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MongoDB：Replica Set 节点切换和 Failover

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前面学习了 Replica Set 的搭建和从节点的添加，删除过程，接下来学习Replica Set 节点的切换以及 failover 相关的内容。
Replica Set 节点切换是指当出现故障或者出于维护需要，需要将主节点切换到另一台从节点，例如主节点主机需要硬件扩容时，那么需要停主节点主机， Replica Set 支持节点成员切换，其中使用的是投票竞选机制，对于投票竞选机制看了下文档，还不是非常明白，但主要是通过 priority 参数来控制的，接下来演示下。

节点切换

1.1 基础环境 ( 三节点 )

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rs0:PRIMARY> rs.status();  
{  
 "set" : "rs0",  
 "date" : ISODate("2012-11-27T16:38:15Z"),  
 "myState" : 1,  
 "members" : [  
 {  
 "_id" : 0,  
 "name" : "redhatB.example.com:27018",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 9143,  
 "optime" : Timestamp(1354025540000, 1),  
 "optimeDate" : ISODate("2012-11-27T14:12:20Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T16:38:15Z"),  
 "pingMs" : 0  
 },  
 {  
 "_id" : 1,  
 "name" : "redhatB.example.com:27019",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 8733,  
 "optime" : Timestamp(1354025540000, 1),  
 "optimeDate" : ISODate("2012-11-27T14:12:20Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T16:38:15Z"),  
 "pingMs" : 0  
 },  
 {  
 "_id" : 2,  
 "name" : "redhatB.example.com:27020",  
 "health" : 1,  
 "state" : 1,  
 "stateStr" : "PRIMARY",  
 "uptime" : 462929,  
 "optime" : Timestamp(1354025540000, 1),  
 "optimeDate" : ISODate("2012-11-27T14:12:20Z"),  
 "self" : true  
 }  
 ],  
 "ok" : 1  
}

备注：三节点 replica set 环境，其中最后一个节点为主节点，假如主节点由于某种原因需要切换，假设需要切换到从节点”redhatB.example.com:27018”，可以通过设置节点 priority 参数来实现，节点的 priority 值越大，切换时的优先级越高。

1.2 设置节点的 priority 值

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cfg = rs.conf()  
cfg.members[0].priority = 2  
cfg.members[1].priority = 1  
cfg.members[2].priority = 0.5  
rs.reconfig(cfg)

1.3 操作日志

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rs0:PRIMARY> cfg.members[0].priority = 2cfg.members[0].priority = 2  
2  
rs0:PRIMARY> cfg.members[1].priority = 1cfg.members[1].priority = 1  
1  
rs0:PRIMARY> cfg.members[2].priority = 0.5cfg.members[2].priority = 0.5  
0.5  

rs0:PRIMARY> rs.reconfig(cfg)  
Wed Nov 28 00:57:30 DBClientCursor::init call() failed  
Wed Nov 28 00:57:30 query failed : admin.$cmd { replSetReconfig: { _id: "rs0", version: 9, members: [ { _id: 0, host: "redhatB.example.com:27018", priority: 2.0 }, { _id: 1, host: "redhatB.example.com:27019", priority: 1.0 }, { _id: 2, host: "redhatB.example.com:27020", priority: 0.5 } ] } } to: 127.0.0.1:27020  
Wed Nov 28 00:57:30 trying reconnect to 127.0.0.1:27020  
Wed Nov 28 00:57:30 reconnect 127.0.0.1:27020 ok  
reconnected to server after rs command (which is normal)

备注：设置好从节点的 priority 值后，调用一次 rs.reconfig 操作会导致当前主库中断，因为需要重新执行主节点竞选，大概几十秒后，新的主节点产生。

1.4 重新查看状态

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rs0:SECONDARY> rs.conf();  
{  
 "_id" : "rs0",  
 "version" : 9,  
 "members" : [  
 {  
 "_id" : 0,  
 "host" : "redhatB.example.com:27018",  
 "priority" : 2  
 },  
 {  
 "_id" : 1,  
 "host" : "redhatB.example.com:27019"  
 },  
 {  
 "_id" : 2,  
 "host" : "redhatB.example.com:27020",  
 "priority" : 0.5  
 }  
 ]  
}

备注：这时可以看到各节点优先级 priority 的值，节点默认的 priority 值为 1，不显示。

1.5 再次查看节点状态

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rs0:SECONDARY> rs.status();  
{  
 "set" : "rs0",  
 "date" : ISODate("2012-11-27T16:58:27Z"),  
 "myState" : 2,  
 "syncingTo" : "redhatB.example.com:27018",  
 "members" : [  
 {  
 "_id" : 0,  
 "name" : "redhatB.example.com:27018",  
 "health" : 1,  
 "state" : 1,  
 "stateStr" : "PRIMARY",  
 "uptime" : 47,  
 "optime" : Timestamp(1354035450000, 1),  
 "optimeDate" : ISODate("2012-11-27T16:57:30Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T16:58:26Z"),  
 "pingMs" : 0  
 },  
 {  
 "_id" : 1,  
 "name" : "redhatB.example.com:27019",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 47,  
 "optime" : Timestamp(1354035450000, 1),  
 "optimeDate" : ISODate("2012-11-27T16:57:30Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T16:58:26Z"),  
 "pingMs" : 0  
 },  
 {  
 "_id" : 2,  
 "name" : "redhatB.example.com:27020",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 464141,  
 "optime" : Timestamp(1354035450000, 1),  
 "optimeDate" : ISODate("2012-11-27T16:57:30Z"),  
 "errmsg" : "syncing to: redhatB.example.com:27018",  
 "self" : true  
 }  
 ],  
 "ok" : 1  
}

备注：此时主节点已漂移到节点 “redhatB.example.com:27018”，达到目标。

测试 Failover

MongoDB 的 replica set 特性支持自动 failover，当主节点由于某种原因掉线时，replica set的其它节点可通过竞选产生新的主节点，接下来测试下这个特性。

2.1 基础信息

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rs0:PRIMARY> rs.status();  
{  
 "set" : "rs0",  
 "date" : ISODate("2012-11-27T17:50:53Z"),  
 "myState" : 1,  
 "members" : [  
 {  
 "_id" : 0,  
 "name" : "redhatB.example.com:27018",  
 "health" : 1,  
 "state" : 1,  
 "stateStr" : "PRIMARY",  
 "uptime" : 200,  
 "optime" : Timestamp(1354037833000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:37:13Z"),  
 "self" : true  
 },  
 {  
 "_id" : 1,  
 "name" : "redhatB.example.com:27019",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 108,  
 "optime" : Timestamp(1354037833000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:37:13Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T17:50:52Z"),  
 "pingMs" : 0,  
 "errmsg" : "syncing to: redhatB.example.com:27018"  
 },  
 {  
 "_id" : 2,  
 "name" : "redhatB.example.com:27020",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 35,  
 "optime" : Timestamp(1354037833000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:37:13Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T17:50:52Z"),  
 "pingMs" : 0,  
 "errmsg" : "syncing to: redhatB.example.com:27018"  
 }  
 ],  
 "ok" : 1  
}

备注：此时的主节点为 “redhatB.example.com:27018”。

2.2 异常关闭主节点。

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[mongo@redhatB data03]$ ps -ef | grep 27018  
mongo 879 1 0 01:47 ? 00:00:02 mongod -f /pgdata_xc/mongodb/data01/mongodb_27018.conf  
mongo 1250 21805 0 01:50 pts/0 00:00:00 mongo 127.0.0.1:27018  
mongo 1296 23173 0 01:51 pts/1 00:00:00 grep 27018  
[mongo@redhatB data03]$ kill -9 879

备注：过一会后，节点 “redhatB.example.com:27019” 竞选成为新主节点。

2.3 查看节点状态

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[mongo@redhatB ~]$ mongo 127.0.0.1:27019  
MongoDB shell version: 2.2.1  
connecting to: 127.0.0.1:27019/test
rs0:PRIMARY> rs.conf();  
{  
 "_id" : "rs0",  
 "version" : 9,  
 "members" : [  
 {  
 "_id" : 0,  
 "host" : "redhatB.example.com:27018",  
 "priority" : 2  
 },  
 {  
 "_id" : 1,  
 "host" : "redhatB.example.com:27019"  
 },  
 {  
 "_id" : 2,  
 "host" : "redhatB.example.com:27020",  
 "priority" : 0.5  
 }  
 ]  
}  
rs0:PRIMARY> rs.status();  
{  
 "set" : "rs0",  
 "date" : ISODate("2012-11-27T17:52:41Z"),  
 "myState" : 1,  
 "members" : [  
 {  
 "_id" : 0,  
 "name" : "redhatB.example.com:27018",  
 "health" : 0,  
 "state" : 8,  
 "stateStr" : "(not reachable/healthy)", "uptime" : 0,  
 "optime" : Timestamp(1354037833000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:37:13Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T17:52:01Z"),  
 "pingMs" : 0,  
 "errmsg" : "socket exception [CONNECT_ERROR] for redhatB.example.com:27018"  
 },  
 {  
 "_id" : 1,  
 "name" : "redhatB.example.com:27019",  
 "health" : 1,  
 "state" : 1,  
 "stateStr" : "PRIMARY",  
 "uptime" : 251,  
 "optime" : Timestamp(1354037833000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:37:13Z"),  
 "self" : true  
 },  
 {  
 "_id" : 2,  
 "name" : "redhatB.example.com:27020",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 143,  
 "optime" : Timestamp(1354037833000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:37:13Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T17:52:41Z"),  
 "pingMs" : 0,  
 "errmsg" : "syncing to: redhatB.example.com:27019"  
 }  
 ],  
 "ok" : 1  
}

备注：可见节点 “redhatB.example.com:27019” 转变成主节点了，此时异常的节点”redhatB.example.com:27018” 不可用，接下来测试，启动异常节点是否能够自动恢复。

2.4 连接新主库，并新表一个集合；

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[mongo@redhatB ~]$ mongo 127.0.0.1:27019  
MongoDB shell version: 2.2.1  
connecting to: 127.0.0.1:27019/test

rs0:PRIMARY> show collections;  
system.indexes  
test_1  
test_2  
test_3  
things

rs0:PRIMARY> db.test_4.save({id:1});
rs0:PRIMARY> db.test_4.find();  
{ "_id" : ObjectId("50b4fe1b15747af472f54831"), "id" : 1 }

2.5 启动原来故障节点

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[mongo@redhatB data03]$ mongod -f /pgdata_xc/mongodb/data01/mongodb_27018.conf  
forked process: 1692  
all output going to: /pgdata_xc/mongodb/data01/mongo.log  
child process started successfully, parent exiting

备注：三个节点都启用日志模式(journal)，否则恢复时会有异常。

2.6 再次查看节点状态

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rs0:PRIMARY> rs.status();rs.status();  
{  
 "set" : "rs0",  
 "date" : ISODate("2012-11-27T18:16:43Z"),  
 "myState" : 1,  
 "members" : [  
 {  
 "_id" : 0,  
 "name" : "redhatB.example.com:27018",  
 "health" : 1,  
 "state" : 1,  
 "stateStr" : "PRIMARY",  
 "uptime" : 1321,  
 "optime" : Timestamp(1354038811000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:53:31Z"),  
 "self" : true  
 },  
 {  
 "_id" : 1,  
 "name" : "redhatB.example.com:27019",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 1313,  
 "optime" : Timestamp(1354038811000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:53:31Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T18:16:42Z"),  
 "pingMs" : 1  
 },  
 {  
 "_id" : 2,  
 "name" : "redhatB.example.com:27020",  
 "health" : 1,  
 "state" : 2,  
 "stateStr" : "SECONDARY",  
 "uptime" : 1313,  
 "optime" : Timestamp(1354038811000, 1),  
 "optimeDate" : ISODate("2012-11-27T17:53:31Z"),  
 "lastHeartbeat" : ISODate("2012-11-27T18:16:42Z"),  
 "pingMs" : 0  
 }  
 ],  
 "ok" : 1  
}

备注：节点 “name” : “redhatB.example.com:27018” 再次竞选成功。

2.7 测试数据

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[mongo@redhatB ~]$ mongo 127.0.0.1:27018  
MongoDB shell version: 2.2.1  
connecting to: 127.0.0.1:27018/test  
rs0:PRIMARY> show collections;  
system.indexes  
test_1  
test_2  
test_3  
test_4  
things
rs0:PRIMARY> db.test_4.find();  
{ "_id" : ObjectId("50b4fe1b15747af472f54831"), "id" : 1 }

备注：恢复后的节点可以查到新数据了，说明已自动恢复。

总结

以上只演示三节点 Replica set 的 failover 的情况，其它情况并没演示；另外 failover 的关键点为投票机制，这点还不是非常清楚，只知道设置节点的 priority ，还需要查阅相关资料。关于奇数节点和偶数节点 Primary 宕机后 SECONDARY 是否可以接管的问题，可以参考以下帖子：
http://www.itpub.net/thread-1740982-1-1.html

四参考

http://docs.mongodb.org/manual/administration/replica-sets/
http://docs.mongodb.org/manual/core/replication/#replica-set-failover

原创文章，作者：745907710，如若转载，请注明出处：https://blog.ytso.com/237924.html

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