MongoDB 主從復制實例講解

主從復制可以用來做數據庫的備份，故障恢復，讀寫分離。

本實驗使用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

主從復制已經是過期的功能，目前使用副本集代替。主從復制和副本集的區別，可以簡單的理解成主從復制不能自動故障轉移，副本集中的集群在主節點宕機後，可以使用選舉的策略選擇一個新的主節點出來。實現自動的故障轉移。

從節點可以是一個，也可以是多個。

下面我們在一台機器上，使用兩個實例的方式實現主從復制。

建立數據庫目錄

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

2. 啟動master實例

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

--master 指定該實例是主服務器 。

3. 啟動從實例

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

--slave 指定該實例為從服務器
--source 指定主服務器是誰？

從服務器啟動後，即不斷的想主服務器請求同步數據

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，主從復制已經配置完成，就是這麼的簡單。

對於從服務器，還有三個參數需要解釋一下。

  --only arg

 從節點指定只復制某個特定的數據庫(默認復制所有數據庫)

  --slavedelay arg

指定從服務器延遲多久時間再同步，此選項在主服務器發生人為操作失誤時，比較有用。發現錯誤時，從服務器還沒有同步錯誤。這樣可以避免錯誤的發生。

  --autoresync

如果從節點的數據與主節點發生斷裂（某些oplog中的數據還未被同步，即被覆蓋了），那麼該選項將是從節點自動的從新從頭開始同步數據庫。

下面我們驗證一下，數據的同步是否有效。
在主庫中插入數據。

[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 })

登錄從數據庫，檢查數據是否同步

[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" }

可以看到數據已經同步啦~

默認情況下，要想在從庫開啟查詢功能，必須告知服務器，你接受從服務器的數據（有可能同步有延遲，數據不一致，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

執行rs.slaveOK

> rs.slaveOk()
> show collections
user
>

在從服務的local數據庫中有個sources集合，記錄了主服務的信息

> 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)
}

我們再次啟動從庫時，就無需指定source參數啦。

[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

主從庫之間利用oplog日志進行同步。oplog存在於主庫的local數據庫，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"
  }
}

該集合屬於固定集合。在一定時間後，舊日志會被覆蓋。如果日志已經被覆蓋，從庫還沒有來的及同步。那麼從庫就無法再同步數據了。只有使用--autoresync讓其重新同步數據。

備注：命令行參數指定的參數值，可以寫到config文件中，啟動時使用

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

mongod 2.4以後的版本使用YAML的格式來編寫配置文件。關於主從復制的配置如何在配置文件中聲明，官方文件沒有給出方法。試了幾種寫法都不正確。 因為mongodb使用副本集代替了主從復制，從而可能配置文件不再支持主從復制。

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