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Replica Set Members 简单介绍：primary. 主副本 提供所有写操作 Secondaries. 次副本 Arbiters 不保持数据，当主副本不能工作时候，Arbiters 作为选举（主副本）角色 A replica set in MongoDB is a group of mongod processes that provide redundancy冗余 and high availability 高可用. The members of a replica set are:
You can also maintain 保持 an arbiter as part of a replica set. Arbiters do not keep a copy of the data . However, arbiters play a role in the elections that select a primary if the current primary is unavailable. Arbiters 不保持数据，当主副本不能工作时候，Arbiters 作为选举（主副本）角色 A replica set can have up to 12 members. [1] However, only 7 members can vote at a time. The minimum requirements for a replica set are: A primary, a secondary, and an arbiter. Most deployments, however, will keep three members that store data: A primary and two secondary members. PrimaryThe primary is the only member in the replica set that receives write operations. MongoDB applies write operations on the primary and then records the operations on the primary’s oplog. Secondary members replicate 复制 this log and apply the operations to their data sets. In the following three-member replica set, the primary accepts all write operations. Then the secondaries replicate the oplog to apply to their data sets.
All members of the replica set can accept read operations. However, by default, an application directs its read operations to the primary member. See Read Preference for details on changing the default read behavior. 所有的成员都可以进行读操作 The replica set can have at most one primary. [2] If the current primary becomes unavailable, an election determines the new primary. See Replica Set Elections for more details. SecondariesA secondary maintains a copy of the primary’s data set. To replicate data, a secondary applies operations from the primary’s oplog to its own data set in an asynchronous process. A replica set can have one or more secondaries. The following three-member replica set has two secondary members. The secondaries replicate the primary’s oplog and apply the operations to their data sets.
Although clients cannot write data to secondaries, clients can read data from secondary members. See Read Preference for more information on how clients direct read operations to replica sets. A secondary can become a primary. If the current primary becomes unavailable, the replica set holds anelection to choose which of the secondaries becomes the new primary. See Replica Set Elections for more details. You can configure a secondary member for a specific purpose. You can configure a secondary to:
ArbiterAn arbiter does not have a copy of data set and cannot become a primary. Replica sets may have arbiters to add a vote in elections of for primary. Arbiters allow replica sets to have an uneven number of members, without the overhead of a member that replicates data. IMPORTANT Do not run an arbiter on systems that also host the primary or the secondary members of the replica set. Only add an arbiter to sets with even numbers of members. If you add an arbiter to a set with an odd number of members, the set may suffer from tied elections. To add an arbiter, see Add an Arbiter to Replica Set.
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