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MongoDB 主從復(fù)制實(shí)例講解

主從復(fù)制可以用來(lái)做數(shù)據(jù)庫(kù)的備份，故障恢復(fù)，讀寫分離。

本實(shí)驗(yàn)使用Mongodb 3.2版本，我們先查看一下mongod的幫助

[root@localhost mongodb]# mongod --help
.....省略
Master/slave options (old; use replica sets instead):
 --master               master mode
 --slave                slave mode
 --source arg             when slave: specify master as 
                    server:port>
 --only arg              when slave: specify a single database 
                    to replicate
 --slavedelay arg           specify delay (in seconds) to be used 
                    when applying master ops to slave
 --autoresync             automatically resync if slave data is 
                    stale
.....省略

主從復(fù)制已經(jīng)是過(guò)期的功能，目前使用副本集代替。主從復(fù)制和副本集的區(qū)別，可以簡(jiǎn)單的理解成主從復(fù)制不能自動(dòng)故障轉(zhuǎn)移，副本集中的集群在主節(jié)點(diǎn)宕機(jī)后，可以使用選舉的策略選擇一個(gè)新的主節(jié)點(diǎn)出來(lái)。實(shí)現(xiàn)自動(dòng)的故障轉(zhuǎn)移。

從節(jié)點(diǎn)可以是一個(gè)，也可以是多個(gè)。

下面我們?cè)谝慌_(tái)機(jī)器上，使用兩個(gè)實(shí)例的方式實(shí)現(xiàn)主從復(fù)制。

建立數(shù)據(jù)庫(kù)目錄

[root@localhost data]# mkdir -p /application/mongodb/data/{master,slave}

2. 啟動(dòng)master實(shí)例

[root@localhost data]# mongod --dbpath=/application/mongodb/data/master/ --port 27017 --master

--master 指定該實(shí)例是主服務(wù)器。

3. 啟動(dòng)從實(shí)例

[root@localhost ~]# mongod --dbpath=/application/mongodb/data/slave/ --port 27018 --slave --source 127.0.0.1:27017

--slave 指定該實(shí)例為從服務(wù)器
--source 指定主服務(wù)器是誰(shuí)？

從服務(wù)器啟動(dòng)后，即不斷的想主服務(wù)器請(qǐng)求同步數(shù)據(jù)

2016-01-16T10:30:10.208+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:11.210+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:12.211+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:14.196+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:15.197+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:16.199+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:17.202+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:18.204+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:19.207+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:30:20.209+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017

至此，主從復(fù)制已經(jīng)配置完成，就是這么的簡(jiǎn)單。

對(duì)于從服務(wù)器，還有三個(gè)參數(shù)需要解釋一下。

--only arg

從節(jié)點(diǎn)指定只復(fù)制某個(gè)特定的數(shù)據(jù)庫(kù)(默認(rèn)復(fù)制所有數(shù)據(jù)庫(kù))

--slavedelay arg

指定從服務(wù)器延遲多久時(shí)間再同步，此選項(xiàng)在主服務(wù)器發(fā)生人為操作失誤時(shí)，比較有用。發(fā)現(xiàn)錯(cuò)誤時(shí)，從服務(wù)器還沒(méi)有同步錯(cuò)誤。這樣可以避免錯(cuò)誤的發(fā)生。

--autoresync

如果從節(jié)點(diǎn)的數(shù)據(jù)與主節(jié)點(diǎn)發(fā)生斷裂（某些oplog中的數(shù)據(jù)還未被同步，即被覆蓋了），那么該選項(xiàng)將是從節(jié)點(diǎn)自動(dòng)的從新從頭開(kāi)始同步數(shù)據(jù)庫(kù)。

下面我們驗(yàn)證一下，數(shù)據(jù)的同步是否有效。
在主庫(kù)中插入數(shù)據(jù)。

[root@localhost ~]# mongo 127.0.0.1:27017
MongoDB shell version: 3.2.1
connecting to: 127.0.0.1:27017/test
> db.user.insert({"name":"jack","age":40,"job":"moive star"})
WriteResult({ "nInserted" : 1 })
> db.user.insert({"name":"vicent","age":25,"job":"teacher"})
WriteResult({ "nInserted" : 1 })

登錄從數(shù)據(jù)庫(kù)，檢查數(shù)據(jù)是否同步

[root@localhost ~]# mongo 127.0.0.1:27018
MongoDB shell version: 3.2.1
connecting to: 127.0.0.1:27018/test
> > db.user.find()
{ "_id" : ObjectId("5699af720102a61caffb76e8"), "name" : "jack", "age" : 40, "job" : "moive star" }
{ "_id" : ObjectId("5699af920102a61caffb76e9"), "name" : "vicent", "age" : 25, "job" : "teacher" }

可以看到數(shù)據(jù)已經(jīng)同步啦~

默認(rèn)情況下，要想在從庫(kù)開(kāi)啟查詢功能，必須告知服務(wù)器，你接受從服務(wù)器的數(shù)據(jù)（有可能同步有延遲，數(shù)據(jù)不一致，你能夠接受這種不一致）

> show collections
2016-01-16T10:52:04.363+0800 E QUERY  [thread1] Error: listCollections failed: { "ok" : 0, "errmsg" : "not master and slaveOk=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

執(zhí)行rs.slaveOK

> rs.slaveOk()
> show collections
user
>

在從服務(wù)的local數(shù)據(jù)庫(kù)中有個(gè)sources集合，記錄了主服務(wù)的信息

> use local
switched to db local
> show collections
me
sources
startup_log
> db.sources.find().pretty()
{
  "_id" : ObjectId("5699aaafa33311c25ab793df"),
  "host" : "127.0.0.1:27017",
  "source" : "main",
  "syncedTo" : Timestamp(1452913003, 1)
}

我們?cè)俅螁?dòng)從庫(kù)時(shí)，就無(wú)需指定source參數(shù)啦。

[root@localhost ~]# mongod --dbpath=/application/mongodb/data/slave/ --port 27018 --slave
2016-01-16T10:57:45.965+0800 I CONTROL [initandlisten] MongoDB starting : pid=21820 port=27018 dbpath=/application/mongodb/data/slave/ slave=1 64-bit host=localhost.localdomain
2016-01-16T10:57:45.967+0800 I CONTROL [initandlisten] db version v3.2.1
2016-01-16T10:57:45.968+0800 I CONTROL [initandlisten] git version: a14d55980c2cdc565d4704a7e3ad37e4e535c1b2
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] OpenSSL version: OpenSSL 1.0.1e-fips 11 Feb 2013
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] allocator: tcmalloc
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] modules: none
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] build environment:
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten]   distmod: rhel62
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten]   distarch: x86_64
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten]   target_arch: x86_64
2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] options: { net: { port: 27018 }, slave: true, storage: { dbPath: "/application/mongodb/data/slave/" } }
2016-01-16T10:57:46.010+0800 I -    [initandlisten] Detected data files in /application/mongodb/data/slave/ created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wiredTiger'.
2016-01-16T10:57:46.011+0800 I STORAGE [initandlisten] wiredtiger_open config: 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+0800 I CONTROL [initandlisten] ** WARNING: You are running this process as the root user, which is not recommended.
2016-01-16T10:57:48.486+0800 I CONTROL [initandlisten] 
2016-01-16T10:57:48.488+0800 I CONTROL [initandlisten] 
2016-01-16T10:57:48.490+0800 I CONTROL [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/enabled is 'always'.
2016-01-16T10:57:48.490+0800 I CONTROL [initandlisten] **    We suggest setting it to 'never'
2016-01-16T10:57:48.490+0800 I CONTROL [initandlisten] 
2016-01-16T10:57:48.490+0800 I CONTROL [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2016-01-16T10:57:48.490+0800 I CONTROL [initandlisten] **    We suggest setting it to 'never'
2016-01-16T10:57:48.490+0800 I CONTROL [initandlisten] 
2016-01-16T10:57:48.493+0800 I FTDC   [initandlisten] Initializing full-time diagnostic data capture with directory '/application/mongodb/data/slave/diagnostic.data'
2016-01-16T10:57:48.494+0800 I NETWORK [initandlisten] waiting for connections on port 27018
2016-01-16T10:57:48.495+0800 I NETWORK [HostnameCanonicalizationWorker] Starting hostname canonicalization worker
2016-01-16T10:57:49.497+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:57:50.503+0800 I REPL   [replslave] sleep 1 sec before next pass
2016-01-16T10:57:51.504+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:57:52.505+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:57:54.295+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017
2016-01-16T10:57:55.296+0800 I REPL   [replslave] syncing from host:127.0.0.1:27017

主從庫(kù)之間利用oplog日志進(jìn)行同步。oplog存在于主庫(kù)的local數(shù)據(jù)庫(kù)，oplog.$main集合。

> use local
switched to db local
> 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" : "moive star"
  }
}

該集合屬于固定集合。在一定時(shí)間后，舊日志會(huì)被覆蓋。如果日志已經(jīng)被覆蓋，從庫(kù)還沒(méi)有來(lái)的及同步。那么從庫(kù)就無(wú)法再同步數(shù)據(jù)了。只有使用--autoresync讓其重新同步數(shù)據(jù)。

備注：命令行參數(shù)指定的參數(shù)值，可以寫到config文件中，啟動(dòng)時(shí)使用

mongod --config /path/to/file.conf

mongod 2.4以后的版本使用YAML的格式來(lái)編寫配置文件。關(guān)于主從復(fù)制的配置如何在配置文件中聲明，官方文件沒(méi)有給出方法。試了幾種寫法都不正確。因?yàn)閙ongodb使用副本集代替了主從復(fù)制，從而可能配置文件不再支持主從復(fù)制。

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