mongodb副本集原理

1.原理：
副本集群每2秒發(fā)一次心跳，如果10秒過(guò)后沒收到則標(biāo)記為不可達(dá)。
MongoDB在主數(shù)據(jù)庫(kù)上應(yīng)用數(shù)據(jù)庫(kù)操作，然后在主數(shù)據(jù)庫(kù)的操作日志中記錄該操作。然后，輔助成員將這些操作復(fù)制并應(yīng)用到異步過(guò)程中。所有副本集成員在local.oplog.rs集合中都包含操作日志的副本 ，默認(rèn)大小為磁盤的5%，但不記錄查詢操作。oplogSize可以在/etc/mongdb.conf中指定大小

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

2.mongdb副本集中的成員角色:
primary: 負(fù)責(zé)寫入數(shù)據(jù)
seconary: 負(fù)責(zé)同步主上的數(shù)據(jù)，提供讀服務(wù)
仲裁節(jié)點(diǎn)： 不保有數(shù)據(jù)，不參與選主，只進(jìn)行選主投票

副本集選舉通過(guò)優(yōu)先級(jí)，范圍0-1000，0為仲裁節(jié)點(diǎn)，不參與選舉。默認(rèn)是1

3.如何設(shè)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值為2的分片優(yōu)先級(jí)為3，當(dāng)primary，則id值為2的成為prmary
config.members[2].priority=3

故意停掉27020進(jìn)程
kill -9 82609

查看id值為2的主機(jī)是否成為primary
進(jìn)入27022的mongdb：

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