MongoDB 復制集的選舉原理

復制的原理

        復制時基于啊哦做日志 oplog , 相當于 MySQL 中的二進制日志，只記錄發生改變的記錄。復制是將主節點的oplog 日志同步并應用到其他從節點的過程。

選舉的原理

      節點類型分為標準（host）節點 、被動（passive）節點和仲裁（arbiter）節點。

（1）只有標準節點可能被選舉為活躍（primary）節點，有選舉權。被動節點有完整副本，不可能成為活躍節點，有選舉權。仲裁節點不復制數據，不可能成為活躍節點，只有選舉權。

（2）標準節點與被動節點的區別：primary 值高者是標準節點，低者則為被動節點。

（3）選舉規則是票數高者獲勝，primary 是優先權為 0~1000 的值，相當于額為增加 0~1000 的票數。選舉結果：票數高者獲勝；若票數相同，數據新者獲勝。

實驗部署

搭建配置4個 MongoDB 實例 mongodb  、mongpdb2 、mongodb3 、mongodb4 。 IP 地址：192.168.213.184

由于MongoDB 數據庫安裝好后，mongodb 為默認第一個實例，因此只要再搭建2 ,3，4 實例即可

1. 配置復制集

（1）創建數據文件和日志文件存儲路徑

[root@localhost ~]# mkdir -p /data/mongodb/mongodb{2,3,4}                #創建數據庫文件2,3,4
[root@localhost ~]# cd /data/mongodb/
[root@localhost mongodb]# ls
mongodb2  mongodb3  mongodb4
[root@localhost mongodb]# mkdir logs
[root@localhost mongodb]# ls
logs  mongodb2  mongodb3  mongodb4
[root@localhost mongodb]# cd logs/
[root@localhost logs]# touch mongodb{2,3,4}.log                            #在 /data/mongodb/logs/ 目錄下的日志存儲路徑
[root@localhost logs]# chmod  - R 777 /data/mongodb/logs/*.log         #修改目錄權限   
[root@localhost logs]# ls -l
總用量 0
-rwxrwxrwx. 1 root root 0 9月  13 18:43 mongodb2.log
-rwxrwxrwx. 1 root root 0 9月  13 18:43 mongodb3.log
-rwxrwxrwx. 1 root root 0 9月  13 18:43 mongodb4.log

（2）配置實例1 的配置文件，啟用復制集名稱

[root@localhost logs]# vim /etc/mongod.conf
replication:
      replSetName: abc                        #去掉注釋，指定復制集名稱（名稱隨便定義）

創建多實例配置文件

[root@localhost logs]# cp -p /etc/mongod.conf /etc/mongod2.conf        復制生成實例2 的配置文件
[root@localhost logs]# vim /etc/mongod2.conf

systemLog:
   destination: file
   logAppend: true
   path: /data/mongodb/logs/mongodb2.log             #日志文件位置    

storage:
   dbPath: /data/mongodb/mongodb2                        #數據文件位置
   journal:
     enabled: true

port: 27018             #端口號，默認端口號為27017，因此實例2 的端口號要與之區別，實例3和4 依次往后排

配置其他實例，與實例2修改一樣，注意端口號

[root@localhost logs]# cp -p /etc/mongod2.conf /etc/mongod3.conf
[root@localhost logs]# cp -p /etc/mongod2.conf /etc/mongod4.conf

（3）啟動服務并查看端口是否開啟

[root@localhost logs]# mongod -f /etc/mongod.conf –shutdown                      #先關閉實例1 的服務，在開啟
killing process with pid: 3997                                   #進程號
[root@localhost logs]# mongod -f /etc/mongod.conf
about to fork child process, waiting until server is ready for connections.
forked process: 4867
child process started successfully, parent exiting

[root@localhost logs]# mongod -f /etc/mongod2.conf                                            #啟動實例2 的服務
about to fork child process, waiting until server is ready for connections.
forked process: 4899
child process started successfully, parent exiting

[root@localhost logs]# mongod -f /etc/mongod3.conf                                             #啟動實例3 的服務
about to fork child process, waiting until server is ready for connections.
forked process: 4927
child process started successfully, parent exiting

[root@localhost logs]# mongod -f /etc/mongod4.conf                                              #啟動實例4 的服務
about to fork child process, waiting until server is ready for connections.
forked process: 4955

查看節點端口是否開啟

[root@localhost logs]# netstat -ntap
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name   
tcp        0      0 0.0.0.0:27019           0.0.0.0:*               LISTEN      4927/mongod        
tcp        0      0 0.0.0.0:27020           0.0.0.0:*               LISTEN      4955/mongod        
tcp        0      0 0.0.0.0:27017           0.0.0.0:*               LISTEN      4867/mongod        
tcp        0      0 0.0.0.0:27018           0.0.0.0:*               LISTEN      4899/mongod      

（4）查看各實例能否進入 mongodb 數據庫 

[root@localhost logs]# mongo
MongoDB shell version v3.6.7
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.6.7

[root@localhost logs]# mongo --port 27018
MongoDB shell version v3.6.7
connecting to: mongodb://127.0.0.1:27018/
MongoDB server version: 3.6.7
Server has startup warnings:

[root@localhost logs]# mongo --port 27019
MongoDB shell version v3.6.7
connecting to: mongodb://127.0.0.1:27019/
MongoDB server version: 3.6.7
Server has startup warnings:

[root@localhost logs]# mongo --port 27020
MongoDB shell version v3.6.7
connecting to: mongodb://127.0.0.1:27020/
MongoDB server version: 3.6.7

2 . 配置復制集的優先級

（1）登錄默認實例 mongo，重新配置4個節點 MongoDB 復制集，設置兩個標準節點，有個被動節點和一個仲裁節點，命令如下：

根據優先級確定節點: 優先級為 100的為標準節點，端口號為 27017和27018  ，優先級為0 的為被動節點，端口號為27019；仲裁節點為27020

> cfg={"_id":"abc","members":[{"_id":0,"host":"192.168.213.184:27017","priority":100},{"_id":1,"host":"192.168.213.184:27018","priority":100},{"_id":2,"host":"192.168.213.184:27019","priority":0},{"_id":3,"host":"192.168.213.184:27020","arbiterOnly":true}]}
{
     "_id" : "abc",
     "members" : [
         {
             "_id" : 0,
             "host" : "192.168.213.184:27017",                   #標準節點，優先級為100
             "priority" : 100
         },
         {
             "_id" : 1,
             "host" : "192.168.213.184:27018",                  #標準節點
             "priority" : 100
         },
         {
             "_id" : 2,
             "host" : "192.168.213.184:27019",                   #被動節點，優先級為0
             "priority" : 0
         },
         {
             "_id" : 3,
             "host" : "192.168.213.184:27020",                   #仲裁節點
             "arbiterOnly" : true
         }
     ]
}

> rs.initiate(cfg)                    # 初始化復制集
{
     "ok" : 1,
     "operationTime" : Timestamp(1536819122, 1),
     "$clusterTime" : {
         "clusterTime" : Timestamp(1536819122, 1),
         "signature" : {
             "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
             "keyId" : NumberLong(0)
         }
     }
}

（2）使用命令 rs.isMaster()  查看各節點身份

abc:PRIMARY> rs.isMaster()
{
     "hosts" : [                                         #標準節點 
         "192.168.213.184:27017",
         "192.168.213.184:27018"
     ],
     "passives" : [                                    #被動節點
         "192.168.213.184:27019"
     ],
     "arbiters" : [                                     #仲裁節點
         "192.168.213.184:27020"
     ],
     "setName" : "abc",
     "setVersion" : 1,
     "ismaster" : true,
     "secondary" : false,
     "primary" : "192.168.213.184:27017",

（3）此時默認實例已經被選舉為活躍（primary）節點，可以在數據庫中插入信息了。

abc:PRIMARY> use school                                                 #活躍節點（primory）
switched to db school
abc:PRIMARY> db.info.insert({"id":1,"name":"lili"})                #插入兩條信息
WriteResult({ "nInserted" : 1 })
abc:PRIMARY> db.info.insert({"id":2,"name":"tom"})
WriteResult({ "nInserted" : 1 })

（4）查看日志記錄所有操作，在默認數據庫 local 中的oplog.rs   查看

abc:PRIMARY> show dbs
admin   0.000GB
config  0.000GB
local   0.000GB
school  0.000GB
abc:PRIMARY> use local                  #進入 local 數據庫
switched to db local
abc:PRIMARY> show collections    #顯示
me
oplog.rs
replset.election
replset.minvalid
startup_log
system.replset
system.rollback.id
abc:PRIMARY> db.oplog.rs.find()     #查看日志記錄所有操作

通過日志記錄，可以找到剛才加入的兩條信息

"school.info", "ui" : UUID("2b9c93b6-a58a-4021-a2b4-33f9b19925d8"), "wall" : ISODate("2018-09-13T06:24:44.537Z"), "o" : { "_id" : ObjectId("5b9a02aca106c3eab9c639e5"), "id" : 1, "name" : "lili" } }
{ "ts" : Timestamp(1536819899, 1), "t" : NumberLong(1), "h" : NumberLong("-1447313909384631008"), "v" : 2, "op" : "i", "ns" : "school.info", "ui" : UUID("2b9c93b6-a58a-4021-a2b4-33f9b19925d8"), "wall" : ISODate("2018-09-13T06:24:59.186Z"), "o" : { "_id" : ObjectId("5b9a02bba106c3eab9c639e6"), "id" : 2, "name" : "tom" } }

3.模擬節點故障，如果主節點出現故障，另一個標準節點會選舉成為新的主節點。

（1）先查看當前狀態，使用命令 rs.status()

abc:PRIMARY> rs.status()

"members" : [
         {
             "_id" : 0,                  
             "name" : "192.168.213.184:27017",               #標準節點，端口27017，作為主節點
             "health" : 1,
             "state" : 1,
             "stateStr" : "PRIMARY",
             "uptime" : 2481,
             "optime" : {
                 "ts" : Timestamp(1536820334, 1),
                 "t" : NumberLong(1)
             },

{
             "_id" : 1,
             "name" : "192.168.213.184:27018",                 #標準節點 ，端口27018 為從節點
             "health" : 1,
             "state" : 2,
             "stateStr" : "SECONDARY",
             "uptime" : 1213,
             "optime" : {
                 "ts" : Timestamp(1536820324, 1),
                 "t" : NumberLong(1)
             },

{
             "_id" : 2,
             "name" : "192.168.213.184:27019",               #被動節點， 端口27019位從節點
             "health" : 1,
             "state" : 2,
             "stateStr" : "SECONDARY",
             "uptime" : 1213,
             "optime" : {
                 "ts" : Timestamp(1536820324, 1),
                 "t" : NumberLong(1)
             },

{
             "_id" : 3,
             "name" : "192.168.213.184:27020",                     #仲裁節點
             "health" : 1,
             "state" : 7,
             "stateStr" : "ARBITER",
             "uptime" : 1213,
             "lastHeartbeat" : ISODate("2018-09-13T06:32:14.152Z"),

（2）模擬主節點故障，關閉主節點服務，登錄另一個標準節點端口 27018 ,查看麟能夠一個標準節點是否被選舉為主節點

abc:PRIMARY> exit
bye
[root@localhost logs]# mongod -f /etc/mongod.conf –shutdown                       #關閉主節點服務
killing process with pid: 4821
[root@localhost logs]# mongo --port 27018                                                           #指定27018 端口進入數據庫

abc:PRIMARY> rs.status()                                                              #查看狀態

"members" : [
         {
             "_id" : 0,
             "name" : "192.168.213.184:27017",
            "health" : 0,                                                 #健康之為 0 ，說明端口27017 已經宕機了
             "state" : 8,
             "stateStr" : "(not reachable/healthy)",
             "uptime" : 0,
             "optime" : {
                 "ts" : Timestamp(0, 0),
                 "t" : NumberLong(-1)
             },

{
             "_id" : 1,
             "name" : "192.168.213.184:27018",               #此時另一臺標準節點被選舉為主節點，端口為 27018
             "health" : 1,
             "state" : 1,
             "stateStr" : "PRIMARY",

             "uptime" : 2812,
             "optime" : {
                 "ts" : Timestamp(1536820668, 1),
                 "t" : NumberLong(2)
             },

{
             "_id" : 2,
             "name" : "192.168.213.184:27019",
             "health" : 1,
             "state" : 2,
             "stateStr" : "SECONDARY",
             "uptime" : 1552,
             "optime" : {
                 "ts" : Timestamp(1536820668, 1),
                 "t" : NumberLong(2)
             },

{
         "_id" : 3,
         "name" : "192.168.213.184:27020",
         "health" : 1,
         "state" : 7,
         "stateStr" : "ARBITER",
         "uptime" : 1552,

其中， health 為 1 代表健康，0 代表宕機。 state 為 1 代表主節點，為 2 代表從節點

（3）將標準節點服務全部關閉，查看被動節點是否會被選舉為主節點

abc:PRIMARY> exit
bye
[root@localhost logs]# mongod -f /etc/mongod2.conf –shutdown                     #關閉主節點服務
killing process with pid: 4853
[root@localhost logs]# mongo --port 27019                                                                #開啟備用節點服務

abc:SECONDARY> rs.status()

"members" : [
         {
             "_id" : 0,
             "name" : "192.168.213.184:27017",                                   #此時兩個標準節點都處于宕機狀態，健康值為 0
             "health" : 0,
             "state" : 8,
             "stateStr" : "(not reachable/healthy)",
             "uptime" : 0,
             "optime" : {
                 "ts" : Timestamp(0, 0),
                 "t" : NumberLong(-1)
             },

{
             "_id" : 1,
             "name" : "192.168.213.184:27018",
             "health" : 0,
             "state" : 8,
             "stateStr" : "(not reachable/healthy)",
             "uptime" : 0,
             "optime" : {
                 "ts" : Timestamp(0, 0),
                 "t" : NumberLong(-1)
             },

{
             "_id" : 2,
             "name" : "192.168.213.184:27019",                            #被動節點并沒有被選舉為主節點，說明被動節點不可能成為活躍節點
             "health" : 1,
             "state" : 2,
             "stateStr" : "SECONDARY",
             "uptime" : 3102,
             "optime" : {
                 "ts" : Timestamp(1536820928, 1),
                 "t" : NumberLong(2)
             },

{
             "_id" : 3,
             "name" : "192.168.213.184:27020",
             "health" : 1,
             "state" : 7,
             "stateStr" : "ARBITER",
             "uptime" : 1849,

（4）再次啟動標準節點服務，查看主節點能否恢復。

abc:SECONDARY> exit
bye
[root@localhost logs]# mongod -f /etc/mongod.conf                                     #啟動實例1 的服務
about to fork child process, waiting until server is ready for connections.
forked process: 39839
child process started successfully, parent exiting
[root@localhost logs]# mongod -f /etc/mongod2.conf                                   #啟動實例2的服務
about to fork child process, waiting until server is ready for connections.
forked process: 39929
child process started successfully, parent exiting
[root@localhost logs]# mongo

{
             "_id" : 0,
             "name" : "192.168.213.184:27017",                           #端口27017 恢復為主節點，（與標準節點啟動順序有關，先啟動的為主節點）
             "health" : 1,
             "state" : 1,
             "stateStr" : "PRIMARY",
             "uptime" : 25,
             "optime" : {
                 "ts" : Timestamp(1536821324, 1),
                 "t" : NumberLong(3)
             },

{
         "_id" : 1,
         "name" : "192.168.213.184:27018",
         "health" : 1,
         "state" : 2,
         "stateStr" : "SECONDARY",
         "uptime" : 14,
         "optime" : {
             "ts" : Timestamp(1536821324, 1),
             "t" : NumberLong(3)
         },

{
         "_id" : 2,
         "name" : "192.168.213.184:27019",
         "health" : 1,
         "state" : 2,
         "stateStr" : "SECONDARY",
         "uptime" : 23,
         "optime" : {

{
         "_id" : 3,
         "name" : "192.168.213.184:27020",
         "health" : 1,
         "state" : 7,
         "stateStr" : "ARBITER",
         "uptime" : 23,

由此可以看出：只有標準節點可能被選舉為活躍節點（主  primary），被動節點不可能成為活躍節點，有選舉權。仲裁節點不可能成為活躍節點。