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cursor.readConcern()

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Definition定义

cursor.readConcern(level)
Important

mongosh Method

This page documents a mongosh method. This is not the documentation for a language-specific driver, such as Node.js.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

Specify a read concern for the db.collection.find() method.为db.collection.find()方法指定一个读取关注。

The readConcern() method has the following form:readConcern()方法的形式如下：

db.collection.find().readConcern(<level>)

The readConcern() method has the following parameter:readConcern()方法具有以下参数：

Parameter参数Type类型Description描述
levelstringRead concern读取关注 level.级别。
Possible read concern levels are: 可能的读取关注级别为：
  • "local". This is the default read concern level for read operations against the primary and secondaries.这是针对主和辅助的读取操作的默认读取关注级别。
  • "available". Available for read operations against the primary and secondaries. 可用于对primary和secondary进行读取操作。"available" behaves the same as "local" against the primary and non-sharded secondaries. 对于主和非分片的辅助，"available"的行为与"local"相同。The query returns the instance's most recent data.查询返回实例的最新数据。
  • "majority". Available for replica sets that use WiredTiger storage engine.适用于使用WiredTiger存储引擎的复制副本集。
  • "linearizable". Available for read operations on the primary only.仅可用于primary上的读取操作。
For more formation on the read concern levels, see Read Concern Levels. 有关读取关注级别的更多信息，请参阅读取关注级别。

Considerations注意事项

Read Your Own Writes阅读自己的文章

Starting in MongoDB 3.6, you can use causally consistent sessions to read your own writes, if the writes request acknowledgement.从MongoDB 3.6开始，如果写入请求确认，您可以使用因果一致会话来读取自己的写入。

Prior to MongoDB 3.6, in order to read your own writes you must issue your write operation with { w: "majority" } write concern, and then issue your read operation with primary read preference, and either "majority" or "linearizable" read concern.在MongoDB 3.6之前，为了读取您自己的写入，您必须发出具有{ w: "majority" }写入关注的写入操作，然后发出具有primary读取偏好的读取操作，以及"majority"或"linearizable"读取关注。

Linearizable Read Concern Performance可线性化的读取关注性能

When specifying linearizable read concern, always use maxTimeMS() in case a majority of data bearing members are unavailable.在指定可线性化的读取关注时，如果大多数数据承载成员不可用，请始终使用maxTimeMS()。

db.restaurants.find( { _id: 5 } ).readConcern("linearizable").maxTimeMS(10000)
Tip

See also: 另请参阅：

Read Concern