mongodb副本集原理

原理：
副本集羣每2秒發一次心跳，若是10秒事後沒收到則標記爲不可達。
MongoDB在主數據庫上應用數據庫操做，而後在主數據庫的操做日誌中記錄該操做。而後，輔助成員將這些操做複製並應用到異步過程當中。全部副本集成員在local.oplog.rs集合中都包含操做日誌的副本 ，默認大小爲磁盤的5%，但不記錄查詢操做。oplogSize能夠在/etc/mongdb.conf中指定大小

replication:
oplogSizeMB: <int>
replSetName: <string>
secondaryIndexPrefetch: <string>
enableMajorityReadConcern: <boolean>

mongdb副本集中的成員:
primary: 負責寫入數據
seconary: 負責同步主上的數據，提供讀服務
仲裁節點： 不保有數據，不參與選主，只進行選主投票

副本集選舉經過優先級，範圍0-1000，0爲仲裁節點，不參與選舉。默認是1

如何設置？

MongoDB Enterprise shard-rs:PRIMARY> config=rs.conf()
MongoDB Enterprise shard-rs:PRIMARY> config
{
"_id" : "shard-rs",
"version" : 3,
"protocolVersion" : NumberLong(1),
"writeConcernMajorityJournalDefault" : true,
"members" : [
{
"_id" : 0,
"host" : "worker2:27020",
"arbiterOnly" : false,
"buildIndexes" : true,
"hidden" : false,
"priority" : 1,
"tags" : {

},
                    "slaveDelay" : NumberLong(0),
                    "votes" : 1
            },
            {
                    "_id" : 1,
                    "host" : "worker2:27021",
                    "arbiterOnly" : false,
                    "buildIndexes" : true,
                    "hidden" : false,
                    "priority" : 1,
                    "tags" : {

                    },
                    "slaveDelay" : NumberLong(0),
                    "votes" : 1
            },
            {
                    "_id" : 2,
                    "host" : "worker2:27022",
                    "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("5dd8bf958b8ea486b84c23b2")
    }

}

修改id值爲2的分片優先級爲3，當primary，則id值爲2的成爲prmary
config.members[2].priority=3

故意停掉27020進程
kill -9 82609

查看id值爲2的主機是否成爲primary
進入27022的mongdb：

MongoDB Enterprise shard-rs:PRIMARY> use adminswitched to db adminMongoDB Enterprise shard-rs:PRIMARY> db.isMaster(){"hosts" : ["worker2:27020","worker2:27021","worker2:27022"],"setName" : "shard-rs","setVersion" : 4,"ismaster" : true, # 成爲primary"secondary" : false,