Replica Set Elections复制集选择

Replication Election Protocol复制选择协议

Replication protocolVersion: 1 reduces replica set failover time and accelerate the detection of multiple simultaneous primaries.复制协议protocolVersion: 1缩短了副本集故障转移时间，并加快了对多个同时出现的主副本的检测。

With protocolVersion 1, you can use catchUpTimeoutMillis to prioritize between faster failovers and preservation of w:1 writes.使用protocolVersion1，您可以使用catchUpTimeoutMillis在更快的故障切换和保存w:1写入之间划分优先级。

For more information on pv1, see Replica Set Protocol Version.有关pv1的详细信息，请参阅副本集协议版本。

Heartbeats心跳

Replica set members send heartbeats (pings) to each other every two seconds. 副本集成员每隔两秒向彼此发送心跳信号（ping）。If a heartbeat does not return within 10 seconds, the other members mark the delinquent member as inaccessible.如果心跳在10秒钟内没有返回，其他成员会将该成员标记为无法访问。

Member Priority成员优先级

After a replica set has a stable primary, the election algorithm will make a "best-effort" attempt to have the secondary with the highest priority available call an election. 在副本集具有稳定的主副本后，选举算法将“尽最大努力”尝试让具有最高优先级的次副本调用选举。Member priority affects both the timing and the outcome of elections; secondaries with higher priority call elections relatively sooner than secondaries with lower priority, and are also more likely to win. 成员优先权影响选举的时间和结果；优先级较高的二级学院比优先级较低的二级院校更快召集选举，也更有可能获胜。However, a lower priority instance can be elected as primary for brief periods, even if a higher priority secondary is available. 然而，即使有较高优先级的次实例可用，也可以选择较低优先级的实例作为短期的主实例。Replica set members continue to call elections until the highest priority member available becomes primary.副本集成员继续进行选举，直到可用的最高优先级成员成为主要成员。

Members with a priority value of 0 cannot become primary and do not seek election. 优先级值为0的成员不能成为主要成员，也不寻求选举。For details, see Priority 0 Replica Set Members.有关详细信息，请参阅优先级0副本集成员。

Mirrored Reads镜像读取

Starting in version 4.4, MongoDB provides mirrored reads to pre-warm electable secondary members' cache with the most recently accessed data. 从4.4版开始，MongoDB使用最近访问的数据向预热可选次要成员的缓存提供镜像读取。With mirrored reads, the primary can mirror a subset of operations that it receives and send them to a subset of electable secondaries. 通过镜像读取，主服务器可以镜像它接收的操作子集，并将其发送到可选辅助服务器的子集。Pre-warming the cache of a secondary can help restore performance more quickly after an election.预热辅助缓存可以帮助在选举后更快地恢复性能。

For details, see Mirrored Reads.有关详细信息，请参阅镜像读取。

Loss of a Data Center失去数据中心

With a distributed replica set, the loss of a data center may affect the ability of the remaining members in other data center or data centers to elect a primary.对于分布式副本集，数据中心的丢失可能会影响其他数据中心或数据中心中其他成员选择主副本的能力。

If possible, distribute the replica set members across data centers to maximize the likelihood that even with a loss of a data center, one of the remaining replica set members can become the new primary.如果可能，将副本集成员分布到各个数据中心，以最大限度地提高即使丢失数据中心，剩余副本集成员之一也可能成为新主副本的可能性。

Network Partition网络分区

A network partition may segregate a primary into a partition with a minority of nodes. 网络分区可以将一个主分区分隔为一个具有少数节点的分区。When the primary detects that it can only see a minority of nodes in the replica set, the primary steps down as primary and becomes a secondary. 当主节点检测到它只能看到副本集中的少数节点时，主节点将作为主节点降级并成为次节点。Independently, a member in the partition that can communicate with a majority of the nodes (including itself) holds an election to become the new primary.独立地，分区中可以与大多数节点（包括其自身）通信的成员将举行选举，成为新的主节点。