Replica Set Members副本集成员

The primary is the only member in the replica set that receives write operations. 主成员是副本集中唯一接收写操作的成员。MongoDB applies write operations on the primary and then records the operations on the primary's oplog. MongoDB在primary上应用写操作，然后将操作记录在primary的oplog上。Secondary members replicate this log and apply the operations to their data sets.辅助成员复制此日志并将操作应用于其数据集。

In the following three-member replica set, the primary accepts all write operations. 在以下三个成员的复制副本集中，主复制副本接受所有写操作。Then the secondaries replicate the oplog to apply to their data sets.然后，辅助设备复制oplog以应用于它们的数据集。

All members of the replica set can accept read operations. 复制副本集的所有成员都可以接受读取操作。However, by default, an application directs its read operations to the primary member. 但是，默认情况下，应用程序将其读取操作定向到主成员。See Read Preference for details on changing the default read behavior.有关更改默认读取行为的详细信息，请参阅读取首选项。

The replica set can have at most one primary. 复制副本集最多可以有一个主副本。[2] If the current primary becomes unavailable, an election determines the new primary. See Replica Set Elections for more details.如果当前初选不可用，则选举将确定新的初选。有关详细信息，请参阅副本集选择。

A secondary maintains a copy of the primary's data set. 辅助服务器维护primary数据集的副本。To replicate data, a secondary applies operations from the primary's oplog to its own data set in an asynchronous process. 要复制数据，辅助进程会在异步进程中将操作从primary的oplog应用到它自己的数据集。[1] A replica set can have one or more secondaries.副本集可以有一个或多个辅助副本。

The following three-member replica set has two secondary members. 以下三个成员的复制副本集有两个辅助成员。The secondaries replicate the primary's oplog and apply the operations to their data sets.辅助设备复制主设备的操作日志，并将操作应用于其数据集。

Although clients cannot write data to secondaries, clients can read data from secondary members. 尽管客户端无法将数据写入辅助成员，但客户端可以从辅助成员读取数据。See Read Preference for more information on how clients direct read operations to replica sets.有关客户端如何将读取操作定向到副本集的详细信息，请参阅读取首选项。

A secondary can become a primary. 次要的可以变成主要的。If the current primary becomes unavailable, the replica set holds an election to choose which of the secondaries becomes the new primary.如果当前主副本不可用，则副本集将进行一次选举，以选择哪个辅助副本将成为新的主副本。

See Replica Set Elections for more details.有关详细信息，请参阅副本集选举。

You can configure a secondary member for a specific purpose. 您可以为特定目的配置辅助成员。You can configure a secondary to:您可以将辅助配置为：

• Prevent it from becoming a primary in an election, which allows it to reside in a secondary data center or to serve as a cold standby. 防止它在选举中成为主要数据中心，这样它就可以驻留在辅助数据中心或充当冷备用。See Priority 0 Replica Set Members.请参阅优先级为0的副本集成员。
• Prevent applications from reading from it, which allows it to run applications that require separation from normal traffic. 阻止应用程序从中读取，这允许它运行需要与正常流量分离的应用程序。See Hidden Replica Set Members.请参见隐藏的副本集成员。
• Keep a running "historical" snapshot for use in recovery from certain errors, such as unintentionally deleted databases. 保留一个正在运行的“历史”快照，以用于从某些错误中恢复，例如无意删除的数据库。See Delayed Replica Set Members.请参阅延迟的副本集成员。

In some circumstances (such as when you have a primary and a secondary, but cost constraints prohibit adding another secondary), you may choose to add an arbiter to your replica set. 在某些情况下（例如，当您有一个主副本和一个辅助副本，但成本限制禁止添加另一个辅助时），您可以选择向复制副本集添加仲裁器。An arbiter participates in elections for primary but an arbiter does not have a copy of the data set and cannot become a primary.仲裁器参与初选，但仲裁器没有数据集的副本，不能成为初选。

An arbiter has exactly 1 election vote. 仲裁者只有1张选举选票。By default an arbiter has priority 0.默认情况下，仲裁器的优先级为0。

To add an arbiter, see Add an Arbiter to Replica Set.要添加仲裁器，请参阅将仲裁器添加到副本集。