November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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从应用程序连接副本集
｜
219
{ "acknowledged" : true, "insertedId" : 10 }
但在这个写操作被复制到副本集的大多数成员之前，服务器端不会发出响应。只有这样，
应用程序才会收到这个写操作成功的确认。如果在指定的超时时间之内没有写入成功，则
服务器端将回复一条错误消息：
WriteConcernError({
"code" : 64,
"errInfo" : {
"wtimeout" : true
},
"errmsg" : "waiting for replication timed out"
})
"majority"
写关注以及副本集选举协议确保了在主节点的选举中，只有那些拥有经过确认
的写操作的从节点才能被选为主节点。通过这种方式，可以保证不会发生回滚。还可以对
超时选项进行调节，以便在应用程序层检测并标记任何长时间运行的写操作。
"w"
的其他选项
"majority"
并不是唯一的
writeConcern
选项。
MongoDB
还允许将
"w"
指定为任意的数字，
如下所示：
db.products.insertOne(
{ "_id": 10, "item": "envelopes", "qty": 100, type: "Self-Sealing" },
{ writeConcern: { "w" : 2, "wtimeout" : 100 } }
);
这个命令会一直等待，直到写操作被复制到两个成员（主节点和一个从节点）中。
注意，
"w"
的值包含了主节点 ...
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