MongoDB副本集(一主兩從)讀寫分離、故障轉移功能環境部署記錄
Mongodb是一種非關係數據庫(NoSQL)，非關係型數據庫的產生就是爲了解決大數據量、高擴展性、高性能、靈活數據模型、高可用性。MongoDB官方已經不建議使用主從模式了，替代方案是採用副本集的模式。主從模式其實就是一個單副本的應用，沒有很好的擴展性和容錯性，而Mongodb副本集具備多個副本保證了容錯性，就算一個副本掛掉了還有不少副本存在，主節點掛掉後，整個集羣內會實現自動切換。linux
Mongodb副本集的工做原理
客戶端鏈接到整個Mongodb副本集，不關心具體哪一臺節點機是否掛掉。主節點機負責整個副本集的讀寫，副本集按期同步數據備份，一但主節點掛掉，副本節點就會選舉一個新的主服務器，這一切對於應用服務器不須要關心。副本集中的副本節點在主節點掛掉後經過心跳機制檢測到後，就會在集羣內發起主節點的選舉機制，自動選舉一位新的主服務器。mongodb
看起來Mongodb副本集很牛X的樣子，下面就演示下副本集環境部署過程，官方推薦的Mongodb副本集機器數量爲至少3個節點，這裏我就選擇三個節點，一個主節點，兩個從節點，暫不使用仲裁節點。shell
1、環境準備數據庫
ip地址 主機名 角色 172.16.60.205 mongodb-master01 副本集主節點 172.16.60.206 mongodb-slave01 副本集副本節點 172.16.60.207 mongodb-slave02 副本集副本節點 三個節點機均設置好各自的主機名，並以下設置好hosts綁定 [root@mongodb-master01 ~]# cat /etc/hosts ............ 172.16.60.205 mongodb-master01 172.16.60.206 mongodb-slave01 172.16.60.207 mongodb-slave02 三個節點機均關閉selinux，爲了測試方便，將iptables也關閉 [root@mongodb-master01 ~]# setenforce 0 [root@mongodb-master01 ~]# cat /etc/sysconfig/selinux ........... SELINUX=disabled [root@mongodb-master01 ~]# iptables -F [root@mongodb-master01 ~]# /etc/init.d/iptables stop [root@mongodb-master01 ~]# /etc/init.d/iptables stop iptables: Setting chains to policy ACCEPT: filter [ OK ] iptables: Flushing firewall rules: [ OK ] iptables: Unloading modules: [ OK ]
2、Mongodb安裝、副本集配置bash
1) 在三個節點機上創建mongodb副本集測試文件夾，用於存放整個副本集文件
[root@mongodb-master01 ~]# mkdir -p /data/mongodb/data/replset/
2）在三個節點機上安裝mongodb
下載地址：https://www.mongodb.org/dl/linux/x86_64-rhel62
[root@mongodb-master01 ~]# wget http://downloads.mongodb.org/linux/mongodb-linux-x86_64-rhel62-v3.6-latest.tgz
[root@mongodb-master01 ~]# tar -zvxf mongodb-linux-x86_64-rhel62-v3.6-latest.tgz
3）分別在每一個節點機上啓動mongodb（啓動時指明--bind_ip地址，默認是127.0.0.1，須要改爲本機ip，不然遠程鏈接時失敗）
[root@mongodb-master01 ~]# mv mongodb-linux-x86_64-rhel62-3.6.11-rc0-2-g2151d1d219 /usr/local/mongodb
[root@mongodb-master01 ~]# nohup /usr/local/mongodb/bin/mongod -dbpath /data/mongodb/data/replset -replSet repset --bind_ip=172.16.60.205 --port=27017 &
[root@mongodb-master01 ~]# ps -ef|grep mongodb
root 7729 6977 1 15:10 pts/1 00:00:01 /usr/local/mongodb/bin/mongod -dbpath /data/mongodb/data/replset -replSet repset
root 7780 6977 0 15:11 pts/1 00:00:00 grep mongodb
[root@mongodb-master01 ~]# lsof -i:27017
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
mongod 7729 root 10u IPv4 6554476 0t0 TCP localhost:27017 (LISTEN)
4）初始化副本集
在三個節點中的任意一個節點機上操做（好比在172.16.60.205節點機）
登錄mongodb
[root@mongodb-master01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.205:27017
.........
#使用admin數據庫
> use admin
switched to db admin
#定義副本集配置變量，這裏的 _id:」repset」 和上面命令參數「 –replSet repset」 要保持同樣。
> config = { _id:"repset", members:[{_id:0,host:"172.16.60.205:27017"},{_id:1,host:"172.16.60.206:27017"},{_id:2,host:"172.16.60.207:27017"}]}
{
"_id" : "repset",
"members" : [
{
"_id" : 0,
"host" : "172.16.60.205:27017"
},
{
"_id" : 1,
"host" : "172.16.60.206:27017"
},
{
"_id" : 2,
"host" : "172.16.60.207:27017"
}
]
}
#初始化副本集配置
> rs.initiate(config);
{
"ok" : 1,
"operationTime" : Timestamp(1551166191, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1551166191, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
}
}
#查看集羣節點的狀態
repset:SECONDARY> rs.status();
{
"set" : "repset",
"date" : ISODate("2019-02-26T07:31:07.766Z"),
"myState" : 1,
"term" : NumberLong(1),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"heartbeatIntervalMillis" : NumberLong(2000),
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"readConcernMajorityOpTime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"appliedOpTime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"durableOpTime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
}
},
"members" : [
{
"_id" : 0,
"name" : "172.16.60.205:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 270,
"optime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-02-26T07:31:03Z"),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "could not find member to sync from",
"electionTime" : Timestamp(1551166202, 1),
"electionDate" : ISODate("2019-02-26T07:30:02Z"),
"configVersion" : 1,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 1,
"name" : "172.16.60.206:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 76,
"optime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"optimeDurable" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-02-26T07:31:03Z"),
"optimeDurableDate" : ISODate("2019-02-26T07:31:03Z"),
"lastHeartbeat" : ISODate("2019-02-26T07:31:06.590Z"),
"lastHeartbeatRecv" : ISODate("2019-02-26T07:31:06.852Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "172.16.60.205:27017",
"syncSourceHost" : "172.16.60.205:27017",
"syncSourceId" : 0,
"infoMessage" : "",
"configVersion" : 1
},
{
"_id" : 2,
"name" : "172.16.60.207:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 76,
"optime" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"optimeDurable" : {
"ts" : Timestamp(1551166263, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-02-26T07:31:03Z"),
"optimeDurableDate" : ISODate("2019-02-26T07:31:03Z"),
"lastHeartbeat" : ISODate("2019-02-26T07:31:06.589Z"),
"lastHeartbeatRecv" : ISODate("2019-02-26T07:31:06.958Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "172.16.60.205:27017",
"syncSourceHost" : "172.16.60.205:27017",
"syncSourceId" : 0,
"infoMessage" : "",
"configVersion" : 1
}
],
"ok" : 1,
"operationTime" : Timestamp(1551166263, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1551166263, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
}
}
如上信息代表：
副本集配置成功後，172.16.60.205爲主節點PRIMARY，172.16.60.206/207爲副本節點SECONDARY。
health：1 1代表狀態是正常，0代表異常
state:1 值小的是primary節點、值大的是secondary節點
3、測試Mongodb副本集數據複製功能 <mongodb默認是從主節點讀寫數據的，副本節點上不容許讀，須要設置副本節點能夠讀>服務器
1）在主節點172.16.60.205上鍊接到終端
[root@mongodb-master01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.205:27017
................
#創建test 數據庫
repset:PRIMARY> use test;
switched to db test
#往testdb表插入測試數據
repset:PRIMARY> db.testdb.insert({"test1":"testval1"})
WriteResult({ "nInserted" : 1 })
2）在副本節點172.16.60.20六、172.16.60.207上鍊接到mongodb查看數據是否複製過來。
這裏在172.16.60.206副本節點上進行查看
[root@mongodb-slave01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.206:27017
................
repset:SECONDARY> use test;
switched to db test
repset:SECONDARY> show tables;
2019-02-26T15:37:46.446+0800 E QUERY [thread1] Error: listCollections failed: {
"operationTime" : Timestamp(1551166663, 1),
"ok" : 0,
"errmsg" : "not master and slaveOk=false",
"code" : 13435,
"codeName" : "NotMasterNoSlaveOk",
"$clusterTime" : {
"clusterTime" : Timestamp(1551166663, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
}
} :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
DB.prototype._getCollectionInfosCommand@src/mongo/shell/db.js:941:1
DB.prototype.getCollectionInfos@src/mongo/shell/db.js:953:19
DB.prototype.getCollectionNames@src/mongo/shell/db.js:964:16
shellHelper.show@src/mongo/shell/utils.js:853:9
shellHelper@src/mongo/shell/utils.js:750:15
@(shellhelp2):1:1
上面出現了報錯！
這是由於mongodb默認是從主節點讀寫數據的，副本節點上不容許讀，須要設置副本節點能夠讀
repset:SECONDARY> db.getMongo().setSlaveOk();
repset:SECONDARY> db.testdb.find();
{ "_id" : ObjectId("5c74ec9267d8c3d06506449b"), "test1" : "testval1" }
repset:SECONDARY> show tables;
testdb
如上發現已經在副本節點上發現了測試數據，即已經從主節點複製過來了。
（在另外一個副本節點172.16.60.207也如上操做便可）
4、測試副本集故障轉移功能
先停掉主節點172.16.60.205，查看mongodb副本集狀態，能夠看到通過一系列的投票選擇操做，172.16.60.206當選主節點，172.16.60.207從172.16.60.206同步數據過來。網絡
1）停掉原來的主節點172.16.60.205的mongodb，模擬故障
[root@mongodb-master01 ~]# ps -ef|grep mongodb|grep -v grep|awk '{print $2}'|xargs kill -9
[root@mongodb-master01 ~]# lsof -i:27017
[root@mongodb-master01 ~]#
2）接着登陸到另外兩個正常的從節點172.16.60.20六、172.16.60.207中的任意一個節點的mongodb，查看副本集狀態
[root@mongodb-slave01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.206:27017
.................
repset:PRIMARY> rs.status();
{
"set" : "repset",
"date" : ISODate("2019-02-26T08:06:02.996Z"),
"myState" : 1,
"term" : NumberLong(2),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"heartbeatIntervalMillis" : NumberLong(2000),
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
},
"readConcernMajorityOpTime" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
},
"appliedOpTime" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
},
"durableOpTime" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
}
},
"members" : [
{
"_id" : 0,
"name" : "172.16.60.205:27017",
"health" : 0,
"state" : 8,
"stateStr" : "(not reachable/healthy)",
"uptime" : 0,
"optime" : {
"ts" : Timestamp(0, 0),
"t" : NumberLong(-1)
},
"optimeDurable" : {
"ts" : Timestamp(0, 0),
"t" : NumberLong(-1)
},
"optimeDate" : ISODate("1970-01-01T00:00:00Z"),
"optimeDurableDate" : ISODate("1970-01-01T00:00:00Z"),
"lastHeartbeat" : ISODate("2019-02-26T08:06:02.917Z"),
"lastHeartbeatRecv" : ISODate("2019-02-26T08:03:37.492Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "Connection refused",
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"configVersion" : -1
},
{
"_id" : 1,
"name" : "172.16.60.206:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 2246,
"optime" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
},
"optimeDate" : ISODate("2019-02-26T08:05:59Z"),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"electionTime" : Timestamp(1551168228, 1),
"electionDate" : ISODate("2019-02-26T08:03:48Z"),
"configVersion" : 1,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 2,
"name" : "172.16.60.207:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 2169,
"optime" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
},
"optimeDurable" : {
"ts" : Timestamp(1551168359, 1),
"t" : NumberLong(2)
},
"optimeDate" : ISODate("2019-02-26T08:05:59Z"),
"optimeDurableDate" : ISODate("2019-02-26T08:05:59Z"),
"lastHeartbeat" : ISODate("2019-02-26T08:06:02.861Z"),
"lastHeartbeatRecv" : ISODate("2019-02-26T08:06:02.991Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "172.16.60.206:27017",
"syncSourceHost" : "172.16.60.206:27017",
"syncSourceId" : 1,
"infoMessage" : "",
"configVersion" : 1
}
],
"ok" : 1,
"operationTime" : Timestamp(1551168359, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1551168359, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
}
}
發現當原來的主節點172.16.60.205宕掉後，通過選舉，原來的從節點172.16.60.206被推舉爲新的主節點。
3）如今在172.16.60.206新主節點上建立測試數據
repset:PRIMARY> use kevin;
switched to db kevin
repset:PRIMARY> db.kevin.insert({"shibo":"hahaha"})
WriteResult({ "nInserted" : 1 })
4）另外一個從節點172.16.60.207上登陸mongodb查看
[root@mongodb-slave02 ~]# /usr/local/mongodb/bin/mongo 172.16.60.207:27017
................
repset:SECONDARY> use kevin;
switched to db kevin
repset:SECONDARY> db.getMongo().setSlaveOk();
repset:SECONDARY> show tables;
kevin
repset:SECONDARY> db.kevin.find();
{ "_id" : ObjectId("5c74f42bb0b339ed6eb68e9c"), "shibo" : "hahaha" }
發現從節點172.16.60.207能夠同步新的主節點172.16.60.206的數據
5）再從新啓動原來的主節點172.16.60.205的mongodb
[root@mongodb-master01 ~]# nohup /usr/local/mongodb/bin/mongod -dbpath /data/mongodb/data/replset -replSet repset --bind_ip=172.16.60.205 --port=27017 &
mongod 9162 root 49u IPv4 6561201 0t0 TCP mongodb-master01:55236->mongodb-slave01:27017 (ESTABLISHED)
[root@mongodb-master01 ~]# ps -ef|grep mongodb
root 9162 6977 4 16:14 pts/1 00:00:01 /usr/local/mongodb/bin/mongod -dbpath /data/mongodb/data/replset -replSet repset --bind_ip=172.16.60.205 --port=27017
root 9244 6977 0 16:14 pts/1 00:00:00 grep mongodb
再次登陸到三個節點中的任意一個的mongodb，查看副本集狀態
[root@mongodb-master01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.205:27017
....................
repset:SECONDARY> rs.status();
{
"set" : "repset",
"date" : ISODate("2019-02-26T08:16:11.741Z"),
"myState" : 2,
"term" : NumberLong(2),
"syncingTo" : "172.16.60.206:27017",
"syncSourceHost" : "172.16.60.206:27017",
"syncSourceId" : 1,
"heartbeatIntervalMillis" : NumberLong(2000),
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"readConcernMajorityOpTime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"appliedOpTime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"durableOpTime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
}
},
"members" : [
{
"_id" : 0,
"name" : "172.16.60.205:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 129,
"optime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"optimeDate" : ISODate("2019-02-26T08:16:09Z"),
"syncingTo" : "172.16.60.206:27017",
"syncSourceHost" : "172.16.60.206:27017",
"syncSourceId" : 1,
"infoMessage" : "",
"configVersion" : 1,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 1,
"name" : "172.16.60.206:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 127,
"optime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"optimeDurable" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"optimeDate" : ISODate("2019-02-26T08:16:09Z"),
"optimeDurableDate" : ISODate("2019-02-26T08:16:09Z"),
"lastHeartbeat" : ISODate("2019-02-26T08:16:10.990Z"),
"lastHeartbeatRecv" : ISODate("2019-02-26T08:16:11.518Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"electionTime" : Timestamp(1551168228, 1),
"electionDate" : ISODate("2019-02-26T08:03:48Z"),
"configVersion" : 1
},
{
"_id" : 2,
"name" : "172.16.60.207:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 127,
"optime" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"optimeDurable" : {
"ts" : Timestamp(1551168969, 1),
"t" : NumberLong(2)
},
"optimeDate" : ISODate("2019-02-26T08:16:09Z"),
"optimeDurableDate" : ISODate("2019-02-26T08:16:09Z"),
"lastHeartbeat" : ISODate("2019-02-26T08:16:10.990Z"),
"lastHeartbeatRecv" : ISODate("2019-02-26T08:16:11.655Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "172.16.60.206:27017",
"syncSourceHost" : "172.16.60.206:27017",
"syncSourceId" : 1,
"infoMessage" : "",
"configVersion" : 1
}
],
"ok" : 1,
"operationTime" : Timestamp(1551168969, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1551168969, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
}
}
發現原來的主節點172.16.60.205在故障恢復後，變成了新的主節點172.16.60.206的從節點
5、Mongodb讀寫分離
目前來看。Mongodb副本集能夠完美支持故障轉移。至於主節點的讀寫壓力過大如何解決？常見的解決方案是讀寫分離。app
通常狀況下，常規寫操做來講並無讀操做多，因此在Mongodb副本集中，一臺主節點負責寫操做，兩臺副本節點負責讀操做。 1）設置讀寫分離須要先在副本節點SECONDARY 設置 setSlaveOk。 2）在程序中設置副本節點負責讀操做，以下代碼：
public class TestMongoDBReplSetReadSplit { public static void main(String[] args) { try { List<ServerAddress> addresses = new ArrayList<ServerAddress>(); ServerAddress address1 = new ServerAddress("172.16.60.205" , 27017); ServerAddress address2 = new ServerAddress("172.16.60.206" , 27017); ServerAddress address3 = new ServerAddress("172.16.60.207" , 27017); addresses.add(address1); addresses.add(address2); addresses.add(address3); MongoClient client = new MongoClient(addresses); DB db = client.getDB( "test" ); DBCollection coll = db.getCollection( "testdb" ); BasicDBObject object = new BasicDBObject(); object.append( "test2" , "testval2" ); //讀操做從副本節點讀取 ReadPreference preference = ReadPreference. secondary(); DBObject dbObject = coll.findOne(object, null , preference); System. out .println(dbObject); } catch (Exception e) { e.printStackTrace(); } } }
讀參數除了secondary一共還有五個參數：primary、primaryPreferred、secondary、secondaryPreferred、nearest。
primary：默認參數，只從主節點上進行讀取操做；
primaryPreferred：大部分從主節點上讀取數據,只有主節點不可用時從secondary節點讀取數據。
secondary：只從secondary節點上進行讀取操做，存在的問題是secondary節點的數據會比primary節點數據「舊」。
secondaryPreferred：優先從secondary節點進行讀取操做，secondary節點不可用時從主節點讀取數據；
nearest：不論是主節點、secondary節點，從網絡延遲最低的節點上讀取數據。性能
讀寫分離作好後，就能夠進行數據分流，減輕壓力，解決了"主節點的讀寫壓力過大如何解決？"這個問題。不過當副本節點增多時，主節點的複製壓力會加大有什麼辦法解決嗎？基於這個問題，Mongodb已有了相應的解決方案 - 引用仲裁節點：
在Mongodb副本集中，仲裁節點不存儲數據，只是負責故障轉移的羣體投票，這樣就少了數據複製的壓力。看起來想的很周到啊，其實不僅是主節點、副本節點、仲裁節點，還有Secondary-Only、Hidden、Delayed、Non-Voting，其中：
Secondary-Only：不能成爲primary節點，只能做爲secondary副本節點，防止一些性能不高的節點成爲主節點。
Hidden：這類節點是不可以被客戶端制定IP引用，也不能被設置爲主節點，可是能夠投票，通常用於備份數據。
Delayed：能夠指定一個時間延遲從primary節點同步數據。主要用於備份數據，若是實時同步，誤刪除數據立刻同步到從節點，恢復又恢復不了。
Non-Voting：沒有選舉權的secondary節點，純粹的備份數據節點。測試
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