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后，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。
下面我們在一臺機器上，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？
從服務器啟動后，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:127.0.0.1:270172016-01-16T10:30:11.210+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:12.211+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:14.196+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:15.197+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:16.199+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:17.202+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:18.204+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:19.207+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:30:20.209+0800 I REPL [replslave] syncing from host:127.0.0.1:27017
至此，主從復制已經配置完成，就是這么的簡單。
對于從服務器，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），那么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 127.0.0.1:27017MongoDB shell version: 3.2.1connecting 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 127.0.0.1:27018MongoDB shell version: 3.2.1connecting 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 collections2016-01-16T10:52:04.363+0800 E QUERY [thread1] Error: listCollections failed: { "ok" : 0, "errmsg" : "not master and slaveOk=false", "code" : 13435 } :_getErrorWithCode@src/mongo/shell/utils.js:23:13DB.prototype._getCollectionInfosCommand@src/mongo/shell/db.js:746:1DB.prototype.getCollectionInfos@src/mongo/shell/db.js:758:15DB.prototype.getCollectionNames@src/mongo/shell/db.js:769:12shellHelper.show@src/mongo/shell/utils.js:695:9shellHelper@src/mongo/shell/utils.js:594:15@(shellhelp2):1:1執行rs.slaveOK
> rs.slaveOk()> show collectionsuser>
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 localswitched to db local> show collectionsmesourcesstartup_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 --slave2016-01-16T10:57:45.965+0800 I CONTROL [initandlisten] MongoDB starting : pid=21820 port=27018 dbpath=/application/mongodb/data/slave/ slave=1 64-bit host=localhost.localdomain2016-01-16T10:57:45.967+0800 I CONTROL [initandlisten] db version v3.2.12016-01-16T10:57:45.968+0800 I CONTROL [initandlisten] git version: a14d55980c2cdc565d4704a7e3ad37e4e535c1b22016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] OpenSSL version: OpenSSL 1.0.1e-fips 11 Feb 20132016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] allocator: tcmalloc2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] modules: none2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] build environment:2016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] distmod: rhel622016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] distarch: x86_642016-01-16T10:57:45.969+0800 I CONTROL [initandlisten] target_arch: x86_642016-01-16T10:57:45.969+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 270182016-01-16T10:57:48.495+0800 I NETWORK [HostnameCanonicalizationWorker] Starting hostname canonicalization worker2016-01-16T10:57:49.497+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:57:50.503+0800 I REPL [replslave] sleep 1 sec before next pass2016-01-16T10:57:51.504+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:57:52.505+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:57:54.295+0800 I REPL [replslave] syncing from host:127.0.0.1:270172016-01-16T10:57:55.296+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存在于主庫的local數據庫，oplog.$main集合。
> use localswitched to db local> db.oplog.$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。在一定時間后，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。那么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以后的版本使用YAML的格式來編寫配置文件。關于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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