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MongoDB副本集(一主两从)读写分离、故障转移功能环境部署记录

22 11月
作者:admin|分类:DBA运维

 

Mongodb是一种非关系数据库(NoSQL)，非关系型数据库的产生就是为了解决大数据量、高扩展性、高性能、灵活数据模型、高可用性。MongoDB官方已经不建议使用主从模式了，替代方案是采用副本集的模式。主从模式其实就是一个单副本的应用，没有很好的扩展性和容错性，而Mongodb副本集具有多个副本保证了容错性，就算一个副本挂掉了还有很多副本存在，主节点挂掉后，整个集群内会实现自动切换。

Mongodb副本集的工作原理
客户端连接到整个Mongodb副本集，不关心具体哪一台节点机是否挂掉。主节点机负责整个副本集的读写，副本集定期同步数据备份，一但主节点挂掉，副本节点就会选举一个新的主服务器，这一切对于应用服务器不需要关心。副本集中的副本节点在主节点挂掉后通过心跳机制检测到后，就会在集群内发起主节点的选举机制，自动选举一位新的主服务器。

看起来Mongodb副本集很牛X的样子，下面就演示下副本集环境部署过程，官方推荐的Mongodb副本集机器数量为至少3个节点，这里我就选择三个节点，一个主节点，两个从节点，暂不使用仲裁节点。

一、环境准备

ip地址                    主机名                               角色
172.16.60.205      mongodb-master01          副本集主节点
172.16.60.206      mongodb-slave01             副本集副本节点
172.16.60.207      mongodb-slave02             副本集副本节点

三个节点机均设置好各自的主机名，并如下设置好hosts绑定
[root@mongodb-master01 ~]# cat /etc/hosts
............
172.16.60.205    mongodb-master01
172.16.60.206    mongodb-slave01
172.16.60.207    mongodb-slave02

三个节点机均关闭selinux，为了测试方便，将iptables也关闭
[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  ]

二、Mongodb安装、副本集配置

1) 在三个节点机上建立mongodb副本集测试文件夹，用于存放整个副本集文件
[root@mongodb-master01 ~]# mkdir -p /data/mongodb/data/replset/

2）在三个节点机上安装mongodb
下载地址：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）分别在每个节点机上启动mongodb（启动时指明--bind_ip地址，默认是127.0.0.1，需要改成本机ip，否则远程连接时失败）
[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
在三个节点中的任意一个节点机上操作（比如在172.16.60.205节点机）

登陆mongodb
[root@mongodb-master01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.205:27017
.........
#使用admin数据库
> use admin
switched to db admin

#定义副本集配置变量，这里的 _id:”repset” 和上面命令参数“ –replSet repset” 要保持一样。
> 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)
                }
        }
}

#查看集群节点的状态
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)
                }
        }
}

如上信息表明：
副本集配置成功后，172.16.60.205为主节点PRIMARY，172.16.60.206/207为副本节点SECONDARY。
health：1   1表明状态是正常，0表明异常
state:1     值小的是primary节点、值大的是secondary节点

三、测试Mongodb副本集数据复制功能 <mongodb默认是从主节点读写数据的，副本节点上不允许读，需要设置副本节点可以读>

1）在主节点172.16.60.205上连接到终端
[root@mongodb-master01 ~]# /usr/local/mongodb/bin/mongo 172.16.60.205:27017
................
#建立test 数据库
repset:PRIMARY> use test;
switched to db test

#往testdb表插入测试数据
repset:PRIMARY> db.testdb.insert({"test1":"testval1"})
WriteResult({ "nInserted" : 1 })

2）在副本节点172.16.60.206、172.16.60.207上连接到mongodb查看数据是否复制过来。
这里在172.16.60.206副本节点上进行查看
[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

上面出现了报错！
这是因为mongodb默认是从主节点读写数据的，副本节点上不允许读，需要设置副本节点可以读
repset:SECONDARY> db.getMongo().setSlaveOk();
repset:SECONDARY> db.testdb.find();
{ "_id" : ObjectId("5c74ec9267d8c3d06506449b"), "test1" : "testval1" }
repset:SECONDARY> show tables;
testdb

如上发现已经在副本节点上发现了测试数据，即已经从主节点复制过来了。
（在另一个副本节点172.16.60.207也如上操作即可）

四、测试副本集故障转移功能
先停掉主节点172.16.60.205，查看mongodb副本集状态，可以看到经过一系列的投票选择操作，172.16.60.206当选主节点，172.16.60.207从172.16.60.206同步数据过来。

1）停掉原来的主节点172.16.60.205的mongodb，模拟故障
[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）接着登录到另外两个正常的从节点172.16.60.206、172.16.60.207中的任意一个节点的mongodb，查看副本集状态
[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)
                }
        }
}

发现当原来的主节点172.16.60.205宕掉后，经过选举，原来的从节点172.16.60.206被推举为新的主节点。

3）现在在172.16.60.206新主节点上创建测试数据
repset:PRIMARY> use kevin;
switched to db kevin
repset:PRIMARY> db.kevin.insert({"shibo":"hahaha"})
WriteResult({ "nInserted" : 1 })

4）另一个从节点172.16.60.207上登录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" }

发现从节点172.16.60.207可以同步新的主节点172.16.60.206的数据

5）再重新启动原来的主节点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

再次登录到三个节点中的任意一个的mongodb，查看副本集状态
[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)
                }
        }
}

发现原来的主节点172.16.60.205在故障恢复后，变成了新的主节点172.16.60.206的从节点

五、Mongodb读写分离
目前来看。Mongodb副本集可以完美支持故障转移。至于主节点的读写压力过大如何解决？常见的解决方案是读写分离。

一般情况下，常规写操作来说并没有读操作多，所以在Mongodb副本集中，一台主节点负责写操作，两台副本节点负责读操作。
1）设置读写分离需要先在副本节点SECONDARY 设置 setSlaveOk。
2）在程序中设置副本节点负责读操作，如下代码：

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" );

//读操作从副本节点读取
ReadPreference preference = ReadPreference. secondary();
DBObject dbObject = coll.findOne(object, null , preference);
System. out .println(dbObject);
} catch (Exception e) {
e.printStackTrace();
}
}
}

读参数除了secondary一共还有五个参数：primary、primaryPreferred、secondary、secondaryPreferred、nearest。
primary：默认参数，只从主节点上进行读取操作；
primaryPreferred：大部分从主节点上读取数据,只有主节点不可用时从secondary节点读取数据。
secondary：只从secondary节点上进行读取操作，存在的问题是secondary节点的数据会比primary节点数据“旧”。
secondaryPreferred：优先从secondary节点进行读取操作，secondary节点不可用时从主节点读取数据；
nearest：不管是主节点、secondary节点，从网络延迟最低的节点上读取数据。

读写分离做好后，就可以进行数据分流，减轻压力，解决了"主节点的读写压力过大如何解决？"这个问题。不过当副本节点增多时，主节点的复制压力会加大有什么办法解决吗？基于这个问题，Mongodb已有了相应的解决方案 - 引用仲裁节点：
在Mongodb副本集中，仲裁节点不存储数据，只是负责故障转移的群体投票，这样就少了数据复制的压力。看起来想的很周到啊，其实不只是主节点、副本节点、仲裁节点，还有Secondary-Only、Hidden、Delayed、Non-Voting，其中：
Secondary-Only：不能成为primary节点，只能作为secondary副本节点，防止一些性能不高的节点成为主节点。
Hidden：这类节点是不能够被客户端制定IP引用，也不能被设置为主节点，但是可以投票，一般用于备份数据。
Delayed：可以指定一个时间延迟从primary节点同步数据。主要用于备份数据，如果实时同步，误删除数据马上同步到从节点，恢复又恢复不了。
Non-Voting：没有选举权的secondary节点，纯粹的备份数据节点。

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Centos 7.5基于MySQL 5.7的 InnoDB Cluster 多节点高可用集群环境部署记录 ProxySQL+MGR实现读写分离和主节点故障无感知切换 - 完整操作记录
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