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Replication Introduction Replication is the process of synchronizing data across multiple servers. Purpose of Replication 副本集目的Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication protects a database from the loss of a single server. Replication also allows you to recover from hardware failure and service interruptions. With additional copies of the data, you can dedicate one to disaster recovery, reporting, or backup.随着数据的附加副本，您可以将一个灾难恢复 In some cases, you can use replication to increase read capacity. Clients have the ability to send read and write operations to different servers. You can also maintain copies in different data centers to increase the locality and availability of data for distributed applications. Replication in MongoDBA replica set is a group of mongod instances that host the same data set. One mongod, the primary, receives all write operations. All other instances, secondaries, apply operations from the primary so that they have the same data set. The primary accepts all write operations from clients. A replica set can have only one primary. [1] To support replication, the primary records all changes to its data sets in its oplog. For more information on primary node operation, see Replica Set Primary. 副本集中只能有一个primary（主副本），记录所有数据改变的动作，然后再同步到其他的副本上。
The secondaries replicate the primary’s oplog and apply the operations to their data sets such that the secondaries’ data sets reflect the primary’s data set. If the primary is unavailable, the replica set will elect a secondary to be primary. For more information on secondary members, see Replica Set Secondary Members. 如果主副本不能利用，副本集将要选出一个次要副本来作为主副本。
You may add an extra mongod instance to a replica set as an arbiter. Arbiters do not maintain a data set. The purpose of an arbiter is to maintain a quorum in a replica set by responding to heartbeat and election requests by other replica set members. Because they do not store a data set, arbiters can be a good way to provide replica set quorum functionality with a cheaper resource cost than a fully functional replica set member with a data set. If your replica set has an even number of members, add an arbiter to obtain a majority of votes in an election for primary.（如果你的副本集成员的一个偶数，添加一个仲裁者在初级选举获得多数票） Arbiters do not require dedicated hardware. For more information on arbiters, see Replica Set Arbiter.
An arbiter will always be an arbiter whereas a primary may step down and become a secondary and asecondary may become the primary during an election. Asynchronous ReplicationSecondaries apply operations from the primary asynchronously. By applying operations after the primary, sets can continue to function despite the failure of one or more members. For more information on replication mechanics, see Replica Set Oplog and Replica Set Data Synchronization. Automatic FailoverWhen a primary does not communicate with the other members of the set for more than 10 seconds, the replica set will attempt to select another member to become the new primary. The first secondary that receives a majority of the votes becomes primary. 当一个主不与其他成员进行通信的10秒以上，副本集将尝试选择另一个成员成为新的主。
See Replica Set Elections and Rollbacks During Replica Set Failover for more information. Read OperationsWhen a replica set has one and only one primary, reads from that primary provide strict consistency. [1] 当一个副本集有一个只有一个主，读取从该主提供严格的一致性 By default, clients read from the primary; however, clients can specify a read preference to send read operations to secondaries. Asynchronous replication to secondaries means that reads from secondaries may return data that does not reflect the state of the data on the primary. For information on reading from replica sets, see Read Preference. In MongoDB, clients can see the results of writes before they are made durable:
Additional FeaturesReplica sets provide a number of options to support application needs. For example, you may deploy a replica set with members in multiple data centers, or control the outcome of elections by adjusting thepriority of some members. Replica sets also support dedicated members for reporting, disaster recovery, or backup functions. See Priority 0 Replica Set Members, Hidden Replica Set Members and Delayed Replica Set Members for more information.
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