MongoDB 副本集切换方法

1、rs.stepDown([stepdownSecs, catchUpSecs])   step down as primary (disconnects)

    这个命令会将primary降级为Secondary节点

    登陆primary节点

[root@127-0-0-1 conf]# mongo --port 2777
    shard1:PRIMARY> show dbs
    admin   0.000GB
    config  0.000GB
    local   0.001GB
    yoon    0.000GB

   通过 help 帮助查看各种命令

shard1:PRIMARY> rs.help()
    rs.status()                                { replSetGetStatus : 1 } checks repl set status
    rs.initiate()                              { replSetInitiate : null } initiates set with default settings
    rs.initiate(cfg)                           { replSetInitiate : cfg } initiates set with configuration cfg
    rs.conf()                                  get the current configuration object from local.system.replset
    rs.reconfig(cfg)                           updates the configuration of a running replica set with cfg (disconnects)
    rs.add(hostportstr)                        add a new member to the set with default attributes (disconnects)
    rs.add(membercfgobj)                       add a new member to the set with extra attributes (disconnects)
    rs.addArb(hostportstr)                     add a new member which is arbiterOnly:true (disconnects)
    rs.stepDown([stepdownSecs, catchUpSecs])   step down as primary (disconnects)
    rs.syncFrom(hostportstr)                   make a secondary sync from the given member
    rs.freeze(secs)                            make a node ineligible to become primary for the time specified
    rs.remove(hostportstr)                     remove a host from the replica set (disconnects)
    rs.slaveOk()                               allow queries on secondary nodes

    rs.printReplicationInfo()                  check oplog size and time range
    rs.printSlaveReplicationInfo()             check replica set members and replication lag
    db.isMaster()                              check who is primary

    reconfiguration helpers disconnect from the database so the shell will display
    an error, even if the command succeeds.

    将primary降级为Secondary节点

shard1:PRIMARY> rs.stepDown() 

   这个命令会让primary降级为Secondary节点并维持30S，如果这段时间没有新的primary选举出来，这个节点重新加入进行选举

shard1:PRIMARY> rs.stepDown(30)

2、通过设置优先级
    A.为了保证数据的一致性，必须先关闭应用的写服务
    B.提升要升级为primary节点的Secondary节点的优先级

查看当前配置信息：

shard1:PRIMARY> rs.conf()
{
    "_id" : "shard1",
    "version" : 4,
    "protocolVersion" : NumberLong(1),
    "writeConcernMajorityJournalDefault" : true,
    "members" : [
        {
            "_id" : 0,
            "host" : "127.0.0.1:2777",
            "arbiterOnly" : false,
            "buildIndexes" : true,
            "hidden" : false,
            "priority" : 3,
            "tags" : {

            },
            "slaveDelay" : NumberLong(0),
            "votes" : 1
        },
        {
            "_id" : 1,
            "host" : "127.0.0.1:3777",
            "arbiterOnly" : false,
            "buildIndexes" : true,
            "hidden" : false,
            "priority" : 1,
            "tags" : {

            },
            "slaveDelay" : NumberLong(0),
            "votes" : 1
        },
        {
            "_id" : 2,
            "host" : "127.0.0.1:4777",
            "arbiterOnly" : false,
            "buildIndexes" : true,
            "hidden" : false,
            "priority" : 1,
            "tags" : {

            },
            "slaveDelay" : NumberLong(0),
            "votes" : 1
        }
    ],
    "settings" : {
        "chainingAllowed" : true,
        "heartbeatIntervalMillis" : 2000,
        "heartbeatTimeoutSecs" : 10,
        "electionTimeoutMillis" : 10000,
        "catchUpTimeoutMillis" : -1,
        "catchUpTakeoverDelayMillis" : 30000,
        "getLastErrorModes" : {

        },
        "getLastErrorDefaults" : {
            "w" : 1,
            "wtimeout" : 0
        },
        "replicaSetId" : ObjectId("5f27de76c8d22581cdeb2f2e")
    }
}

    查看当前配置，存入config变量中

shard1:PRIMARY> config=rs.conf() 

    修改config变量，将第三组端口为4777的成员优先级设置为3

hard1:PRIMARY> config.members[2].priority = 3 

    配置生效

shard1:PRIMARY> rs.reconfig(config)

   查看切换后的节点信息

shard1:PRIMARY> db.isMaster()
{
    "hosts" : [
        "127.0.0.1:2777",
        "127.0.0.1:3777",
        "127.0.0.1:4777"
    ],
    "setName" : "shard1",
    "setVersion" : 7,
    "ismaster" : true,
    "secondary" : false,
    "primary" : "127.0.0.1:4777",    
    "me" : "127.0.0.1:4777",
    "electionId" : ObjectId("7fffffff0000000000000004"),
    "lastWrite" : {
        "opTime" : {
            "ts" : Timestamp(1596599298, 1),
            "t" : NumberLong(4)
        },
        "lastWriteDate" : ISODate("2020-08-05T03:48:18Z"),
        "majorityOpTime" : {
            "ts" : Timestamp(1596599298, 1),
            "t" : NumberLong(4)
        },
        "majorityWriteDate" : ISODate("2020-08-05T03:48:18Z")
    }..
    ...