mongo之Wtire Concern(安全写级别)

The mongo shell and the MongoDB drivers use Acknowledged as the default write concern.
//mongo shell和monogdb驱动将Acknowledged 作为默认的写安全级别

Read Isolation(读隔离)
MongoDB allows clients to read documents inserted or modified before it commits these modifications to disk, 
regardless of write concern level or journaling configuration
//mongodb允许客户端读取插入或修改的文档,在文档写到磁盘之前。

For systems with multiple concurrent readers and writers, 
MongoDB will allow clients to read the results of a write operation before the write operation returns.
//在多并发的读写系统中,mongodb将允许客户端读取写操作的结果,在写操作返回之前。

If the mongod terminates before the journal commits, even if a write returns successfully, 
queries may have read data that will not exist after the mongod restarts.
//如果mongod在日志提交前中断了,尽管写操作返回成功,在重启mongod之后,数据将不存在。

For replica sets, write operations are durable only after a write replicates and commits 
to the journal of a majority of the voting members of the set.
MongoDB regularly commits data to the journal regardless of journaled write concern: 
use the commitIntervalMs to control how often a mongod commits the journal.
//对于复制集中,只有写操作复制并且提交到大多数的投票复制集成员,写操作才能继续。
//对于复制集中,mongodb定期提交数据到日志中不受Journaled安全写级别的控制,通过commitIntervalMs来控制提交日志的频率。

MongoDB does not “rollback” or undo modifications made before the wtimeout interval expired
//mongodb 不能撤销做出的修改在wtimeout超时到期前。

write Concern Levels(安全写级别分类):

1.Unacknowledged: {w: 0}
 Unacknowledged is similar to errors ignored; 
 however,drivers will attempt to receive and handle network errors when possible
//不确认 类似于忽略错误,但是Mongo的驱动会尝试接收和处理网络的错误

2.Acknowledged: {w: 1}
 With a receipt acknowledged write concern, 
 the mongod confirms that it received the write operation and applied the change to the in-memory view of data.
 Acknowledged write concern allows clients to catch network, duplicate key, and other errors.
 Acknowledged write concern does not confirm that the write operation has persisted to the disk system.
//mongod确保客户端接收到写操作返回的结果并将改变写入到数据内存区域中,
//Acknowledged级别不保证写操作是否写入到磁盘中,它是默认级别

3.Journaled: {w: 1, j: true}
With a journaled write concern, the MongoDB acknowledges the write operation only after committing the data to the journal
Specifying a write concern that includes j: true to a mongod or mongos running with --nojournal option now errors. Previous versions would ignore the j: true.

4.Replica Acknowledged: {w: 2} {w: "majority"}
The default write concern only requires acknowledgement from the primary.
With replica acknowledged write concern, you can guarantee that the write operation propagates to additional members of the replica set. 
See Write Concern for Replica Sets for more information.
Write operation to a replica set with write concern level of w:2 or write to the primary and at least one secondary.

Confirms that write operations have propagated to the majority of configured replica set: 
a majority of the set’s configured members must acknowledge the write operation before it succeeds. 
This allows you to avoid hard coding assumptions about the size of your replica set into your application.
Changed in version 2.6: In Master/Slave deployments, MongoDB treats w: "majority" as equivalent to w: 1. 
In earlier versions of MongoDB, w: "majority" produces an error in master/slave deployments.

最后附一下 这个参数的说明:
storage.journal.commitIntervalMs
Type: number
Default: 100 or 30
The maximum amount of time the mongod process allows between journal operations. Values can range from 2 to 300 milliseconds. 
Lower values increase the durability of the journal, at the expense of disk performance.
The default journal commit interval is 100 milliseconds if a single block device 
(e.g. physical volume, RAID device, or LVM volume) contains both the journal and the data files.
If the journal is on a different block device than the data files the default journal commit interval is 30 milliseconds.
To force mongod to commit to the journal more frequently, you can specify j:true. 
When a write operation with j:true is pending, mongod will reduce commitIntervalMs to a third of the set value.
The storage.journal.commitIntervalMs option is available only for mongod.