（一）MongDB复制（副本集）
MongDB复制是将数据同步到多个服务器的过程。
复制提供了数据的冗余备份，并在服务器上存储数据副本，提高数据的可用性，保证数据的安全性，
复制允许你从硬件和服务的中断中恢复数据，能随时应对数据丢失、机器损坏带来的风险。
而且复制还能提高读取能力，用户读取服务器和写入服务器在不同的地方，提高整个系统的负载。
1、复制的特性：
保障数据的安全性
数据的高可用性7*24
灾难恢复
无需停机维护
分布式读取数据
2、复制的原理
Mongodb复制集由一组Mongod实例（进程）组成。mongodb复制至少需要二个节点，一个是Primary主节点用于处理客户端的请求，其余都是从节点Secondary用于复制主节点上的数据.
Primary会记录那些写的操作到一个日志oplog中，其他的Secondary会定期轮询主节点这个日志，去操作他们的数据库来保持数据库的一致。
客户端从主节点读取数据，在客户端写入数据到主节点时， 主节点与从节点进行数据交互保障数据的一致性。
MongoDB（3.2以上）中，官方已经不推荐使用主从模式，取而代之的，是使用复制集（Replica Set）的方式做主备处理。
3、复制集（Replica Set），在复制集中，有且只有一个主节点(primary)，可以包含一个或多个从节点(secondary)，主从节点直接会通过心跳检测来确定节点是否健康或存活。所有的读写操作都是在主节点上进行的，如果要实现读写分离，需要进行相应的处理，这个最后会说。从节点会根据oplog（也就是操作日志）来复制主节点的数据。原理：而 MongoDB 复制集,集群中的任何节点都可能成为 Master 节点。一旦 Master 节点故障,则会在其余节点中选举出一个新的 Master 节点。 并引导剩余节点连接到新的 Master 节点。这个过程对于应用是透明的。
在生产环境，复制集至少包括三个节点，其中一个必须为主节点，一个从节点，一个仲裁节点。其中每一个节点都是mongod进程对应的实例，节点间通过心跳检查对方的状态。
primary节点：负责数据库的读写操作。
secondary节点：备份primary节点上的数据，可以有多个。
arbiter节点：主节点故障时，参与复制集剩下节点中选举一个新的primary节点。
（二）、mongodb环境配置
1、主机配置
192.168.4.203 db1 ##primary节点
192.168.4.97 db2 ##secondary节点
192.168.4.200 db3 ##arbriter节点
2、安装（略）请参考https://blog.51cto.com/liqingbiao/2401942
3、primary节点的相关操作。
3.1、相关主节点的配置[root@otrs ~]# vim /usr/local/mongodb/conf/master.conf
bind_ip = 0.0.0.0
port = 27017
dbpath = /data/mongodb/db
logpath = /data/mongodb/mongodb.log
logappend = true
fork = true
journal=true
maxConns=1000
auth = true
keyFile = /usr/local/mongodb/mongodb-keyfile
storageEngine=wiredTiger
oplogSize=2048
replSet=RS
keyFile = /usr/local/mongodb/mongodb-keyfile ##### 集群的私钥的完整路径，只对于Replica Set 架构有效（noauth = true时不用配置此项）
3.2、在服务器上生成私钥，并设置成600，把该私钥scp到其它远程服务器上。 [root@otrs ~]# openssl rand -base64 745 > /usr/local/mongodb/mongodb-keyfile
[root@otrs ~]# chmod 600 /usr/local/mongodb/mongodb-keyfile
[root@otrs ~]# cat /usr/local/mongodb/mongodb-keyfile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[root@otrs ~]# scp /usr/local/mongodb/mongodb-keyfile [email protected]:/usr/local/mongodb/
mongodb-keyfile 100% 1012 1.0KB/s 00:00 3.3、创建认证用户。由于配置文件启用认证模式，现在需要以非认证的模式来创建用户。
#```
#######先以非验证模式启动数据库
[root@otrs ~]# /usr/local/mongodb/bin/mongod --fork --dbpath=/data/mongodb/db --logpath=/data/mongodb/mongodb.log
about to fork child process, waiting until server is ready for connections.
forked process: 28389
child process started successfully, parent exiting
[root@otrs ~]# netstat -lntp|grep mongod
tcp 0 0 127.0.0.1:27017 0.0.0.0:* LISTEN 28389/mongod
############创建root用户并配置管理员权限
[root@otrs ~]# mongo
MongoDB shell version v4.0.9
connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("4df82588-c03c-49e5-8839-92dafd7ff8ce") }
MongoDB server version: 4.0.9
Welcome to the MongoDB shell.
For interactive help, type "help".
For more comprehensive documentation, see
http://docs.mongodb.org/
Questions? Try the support group
http://groups.google.com/group/mongodb-user
Server has startup warnings:
2019-06-14T16:02:56.903+0800 I CONTROL [initandlisten]
2019-06-14T16:02:56.903+0800 I CONTROL [initandlisten] WARNING: Access control is not enabled for the database.
2019-06-14T16:02:56.903+0800 I CONTROL [initandlisten] Read and write access to data and configuration is unrestricted.
2019-06-14T16:02:56.903+0800 I CONTROL [initandlisten]
show dbs
admin 0.000GB
config 0.000GB
local 0.000GB
use admin
switched to db admin
db.createUser({ user: "root", pwd: "root", roles: [ { role: "root", db: "admin" } ] });
"user" : "root",
"roles" : [
{
"role" : "root",
"db" : "admin"
}
]
}3.4、启动以验证权限的配置文件[root@otrs ~]# mongod --config /usr/local/mongodb/conf/master.conf
about to fork child process, waiting until server is ready for connections.
forked process: 28698
child process started successfully, parent exiting
[root@otrs ~]# ps -ef|grep mongod
root 28698 1 19 16:19 ? 00:00:01 mongod --config /usr/local/mongodb/conf/master.conf
root 28731 28040 0 16:19 pts/0 00:00:00 grep --color=auto mongod4、配置secondary节点 4.1、新建目录编辑配置文件[root@otrs004097 opt]# mkdir /data/mongodb/standard
[root@otrs004097 opt]# cat /usr/local/mongodb/conf/standard.conf
bind_ip = 0.0.0.0
port = 27017
dbpath = /data/mongodb/standard
logpath = /data/mongodb/mongodb.log
logappend = true
fork = true
journal=true
maxConns=1000
auth = true
keyFile = /usr/local/mongodb/mongodb-keyfile
storageEngine=wiredTiger
oplogSize=2048
replSet=RS4.2、启动服务[root@otrs004097 opt]# /usr/local/mongodb/bin/mongod --fork --config /usr/local/mongodb/conf/standard.conf
[root@otrs004097 opt]# netstat -lntp|grep mongod
tcp 0 0 0.0.0.0:27017 0.0.0.0:* LISTEN 1045/mongod5、配置arbriter节点[root@NginxServer01 mongodb]# cat /usr/local/mongodb/conf/arbiter.conf
bind_ip = 0.0.0.0
port = 27017
dbpath = /data/mongodb/arbiter
logpath = /data/mongodb/mongodb.log
logappend = true
fork = true
journal=true
maxConns=1000
auth = true
keyFile = /usr/local/mongodb/mongodb-keyfile
storageEngine=wiredTiger
oplogSize=2048
replSet=RS
[root@NginxServer01 mongodb]# /usr/local/mongodb/bin/mongod --config /usr/local/mongodb/conf/arbiter.conf
about to fork child process, waiting until server is ready for connections.
forked process: 26321
child process started successfully, parent exiting
[root@NginxServer01 mongodb]# netstat -lntp|grep mongod
tcp 0 0 0.0.0.0:27017 0.0.0.0:* LISTEN 26321/mongod6、在primary节点服务器上，初始化复制集primary节点、添加secondary节点和arbriter节点。 6.1、初始化primary节点[root@otrs ~]# mongo -uroot -p
MongoDB shell version v4.0.9
Enter password:
#########查看副本集的状态，发现并未初始化，接下来进行初始化。
rs.status()
{
"ok" : 0,
"errmsg" : "no replset config has been received",
"code" : 94,
"codeName" : "NotYetInitialized"
}#########进行config的配置
config={"_id":"RS","members":[ {"_id":0,"host":"192.168.4.203:27017"},{"_id":1,"host":"192.168.4.97:27017"}]}
{
"_id" : "RS",
"members" : [
{
"_id" : 0,
"host" : "192.168.4.203:27017"
},
{
"_id" : 1,
"host" : "192.168.4.97:27017"
}
]
}rs.initiate(config); #######初始化
{ "ok" : 1 }
6.2、查看副本集的状态
RS:SECONDARY> rs.status()
{
"set" : "RS",
"date" : ISODate("2019-06-14T09:01:19.722Z"),
"myState" : 2,
"term" : NumberLong(0),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"heartbeatIntervalMillis" : NumberLong(2000),
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(0, 0),
"t" : NumberLong(-1)
},
"appliedOpTime" : {
"ts" : Timestamp(1560502874, 1),
"t" : NumberLong(-1)
},
"durableOpTime" : {
"ts" : Timestamp(1560502874, 1),
"t" : NumberLong(-1)
}
},
"lastStableCheckpointTimestamp" : Timestamp(0, 0),
"members" : [
{
"_id" : 0,
"name" : "192.168.4.203:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 2525,
"optime" : {
"ts" : Timestamp(1560502874, 1),
"t" : NumberLong(-1)
},
"optimeDate" : ISODate("2019-06-14T09:01:14Z"),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "could not find member to sync from",
"configVersion" : 1,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 1,
"name" : "192.168.4.97:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 5,
"optime" : {
"ts" : Timestamp(1560502874, 1),
"t" : NumberLong(-1)
},
"optimeDurable" : {
"ts" : Timestamp(1560502874, 1),
"t" : NumberLong(-1)
},
"optimeDate" : ISODate("2019-06-14T09:01:14Z"),
"optimeDurableDate" : ISODate("2019-06-14T09:01:14Z"),
"lastHeartbeat" : ISODate("2019-06-14T09:01:19.645Z"),
"lastHeartbeatRecv" : ISODate("2019-06-14T09:01:19.489Z"),
"pingMs" : NumberLong(1),
"lastHeartbeatMessage" : "",
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"configVersion" : 1
}
],
"ok" : 1
}6.3、由初始化结果查看，两个都是secondary节点，由于数据库同步没有完成，等同步完成的时候会变成primary节点。等两分钟再次查看下
RS:PRIMARY> rs.status()
{
"set" : "RS",
"date" : ISODate("2019-06-14T09:11:17.382Z"),
"myState" : 1,
"term" : NumberLong(1),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"heartbeatIntervalMillis" : NumberLong(2000),
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(1560503477, 1),
"t" : NumberLong(1)
},
"readConcernMajorityOpTime" : {
"ts" : Timestamp(1560503477, 1),
"t" : NumberLong(1)
},
"appliedOpTime" : {
"ts" : Timestamp(1560503477, 1),
"t" : NumberLong(1)
},
"durableOpTime" : {
"ts" : Timestamp(1560503477, 1),
"t" : NumberLong(1)
}
},
"lastStableCheckpointTimestamp" : Timestamp(1560503427, 1),
"members" : [
{
"_id" : 0,
"name" : "192.168.4.203:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 3123,
"optime" : {
"ts" : Timestamp(1560503477, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-06-14T09:11:17Z"),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"electionTime" : Timestamp(1560502885, 1),
"electionDate" : ISODate("2019-06-14T09:01:25Z"),
"configVersion" : 1,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 1,
"name" : "192.168.4.97:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 603,
"optime" : {
"ts" : Timestamp(1560503467, 1),
"t" : NumberLong(1)
},
"optimeDurable" : {
"ts" : Timestamp(1560503467, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-06-14T09:11:07Z"),
"optimeDurableDate" : ISODate("2019-06-14T09:11:07Z"),
"lastHeartbeat" : ISODate("2019-06-14T09:11:15.995Z"),
"lastHeartbeatRecv" : ISODate("2019-06-14T09:11:16.418Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "192.168.4.203:27017",
"syncSourceHost" : "192.168.4.203:27017",
"syncSourceId" : 0,
"infoMessage" : "",
"configVersion" : 1
}
],
"ok" : 1,
"operationTime" : Timestamp(1560503477, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1560503477, 1),
"signature" : {
"hash" : BinData(0,"iR63R/X7QanbrWvuDJNkpdPgcVY="),
"keyId" : NumberLong("6702308864978583553")
}
}
}6.4、在secondary服务器上查看同步日志
[root@otrs004097 opt]# tail -f /data/mongodb/mongodb.log
2019-06-14T17:01:15.870+0800 I REPL [replexec-0] This node is 192.168.4.97:27017 in the config
2019-06-14T17:01:15.870+0800 I REPL [replexec-0] transition to STARTUP2 from STARTUP
2019-06-14T17:01:15.871+0800 I REPL [replexec-0] Starting replication storage threads
2019-06-14T17:01:15.872+0800 I REPL [replexec-2] Member 192.168.4.203:27017 is now in state SECONDARY
2019-06-14T17:01:15.872+0800 I STORAGE [replexec-0] createCollection: local.temp_oplog_buffer with generated UUID: 2e5c6683-a67b-4a16-bd9b-8672ee4db900
2019-06-14T17:01:15.880+0800 I REPL [replication-0] Starting initial sync (attempt 1 of 10)
2019-06-14T17:01:15.881+0800 I STORAGE [replication-0] Finishing collection drop for local.temp_oplog_buffer (2e5c6683-a67b-4a16-bd9b-8672ee4db900).
2019-06-14T17:01:15.882+0800 I STORAGE [replication-0] createCollection: local.temp_oplog_buffer with generated UUID: c25fa3cf-cae9-430b-b514-f13c3ab1e247
2019-06-14T17:01:15.889+0800 I REPL [replication-0] sync source candidate: 192.168.4.203:27017
2019-06-14T17:01:15.889+0800 I REPL [replication-0] Initial syncer oplog truncation finished in: 0ms
2019-06-14T17:01:15.889+0800 I REPL [replication-0] ******
2019-06-14T17:01:15.889+0800 I REPL [replication-0] creating replication oplog of size: 2048MB...
2019-06-14T17:01:15.889+0800 I STORAGE [replication-0] createCollection: local.oplog.rs with generated UUID: f35caf0a-f2da-4cf7-b16b-34a94f1e25a7
2019-06-14T17:01:15.892+0800 I STORAGE [replication-0] Starting OplogTruncaterThread local.oplog.rs
2019-06-14T17:01:15.892+0800 I STORAGE [replication-0] The size storer reports that the oplog conta××× 0 records totaling to 0 bytes
2019-06-14T17:01:15.892+0800 I STORAGE [replication-0] Scanning the oplog to determine where to place markers for truncation
2019-06-14T17:01:15.905+0800 I REPL [replication-0] ******
2019-06-14T17:01:15.905+0800 I STORAGE [replication-0] dropAllDatabasesExceptLocal 1 6.5、添加arbriter仲裁节点RS:PRIMARY> rs.addArb("192.168.4.45:27017")
{
"ok" : 1,
"operationTime" : Timestamp(1560504144, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1560504144, 1),
"signature" : {
"hash" : BinData(0,"um2WmD60Gh9q/43qUff8yN2abIw="),
"keyId" : NumberLong("6702308864978583553")
}
}
}##########查看复制集的状态
RS:PRIMARY> rs.status()
{
"set" : "RS",
"date" : ISODate("2019-06-14T09:40:04.334Z"),
"myState" : 1,
"term" : NumberLong(1),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"heartbeatIntervalMillis" : NumberLong(2000),
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
},
"readConcernMajorityOpTime" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
},
"appliedOpTime" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
},
"durableOpTime" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
}
},
"lastStableCheckpointTimestamp" : Timestamp(1560505167, 1),
"members" : [
{
"_id" : 0,
"name" : "192.168.4.203:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 4850,
"optime" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-06-14T09:39:57Z"),
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"electionTime" : Timestamp(1560502885, 1),
"electionDate" : ISODate("2019-06-14T09:01:25Z"),
"configVersion" : 2,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 1,
"name" : "192.168.4.97:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 2330,
"optime" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
},
"optimeDurable" : {
"ts" : Timestamp(1560505197, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2019-06-14T09:39:57Z"),
"optimeDurableDate" : ISODate("2019-06-14T09:39:57Z"),
"lastHeartbeat" : ISODate("2019-06-14T09:40:02.717Z"),
"lastHeartbeatRecv" : ISODate("2019-06-14T09:40:02.767Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "192.168.4.203:27017",
"syncSourceHost" : "192.168.4.203:27017",
"syncSourceId" : 0,
"infoMessage" : "",
"configVersion" : 2
},
{
"_id" : 2,
"name" : "192.168.4.45:27017",
"health" : 1,
"state" : 7,
"stateStr" : "ARBITER",
"uptime" : 11,
"lastHeartbeat" : ISODate("2019-06-14T09:40:02.594Z"),
"lastHeartbeatRecv" : ISODate("2019-06-14T09:40:04.186Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncingTo" : "",
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"configVersion" : 2
}
],
"ok" : 1,
"operationTime" : Timestamp(1560505197, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1560505197, 1),
"signature" : {
"hash" : BinData(0,"Ff00RyXUvxDPc5nzFQYXGZIlnBc="),
"keyId" : NumberLong("6702308864978583553")
}
}
}
备注：
"health" : 1,——————是1，表示健康状态
"stateStr" : "PRIMARY",——————主
"stateStr" : "SECONDARY",——————从
"stateStr" : "ARBITER",——————仲裁7、查看primary节点、secondary节点、arbriter节点的数据的变化
7.1、主节点Parimary操作，创建表并插入数据。
8.RS:PRIMARY> use lqb
switched to db lqb
RS:PRIMARY> db.object.×××ert([{"language":"C"},{"language":"C++"}])
BulkWriteResult({
"writeErrors" : [ ],
"writeConcernErrors" : [ ],
"nInserted" : 2,
"nUpserted" : 0,
"nMatched" : 0,
"nModified" : 0,
"nRemoved" : 0,
"upserted" : [ ]
})
RS:PRIMARY> db.object.find()
{ "_id" : ObjectId("5d0370424e7535b767bb7098"), "language" : "C" }
{ "_id" : ObjectId("5d0370424e7535b767bb7099"), "language" : "C++" }
7.2、从节点操作。查看有数据同步到
RS:SECONDARY> use lqb;
switched to db lqb
RS:SECONDARY> rs.slaveOk();
RS:SECONDARY> db.object.find()
{ "_id" : ObjectId("5d0370424e7535b767bb7099"), "language" : "C++" }
{ "_id" : ObjectId("5d0370424e7535b767bb7098"), "language" : "C" }
7.3、仲裁节点aribriter。仲裁节点不会存储数据
RS:ARBITER> use lqb;
switched to db lqb
RS:ARBITER> show tables
Warning: unable to run listCollections, attempting to approximate collection names by parsing connectionStatus
RS:ARBITER> db.object.find()
Error: error: {
"ok" : 0,
"errmsg" : "not authorized on lqb to execute command { find: \"object\", filter: {}, lsid: { id: UUID(\"d2d7e624-8f30-468a-a3b0-79728b0cabbd\") }, $readPreference: { mode: \"secondaryPreferred\" }, $db: \"lqb\" }",
"code" : 13,
8、结论：
结论一：当主节点挂了得时候，从节点会变成主节点，当原先的主节点恢复的时候，会变成从节点，主节点不变。
结论二：主节点能写，从节点不能写。
结论三：主从节点的角色换了得话，原先的读写也就换了。