MongoDB 主从复制实例讲解
主从复制可以用来做数据库的备份,故障恢复,读写分离。
本实验使用Mongodb3.2版本,我们先查看一下mongod的帮助
[root@localhostmongodb]#mongod--help .....省略 Master/slaveoptions(old;usereplicasetsinstead): --mastermastermode --slaveslavemode --sourceargwhenslave:specifymasteras <server:port> --onlyargwhenslave:specifyasingledatabase toreplicate --slavedelayargspecifydelay(inseconds)tobeused whenapplyingmasteropstoslave --autoresyncautomaticallyresyncifslavedatais stale .....省略
主从复制已经是过期的功能,目前使用副本集代替。主从复制和副本集的区别,可以简单的理解成主从复制不能自动故障转移,副本集中的集群在主节点宕机后,可以使用选举的策略选择一个新的主节点出来。实现自动的故障转移。
从节点可以是一个,也可以是多个。
下面我们在一台机器上,使用两个实例的方式实现主从复制。
建立数据库目录
[root@localhostdata]#mkdir-p/application/mongodb/data/{master,slave}
2.启动master实例
[root@localhostdata]#mongod--dbpath=/application/mongodb/data/master/--port27017--master
--master指定该实例是主服务器。
3.启动从实例
[root@localhost~]#mongod--dbpath=/application/mongodb/data/slave/--port27018--slave--source127.0.0.1:27017
--slave指定该实例为从服务器
--source指定主服务器是谁?
从服务器启动后,即不断的想主服务器请求同步数据
2016-01-16T10:30:10.208+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:11.210+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:12.211+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:14.196+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:15.197+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:16.199+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:17.202+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:18.204+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:19.207+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:30:20.209+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017
至此,主从复制已经配置完成,就是这么的简单。
对于从服务器,还有三个参数需要解释一下。
--onlyarg
从节点指定只复制某个特定的数据库(默认复制所有数据库)
--slavedelayarg
指定从服务器延迟多久时间再同步,此选项在主服务器发生人为操作失误时,比较有用。发现错误时,从服务器还没有同步错误。这样可以避免错误的发生。
--autoresync
如果从节点的数据与主节点发生断裂(某些oplog中的数据还未被同步,即被覆盖了),那么该选项将是从节点自动的从新从头开始同步数据库。
下面我们验证一下,数据的同步是否有效。
在主库中插入数据。
[root@localhost~]#mongo127.0.0.1:27017 MongoDBshellversion:3.2.1 connectingto:127.0.0.1:27017/test >db.user.insert({"name":"jack","age":40,"job":"moivestar"}) WriteResult({"nInserted":1}) >db.user.insert({"name":"vicent","age":25,"job":"teacher"}) WriteResult({"nInserted":1})
登录从数据库,检查数据是否同步
[root@localhost~]#mongo127.0.0.1:27018 MongoDBshellversion:3.2.1 connectingto:127.0.0.1:27018/test >>db.user.find() {"_id":ObjectId("5699af720102a61caffb76e8"),"name":"jack","age":40,"job":"moivestar"} {"_id":ObjectId("5699af920102a61caffb76e9"),"name":"vicent","age":25,"job":"teacher"}
可以看到数据已经同步啦~
默认情况下,要想在从库开启查询功能,必须告知服务器,你接受从服务器的数据(有可能同步有延迟,数据不一致,你能够接受这种不一致)
>showcollections 2016-01-16T10:52:04.363+0800EQUERY[thread1]Error:listCollectionsfailed:{"ok":0,"errmsg":"notmasterandslaveOk=false","code":13435}: _getErrorWithCode@src/mongo/shell/utils.js:23:13 DB.prototype._getCollectionInfosCommand@src/mongo/shell/db.js:746:1 DB.prototype.getCollectionInfos@src/mongo/shell/db.js:758:15 DB.prototype.getCollectionNames@src/mongo/shell/db.js:769:12 shellHelper.show@src/mongo/shell/utils.js:695:9 shellHelper@src/mongo/shell/utils.js:594:15 @(shellhelp2):1:1
执行rs.slaveOK
>rs.slaveOk() >showcollections user >
在从服务的local数据库中有个sources集合,记录了主服务的信息
>uselocal switchedtodblocal >showcollections me sources startup_log >db.sources.find().pretty() { "_id":ObjectId("5699aaafa33311c25ab793df"), "host":"127.0.0.1:27017", "source":"main", "syncedTo":Timestamp(1452913003,1) }
我们再次启动从库时,就无需指定source参数啦。
[root@localhost~]#mongod--dbpath=/application/mongodb/data/slave/--port27018--slave 2016-01-16T10:57:45.965+0800ICONTROL[initandlisten]MongoDBstarting:pid=21820port=27018dbpath=/application/mongodb/data/slave/slave=164-bithost=localhost.localdomain 2016-01-16T10:57:45.967+0800ICONTROL[initandlisten]dbversionv3.2.1 2016-01-16T10:57:45.968+0800ICONTROL[initandlisten]gitversion:a14d55980c2cdc565d4704a7e3ad37e4e535c1b2 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]OpenSSLversion:OpenSSL1.0.1e-fips11Feb2013 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]allocator:tcmalloc 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]modules:none 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]buildenvironment: 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]distmod:rhel62 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]distarch:x86_64 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]target_arch:x86_64 2016-01-16T10:57:45.969+0800ICONTROL[initandlisten]options:{net:{port:27018},slave:true,storage:{dbPath:"/application/mongodb/data/slave/"}} 2016-01-16T10:57:46.010+0800I-[initandlisten]Detecteddatafilesin/application/mongodb/data/slave/createdbythe'wiredTiger'storageengine,sosettingtheactivestorageengineto'wiredTiger'. 2016-01-16T10:57:46.011+0800ISTORAGE[initandlisten]wiredtiger_openconfig:create,cache_size=1G,session_max=20000,eviction=(threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),checkpoint=(wait=60,log_size=2GB),statistics_log=(wait=0), 2016-01-16T10:57:48.485+0800ICONTROL[initandlisten]**WARNING:Youarerunningthisprocessastherootuser,whichisnotrecommended. 2016-01-16T10:57:48.486+0800ICONTROL[initandlisten] 2016-01-16T10:57:48.488+0800ICONTROL[initandlisten] 2016-01-16T10:57:48.490+0800ICONTROL[initandlisten]**WARNING:/sys/kernel/mm/transparent_hugepage/enabledis'always'. 2016-01-16T10:57:48.490+0800ICONTROL[initandlisten]**Wesuggestsettingitto'never' 2016-01-16T10:57:48.490+0800ICONTROL[initandlisten] 2016-01-16T10:57:48.490+0800ICONTROL[initandlisten]**WARNING:/sys/kernel/mm/transparent_hugepage/defragis'always'. 2016-01-16T10:57:48.490+0800ICONTROL[initandlisten]**Wesuggestsettingitto'never' 2016-01-16T10:57:48.490+0800ICONTROL[initandlisten] 2016-01-16T10:57:48.493+0800IFTDC[initandlisten]Initializingfull-timediagnosticdatacapturewithdirectory'/application/mongodb/data/slave/diagnostic.data' 2016-01-16T10:57:48.494+0800INETWORK[initandlisten]waitingforconnectionsonport27018 2016-01-16T10:57:48.495+0800INETWORK[HostnameCanonicalizationWorker]Startinghostnamecanonicalizationworker 2016-01-16T10:57:49.497+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:57:50.503+0800IREPL[replslave]sleep1secbeforenextpass 2016-01-16T10:57:51.504+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:57:52.505+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:57:54.295+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017 2016-01-16T10:57:55.296+0800IREPL[replslave]syncingfromhost:127.0.0.1:27017
主从库之间利用oplog日志进行同步。oplog存在于主库的local数据库,oplog.$main集合。
>uselocal switchedtodblocal >db.oplog.$main.find({"op":"i"}).sort({"ts":-1}).pretty() { "ts":Timestamp(1452916694,1), "h":NumberLong(0), "v":2, "op":"i", "ns":"test.user", "o":{ "_id":ObjectId("5699bfd6647c735cb3a50e0c"), "name":"zhangcong" } } { "ts":Timestamp(1452913156,1), "h":NumberLong(0), "v":2, "op":"i", "ns":"test.user", "o":{ "_id":ObjectId("5699b204358c4672cad1cc6e"), "name":"zhangdd", "age":30, "job":"teacher" } } { "ts":Timestamp(1452912530,1), "h":NumberLong(0), "v":2, "op":"i", "ns":"test.user", "o":{ "_id":ObjectId("5699af920102a61caffb76e9"), "name":"vicent", "age":25, "job":"teacher" } } { "ts":Timestamp(1452912498,2), "h":NumberLong(0), "v":2, "op":"i", "ns":"test.user", "o":{ "_id":ObjectId("5699af720102a61caffb76e8"), "name":"jack", "age":40, "job":"moivestar" } }
该集合属于固定集合。在一定时间后,旧日志会被覆盖。如果日志已经被覆盖,从库还没有来的及同步。那么从库就无法再同步数据了。只有使用--autoresync让其重新同步数据。
备注:命令行参数指定的参数值,可以写到config文件中,启动时使用
mongod--config/path/to/file.conf
mongod2.4以后的版本使用YAML的格式来编写配置文件。关于主从复制的配置如何在配置文件中声明,官方文件没有给出方法。试了几种写法都不正确。因为mongodb使用副本集代替了主从复制,从而可能配置文件不再支持主从复制。
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