018 --smallfiles --oplogSize=200 &mongod --replSet mydb --dbpath ~/data/rs3 --logpath ~/logs/rs3/log --port 27019 --smallfiles --oplogSize=200 &步骤三把副本连接到一起，副本集配置传递:
mongo --port 27017初始化副本集配置:
rsconf={ "_id" : "mydb", "members" : [{"_id" : 0,"host" : "localhost:27017"},{"_id" : 1,"host" : "localhost:27018"},{"_id" : 2,"host" : "localhost:27019"} ]}rs.initiate(rsconf)这个配置文档就是副本集的配置 。在localhost:27017 上运行的成员会解析配置并将消息发送给其他成员，提醒他们存在新的配置 。一旦所有成员都加载了配置，他们就会选择一个主节点并开始处理读写操作 。
注意：
不能在不停机的情况下将单机服务器转换为副本集，以重新启动并初始化该副本集 。因此，即使一开始只有一台服务器 ， 你也希望将其配置为一个单成员的副本集 。这样，如果以后想添加更多成员 ， 则可以在不停止运行的情况下进行添加 。然后使用rs.status()查看副本集的状态
{ "set" : "mydb", "date" : ISODate("2022-10-16T02:51:57.670Z"), "myState" : 1, "term" : NumberLong(1), "syncingTo" : "", "syncSourceHost" : "", "syncSourceId" : -1, "heartbeatIntervalMillis" : NumberLong(2000), "optimes" : {"lastCommittedOpTime" : {"ts" : Timestamp(1665888717, 1),"t" : NumberLong(1)},"readConcernMajorityOpTime" : {"ts" : Timestamp(1665888717, 1),"t" : NumberLong(1)},"appliedOpTime" : {"ts" : Timestamp(1665888717, 1),"t" : NumberLong(1)},"durableOpTime" : {"ts" : Timestamp(1665888717, 1),"t" : NumberLong(1)} }, "members" : [{"_id" : 0,"name" : "localhost:27017","health" : 1,"state" : 1,"stateStr" : "PRIMARY","uptime" : 955,"optime" : {"ts" : Timestamp(1665888717, 1),"t" : NumberLong(1)},"optimeDate" : ISODate("2022-10-16T02:51:57Z"),"syncingTo" : "","syncSourceHost" : "","syncSourceId" : -1,"infoMessage" : "","electionTime" : Timestamp(1665888285, 1),"electionDate" : ISODate("2022-10-16T02:44:45Z"),"configVersion" : 1,"self" : true,"lastHeartbeatMessage" : ""},{"_id" : 1,"name" : "localhost:27018","health" : 1,"state" : 2,"stateStr" : "SECONDARY","uptime" : 443,"optime" : {"ts" : Timestamp(1665888707, 1),"t" : NumberLong(1)},"optimeDurable" : {"ts" : Timestamp(1665888707, 1),"t" : NumberLong(1)},"optimeDate" : ISODate("2022-10-16T02:51:47Z"),"optimeDurableDate" : ISODate("2022-10-16T02:51:47Z"),"lastHeartbeat" : ISODate("2022-10-16T02:51:55.983Z"),"lastHeartbeatRecv" : ISODate("2022-10-16T02:51:57.128Z"),"pingMs" : NumberLong(0),"lastHeartbeatMessage" : "","syncingTo" : "localhost:27017","syncSourceHost" : "localhost:27017","syncSourceId" : 0,"infoMessage" : "","configVersion" : 1},{"_id" : 2,"name" : "localhost:27019","health" : 1,"state" : 2,"stateStr" : "SECONDARY","uptime" : 443,"optime" : {"ts" : Timestamp(1665888707, 1),"t" : NumberLong(1)},"optimeDurable" : {"ts" : Timestamp(1665888707, 1),"t" : NumberLong(1)},"optimeDate" : ISODate("2022-10-16T02:51:47Z"),"optimeDurableDate" : ISODate("2022-10-16T02:51:47Z"),"lastHeartbeat" : ISODate("2022-10-16T02:51:56.020Z"),"lastHeartbeatRecv" : ISODate("2022-10-16T02:51:57.065Z"),"pingMs" : NumberLong(0),"lastHeartbeatMessage" : "","syncingTo" : "localhost:27017","syncSourceHost" : "localhost:27017","syncSourceId" : 0,"infoMessage" : "","configVersion" : 1} ], "ok" : 1, "operationTime" : Timestamp(1665888717, 1), "$clusterTime" : {"clusterTime" : Timestamp(1665888717, 1),"signature" : {"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),"keyId" : NumberLong(0)} }}这样就可以看到27017 是主节点，其他的不是 。
然后rs 是一个mongo 命令的封装:
mydb:PRIMARY> rs.help() rs.status(){ replSetGetStatus : 1 } checks repl set status rs.initiate(){ replSetInitiate : null } initiates set with default settings rs.initiate(cfg){ replSetInitiate : cfg } initiates set with configuration cfg rs.conf()get the current configuration object from local.system.replset rs.reconfig(cfg)updates the configuration of a running replica set with cfg (disconnects) rs.add(hostportstr)add a new member to the set with default attributes (disconnects) rs.add(membercfgobj)add a new member to the set with extra attributes (disconnects) rs.addArb(hostportstr)add a new member which is arbiterOnly:true (disconnects) rs.stepDown([stepdownSecs, catchUpSecs])step down as primary (disconnects) rs.syncFrom(hostportstr)make a secondary sync from the given member rs.freeze(secs)make a node ineligible to become primary for the time specified rs.remove(hostportstr)remove a host from the replica set (disconnects) rs.secondaryOk()allow queries on secondary nodes rs.printReplicationInfo()check oplog size and time range rs.printSecondaryReplicationInfo()check replica set members and replication lag db.isMaster()check who is primary db.hello()check who is primary reconfiguration helpers disconnect from the database so the shell will display an error, even if the command succeeds.