副本集 是一组 MongoDB 服务器,维护相同的数据 — 提供 高可用性(自动故障转移)和 冗余性。一个节点是 主节点(处理写入),其他节点是 从节点(复制主节点的数据并可以提供读取)。这是 MongoDB 的标准可靠性机制。
副本集的结构
PRIMARY (1) → receives all WRITES; replicates changes to secondaries
SECONDARIES (2+) → copy the primary's data (via the oplog); can serve READS
ARBITER (optional) → votes in elections but holds no data (for odd vote counts)
Write → Primary → replicated to Secondaries (via the operations log "oplog")
