复制的原理
复制时基于啊哦做日志 oplog , 相当于 MySQL 中的二进制日志，只记录发生改变的记录。复制是将主节点的oplog 日志同步并应用到其他从节点的过程。
选举的原理
节点类型分为标准（host）节点 、被动（passive）节点和仲裁（arbiter）节点。
（1）只有标准节点可能被选举为活跃（primary）节点，有选举权。被动节点有完整副本，不可能成为活跃节点，有选举权。仲裁节点不复制数据，不可能成为活跃节点，只有选举权。
（2）标准节点与被动节点的区别：primary 值高者是标准节点，低者则为被动节点。
（3）选举规则是票数高者获胜，primary 是优先权为 0~1000 的值，相当于额为增加 0~1000 的票数。选举结果：票数高者获胜；若票数相同，数据新者获胜。
实验部署
搭建配置4个 MongoDB 实例 mongodb 、mongpdb2 、mongodb3 、mongodb4 。 IP 地址：192.168.213.184
由于MongoDB 数据库安装好后，mongodb 为默认第一个实例，因此只要再搭建2 ,3，4 实例即可
1. 配置复制集
（1）创建数据文件和日志文件存储路径
[root@localhost ~]# mkdir -p /data/mongodb/mongodb{2,3,4} #创建数据库文件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/ 目录下的日志存储路径
[root@localhost logs]# chmod - R 777 /data/mongodb/logs/*.log #修改目录权限
[root@localhost logs]# ls -l
总用量 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）配置实例1 的配置文件，启用复制集名称
[root@localhost logs]# vim /etc/mongod.conf
replication:
replSetName: abc #去掉注释，指定复制集名称（名称随便定义）
创建多实例配置文件
[root@localhost logs]# cp -p /etc/mongod.conf /etc/mongod2.conf 复制生成实例2 的配置文件
[root@localhost logs]# vim /etc/mongod2.confsystemLog:
destination: file
logAppend: true
path: /data/mongodb/logs/mongodb2.log #日志文件位置storage:
dbPath: /data/mongodb/mongodb2 #数据文件位置
journal:
enabled: trueport: 27018 #端口号，默认端口号为27017，因此实例2 的端口号要与之区别，实例3和4 依次往后排
配置其他实例，与实例2修改一样，注意端口号
[root@localhost logs]# cp -p /etc/mongod2.conf /etc/mongod3.conf
[root@localhost logs]# cp -p /etc/mongod2.conf /etc/mongod4.conf
（3）启动服务并查看端口是否开启
[root@localhost logs]# mongod -f /etc/mongod.conf –shutdown #先关闭实例1 的服务，在开启
killing process with pid: 3997 #进程号
[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 #启动实例2 的服务
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 #启动实例3 的服务
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 #启动实例4 的服务
about to fork child process, waiting until server is ready for connections.
forked process: 4955查看节点端口是否开启
[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）查看各实例能否进入 mongodb 数据库
[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 . 配置复制集的优先级
（1）登录默认实例 mongo，重新配置4个节点 MongoDB 复制集，设置两个标准节点，有个被动节点和一个仲裁节点，命令如下：
根据优先级确定节点: 优先级为 100的为标准节点，端口号为 27017和27018 ，优先级为0 的为被动节点，端口号为27019；仲裁节点为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", #标准节点，优先级为100
"priority" : 100
},
{
"_id" : 1,
"host" : "192.168.213.184:27018", #标准节点
"priority" : 100
},
{
"_id" : 2,
"host" : "192.168.213.184:27019", #被动节点，优先级为0
"priority" : 0
},
{
"_id" : 3,
"host" : "192.168.213.184:27020", #仲裁节点
"arbiterOnly" : true
}
]
}
> rs.initiate(cfg) # 初始化复制集
{
"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() 查看各节点身份
abc:PRIMARY> rs.isMaster()
{
"hosts" : [ #标准节点
"192.168.213.184:27017",
"192.168.213.184:27018"
],
"passives" : [ #被动节点
"192.168.213.184:27019"
],
"arbiters" : [ #仲裁节点
"192.168.213.184:27020"
],
"setName" : "abc",
"setVersion" : 1,
"ismaster" : true,
"secondary" : false,
"primary" : "192.168.213.184:27017",
（3）此时默认实例已经被选举为活跃（primary）节点，可以在数据库中插入信息了。
abc:PRIMARY> use school #活跃节点（primory）
switched to db school
abc:PRIMARY> db.info.insert({"id":1,"name":"lili"}) #插入两条信息
WriteResult({ "nInserted" : 1 })
abc:PRIMARY> db.info.insert({"id":2,"name":"tom"})
WriteResult({ "nInserted" : 1 })
（4）查看日志记录所有操作，在默认数据库 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 #进入 local 数据库
switched to db local
abc:PRIMARY> show collections #显示
me
oplog.rs
replset.election
replset.minvalid
startup_log
system.replset
system.rollback.id
abc:PRIMARY> db.oplog.rs.find() #查看日志记录所有操作
通过日志记录，可以找到刚才加入的两条信息
"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.模拟节点故障，如果主节点出现故障，另一个标准节点会选举成为新的主节点。
（1）先查看当前状态，使用命令 rs.status()
abc:PRIMARY> rs.status()
"members" : [
{
"_id" : 0,
"name" : "192.168.213.184:27017", #标准节点，端口27017，作为主节点
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 2481,
"optime" : {
"ts" : Timestamp(1536820334, 1),
"t" : NumberLong(1)
},{
"_id" : 1,
"name" : "192.168.213.184:27018", #标准节点 ，端口27018 为从节点
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 1213,
"optime" : {
"ts" : Timestamp(1536820324, 1),
"t" : NumberLong(1)
},{
"_id" : 2,
"name" : "192.168.213.184:27019", #被动节点， 端口27019位从节点
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 1213,
"optime" : {
"ts" : Timestamp(1536820324, 1),
"t" : NumberLong(1)
},{
"_id" : 3,
"name" : "192.168.213.184:27020", #仲裁节点
"health" : 1,
"state" : 7,
"stateStr" : "ARBITER",
"uptime" : 1213,
"lastHeartbeat" : ISODate("2018-09-13T06:32:14.152Z"),
（2）模拟主节点故障，关闭主节点服务，登录另一个标准节点端口 27018 ,查看麟能够一个标准节点是否被选举为主节点
abc:PRIMARY> exit
bye
[root@localhost logs]# mongod -f /etc/mongod.conf –shutdown #关闭主节点服务
killing process with pid: 4821
[root@localhost logs]# mongo --port 27018 #指定27018 端口进入数据库abc:PRIMARY> rs.status() #查看状态
"members" : [
{
"_id" : 0,
"name" : "192.168.213.184:27017",
"health" : 0, #健康之为 0 ，说明端口27017 已经宕机了
"state" : 8,
"stateStr" : "(not reachable/healthy)",
"uptime" : 0,
"optime" : {
"ts" : Timestamp(0, 0),
"t" : NumberLong(-1)
},{
"_id" : 1,
"name" : "192.168.213.184:27018", #此时另一台标准节点被选举为主节点，端口为 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,
（3）将标准节点服务全部关闭，查看被动节点是否会被选举为主节点
abc:PRIMARY> exit
bye
[root@localhost logs]# mongod -f /etc/mongod2.conf –shutdown #关闭主节点服务
killing process with pid: 4853
[root@localhost logs]# mongo --port 27019 #开启备用节点服务abc:SECONDARY> rs.status()
"members" : [
{
"_id" : 0,
"name" : "192.168.213.184:27017", #此时两个标准节点都处于宕机状态，健康值为 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", #被动节点并没有被选举为主节点，说明被动节点不可能成为活跃节点
"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）再次启动标准节点服务，查看主节点能否恢复。
abc:SECONDARY> exit
bye
[root@localhost logs]# mongod -f /etc/mongod.conf #启动实例1 的服务
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 #启动实例2的服务
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 恢复为主节点，（与标准节点启动顺序有关，先启动的为主节点）
"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：只有标准节点可能被选举为活跃节点（主 primary），被动节点不可能成为活跃节点，有选举权。仲裁节点不可能成为活跃节点。