Replica Sets一般歸入到高可用範疇內，提供服務的高度連續可用，降低服務的間斷時間；而且Replica Sets也可以用在容災、備份/恢復、負載均衡等方面。
1、Slave可以執行查詢，降低Master壓力；
2、在Slave上進行備份，避免備份中將Master鎖定，造成Master不可方面的情況；
3、Master出現故障，可以快速切換到Slave上，使得服務不間斷；
4、多臺Slave可以和負載均衡器組合，提供高效能的可訪問服務。

一、規劃：
按照上圖，安裝配置3節點的Replica Sets，使用的作業系統是RHEL6.4，具體安裝軟體請參見“MongoDB系列一”。
相應的儲存路徑和埠定義，如下表：

	R0	R1	R2
資料儲存路徑	/mongodb/data/r0	/mongodb/data/r1	/mongodb/data/r2
日誌儲存路徑	/mongodb/log/r0.log	/mongodb/log/r1.log	/mongodb/log/r2.log
Port	28010	28011	28012

二、準備：
# echo "never" > /sys/kernel/mm/transparent_hugepage/enabled
# echo "never" > /sys/kernel/mm/transparent_hugepage/defrag
如果不執行，啟動mongod，日誌中會顯示相應的警告。

三、建立相關目錄
# su - mongodb
$ mkdir /mongodb/data/r{0,1,2}
$ mkdir /mongodb/log

四、啟動每個節點的mongod
$ mongod --replSet rs1 --fork --port 28010 --dbpath /mongodb/data/r0/ --logpath /mongodb/log/r0.log --logappend --smallfiles --oplogSize 128
about to fork child process, waiting until server is ready for connections.
forked process: 11552
child process started successfully, parent exiting

$ mongod --replSet rs1 --fork --port 28011 --dbpath /mongodb/data/r1/ --logpath /mongodb/log/r1.log --logappend --smallfiles --oplogSize 128

$ mongod --replSet rs1 --fork --port 28012 --dbpath /mongodb/data/r2/ --logpath /mongodb/log/r2.log --logappend --smallfiles --oplogSize 128

說明：
--replSet        Replica Sets的名稱
--fork           以建立子程式的方式執行
--port           服務使用的埠
--dbpath         資料檔案儲存的路徑
--logpath        日誌檔案儲存的路徑
--logappend      日誌寫入日誌問題的方式“追加”
--smallfiles     使用小檔案的預設大小
--oplogSize      複製日誌的空間大小（MB）

五、配置Replica Sets
連線R0，在R0做配置。
$ mongo --port 28010
MongoDB shell version: 3.2.6
connecting to: 127.0.0.1:28010/test
> rsconf = {
   "_id" : "rs1",
   "members" : [
       {
           "_id" : 0,
           "host" : "localhost:28010"
       }
   ]
}
> rs.initiate(rsconf);
{ "ok" : 1 }
rs1:OTHER> rs.conf();
{
   "_id" : "rs1",
   "version" : 1,
   "protocolVersion" : NumberLong(1),
   "members" : [
       {
           "_id" : 0,
           "host" : "localhost:28010",
           "arbiterOnly" : false,
           "buildIndexes" : true,
           "hidden" : false,
           "priority" : 1,
           "tags" : {

           },
           "slaveDelay" : NumberLong(0),
           "votes" : 1
       }
   ],
   "settings" : {
       "chainingAllowed" : true,
       "heartbeatIntervalMillis" : 2000,
       "heartbeatTimeoutSecs" : 10,
       "electionTimeoutMillis" : 10000,
       "getLastErrorModes" : {

       },
       "getLastErrorDefaults" : {
           "w" : 1,
           "wtimeout" : 0
       },
       "replicaSetId" : ObjectId("57c9095e7bf8b5d9980b2e9d")
   }
}
rs1:PRIMARY> rs.add("localhost:28011");
{ "ok" : 1 }
rs1:PRIMARY> rs.add("localhost:28012");
{ "ok" : 1 }
rs1:PRIMARY> rs.conf();
{
   "_id" : "rs1",
   "version" : 3,
   "protocolVersion" : NumberLong(1),
   "members" : [
       {
           "_id" : 0,
           "host" : "localhost:28010",
           "arbiterOnly" : false,
           "buildIndexes" : true,
           "hidden" : false,
           "priority" : 1,
           "tags" : {

           },
           "slaveDelay" : NumberLong(0),
           "votes" : 1
       },
       {
           "_id" : 1,
           "host" : "localhost:28011",
           "arbiterOnly" : false,
           "buildIndexes" : true,
           "hidden" : false,
           "priority" : 1,
           "tags" : {

           },
           "slaveDelay" : NumberLong(0),
           "votes" : 1
       },
       {
           "_id" : 2,
           "host" : "localhost:28012",
           "arbiterOnly" : false,
           "buildIndexes" : true,
           "hidden" : false,
           "priority" : 1,
           "tags" : {

           },
           "slaveDelay" : NumberLong(0),
           "votes" : 1
       }
   ],
   "settings" : {
       "chainingAllowed" : true,
       "heartbeatIntervalMillis" : 2000,
       "heartbeatTimeoutSecs" : 10,
       "electionTimeoutMillis" : 10000,
       "getLastErrorModes" : {

       },
       "getLastErrorDefaults" : {
           "w" : 1,
           "wtimeout" : 0
       },
       "replicaSetId" : ObjectId("57c9095e7bf8b5d9980b2e9d")
   }
}
配置完成。

六、檢視狀態，以及檢查Replica Sets部署是否成功
1）檢視狀態
rs1:PRIMARY> rs.status();
{
   "set" : "rs1",
   "date" : ISODate("2016-09-02T05:12:10.162Z"),
   "myState" : 1,
   "term" : NumberLong(1),
   "heartbeatIntervalMillis" : NumberLong(2000),
   "members" : [
       {
           "_id" : 0,
           "name" : "localhost:28010",
           "health" : 1,
           "state" : 1,
           "stateStr" : "PRIMARY",
           "uptime" : 450,
           "optime" : {
               "ts" : Timestamp(1472793094, 1),
               "t" : NumberLong(1)
           },
           "optimeDate" : ISODate("2016-09-02T05:11:34Z"),
           "electionTime" : Timestamp(1472792926, 2),
           "electionDate" : ISODate("2016-09-02T05:08:46Z"),
           "configVersion" : 3,
           "self" : true
       },
       {
           "_id" : 1,
           "name" : "localhost:28011",
           "health" : 1,
           "state" : 2,
           "stateStr" : "SECONDARY",
           "uptime" : 39,
           "optime" : {
               "ts" : Timestamp(1472793094, 1),
               "t" : NumberLong(1)
           },
           "optimeDate" : ISODate("2016-09-02T05:11:34Z"),
           "lastHeartbeat" : ISODate("2016-09-02T05:12:08.894Z"),
           "lastHeartbeatRecv" : ISODate("2016-09-02T05:12:09.898Z"),
           "pingMs" : NumberLong(0),
           "syncingTo" : "localhost:28010",
           "configVersion" : 3
       },
       {
           "_id" : 2,
           "name" : "localhost:28012",
           "health" : 1,
           "state" : 2,
           "stateStr" : "SECONDARY",
           "uptime" : 35,
           "optime" : {
               "ts" : Timestamp(1472793094, 1),
               "t" : NumberLong(1)
           },
           "optimeDate" : ISODate("2016-09-02T05:11:34Z"),
           "lastHeartbeat" : ISODate("2016-09-02T05:12:08.894Z"),
           "lastHeartbeatRecv" : ISODate("2016-09-02T05:12:06.897Z"),
           "pingMs" : NumberLong(0),
           "configVersion" : 3
       }
   ],
   "ok" : 1
}

2）檢查
（a）插入記錄
rs1:PRIMARY> use test
switched to db test
rs1:PRIMARY> db.t1.insert({name:"john",age:10});
WriteResult({ "nInserted" : 1 })
rs1:PRIMARY> db.t1.find();
{ "_id" : ObjectId("57c923dc296d0a6382e17195"), "name" : "john", "age" : 10 }

（b）檢查同步日誌
rs1:PRIMARY> use local
switched to db local

rs1:PRIMARY> db.oplog.rs.find();
{ "ts" : Timestamp(1472792926, 1), "h" : NumberLong("-1551070289351464931"), "v" : 2, "op" : "n", "ns" : "", "o" : { "msg" : "initiating set" } }
{ "ts" : Timestamp(1472792927, 1), "t" : NumberLong(1), "h" : NumberLong("-6644373458713625013"), "v" : 2, "op" : "n", "ns" : "", "o" : { "msg" : "new primary" } }
{ "ts" : Timestamp(1472793091, 1), "t" : NumberLong(1), "h" : NumberLong("-3467321571589321913"), "v" : 2, "op" : "n", "ns" : "", "o" : { "msg" : "Reconfig set", "version" : 2 } }
{ "ts" : Timestamp(1472793094, 1), "t" : NumberLong(1), "h" : NumberLong("5469898305152102871"), "v" : 2, "op" : "n", "ns" : "", "o" : { "msg" : "Reconfig set", "version" : 3 } }
{ "ts" : Timestamp(1472799708, 1), "t" : NumberLong(1), "h" : NumberLong("-2309306902204899951"), "v" : 2, "op" : "c", "ns" : "test.$cmd", "o" : { "create" : "t1" } }
{ "ts" : Timestamp(1472799708, 2), "t" : NumberLong(1), "h" : NumberLong("-7169457093293246964"), "v" : 2, "op" : "i", "ns" : "test.t1", "o" : { "_id" : ObjectId("57c923dc296d0a6382e17195"), "name" : "john", "age" : 10 } }

注意：紅色部分即是插入的記錄資料，在此出現，說明在Master插入的記錄，已經進入同步日誌，進行同步，需要到Slave去驗證是否接收到該記錄。

（c）連線Slave，檢查該記錄是否可以查詢
$ mongo --port 28011
MongoDB shell version: 3.2.6
connecting to: 127.0.0.1:28011/test
rs1:SECONDARY> use test
switched to db test
rs1:SECONDARY> db.t1.find();
Error: error: { "ok" : 0, "errmsg" : "not master and slaveOk=false", "code" : 13435 }
注：MongoDB預設Slave是不能讀的，所以需要進行如下配置。
rs1:SECONDARY> db.getMongo().setSlaveOk();

查詢：
rs1:SECONDARY> db.t1.find();
{ "_id" : ObjectId("57c923dc296d0a6382e17195"), "name" : "john", "age" : 10 }

至此，可以證明Replica Sets是安裝配置成功的。

MongoDB系列二：Replica Sets安裝與配置

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