MongoDB 复制集的选举原理
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。
轉載于:https://blog.51cto.com/13706703/2175076
總結
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