cursor.tailable() (mongosh method方法)

Definition定义

cursor.tailable()

Important

mongosh Method方法

This page documents a mongosh method. This is not the documentation for a language-specific driver, such as Node.js.本页记录了一种mongosh方法。这不是针对特定语言驱动程序（如Node.js）的文档。

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.有关MongoDB API驱动程序，请参阅特定语言的MongoDB驱动程序文档。

Marks the cursor as tailable keeping it open even when the client exhausts all results.将游标标记为可尾随，即使客户端用尽所有结果，游标也会保持打开状态。

For use against a capped collection only. Using tailable() against a non-capped collection returns an error.仅用于封顶集合。对非上限集合使用tailable()会返回错误。

cursor.tailable() uses the following syntax:使用以下语法：

cursor.tailable( { awaitData : <boolean> } )

~cursor.tailable() has the following parameter:具有以下参数：

Parameter参数	Type类型	Description描述
awaitData	boolean布尔值	Optional. 可选。For use with DBQuery.Option.tailable. Sets the cursor to block the query thread when no data is available and await data for a set time instead of immediately returning no data. 与DBQuery.Option.tailable一起使用。将游标设置为在没有数据可用时阻止查询线程，并在设定的时间内等待数据，而不是立即返回无数据。The cursor returns no data only if the timeout expires.只有当超时到期时，游标才会返回任何数据。 By default, if maxTimeMS is set on the command that created the cursor, then the timeout for awaitData is the remaining time. Otherwise, the default timeout is 1000 milliseconds.默认情况下，如果在创建游标的命令上设置了maxTimeMS，则awaitData的超时时间是剩余时间。否则，默认超时为1000毫秒。 You can set a timeout when running getMore on a cursor with awaitData enabled.在启用了awaitData的游标上运行getMore时，可以设置超时。 Defaults to false.默认为false。

Returns:返回	The cursor that ~cursor.tailable() is attached to.~cursor.tailable()所连接的游标。

Compatibility兼容性

This method is available in deployments hosted in the following environments:此方法在以下环境中托管的部署中可用：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud：云中MongoDB部署的完全托管服务

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB：MongoDB的基于订阅的自我管理版本
• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB：MongoDB的源代码可用、免费使用和自我管理版本

Behavior行为

A tailable cursor performs a collection scan over a capped collection. It remains open even after reaching the end of the collection. Applications can continue to iterate the tailable cursor as new data is inserted into the collection.可尾随游标在封顶集合上执行集合扫描。即使在集合结束后，它仍然开放。当新数据插入到集合中时，应用程序可以继续迭代可跟踪游标。

If awaitData is true, when the cursor reaches the end of the capped collection, mongod blocks the query thread for the timeout interval and waits for new data to arrive. 如果awaitData为true，当游标到达上限集合的末尾时，mongod会在超时间隔内阻塞查询线程，并等待新数据到达。When new data is inserted into the capped collection, mongod signals the blocked thread to wake and return the next batch to the client.当新数据被插入到封顶的集合中时，mongod会向被阻塞的线程发出信号，要求其唤醒并将下一批数据返回给客户端。

See Tailable Cursors.请参阅可尾随游标。