MongoDB Tailable Cursors

刺骨的言语ヽ痛彻心扉 2022-02-03 08:11 130阅读 0赞

MongoDB 有一个叫 Tailable Cursors的特性，它类似于tail -f 命令，你在一个Capped Collection上面执行查询操作，当操作完成后，你可以不关闭返回的数据Cursor，并持续地从中读出新加入的数据。  
  
在高写入的Capped Collection上，索引不可用时，可使用Tailable Cursors。例如，MongoDB复制使用了Tailable Cursors来获取Primary的尾oplog日志。  
  
考虑以下与Tailable Cursors相关的行为：

*  Tailable Cursors不使用索引，并以自然排序返回文档。
 *  因为Tailable Cursors不使用索引，查询的初始扫描非常耗性能；但是，游标初始化完后，随后获取到的新增加的文档是很快速的。
 *  Tailable Cursors如果遇到以下情况之一将会僵死或无效：
    
     *  查询无匹配结果。
     *  游标在集合尾部返回文档，随后应用程序删除了该文档。

僵死的游标id为0。

**DBQuery.Option.awaitData**  
  
在使用TailableCursor时，此参数会在数据读尽时先阻塞一小段时间后再读取一次并进行返回。  
  
**跟踪oplog的示例**

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**2.6版的游标方法：**  
cursor.addOption()  
[https://docs.mongodb.com/v2.6/reference/method/cursor.addOption/][https_docs.mongodb.com_v2.6_reference_method_cursor.addOption]  
  
**3.2版的游标方法：**  
cursor.tailable()  
[https://docs.mongodb.com/manual/reference/method/cursor.tailable/][https_docs.mongodb.com_manual_reference_method_cursor.tailable]

本文转自UltraSQL51CTO博客，原文链接：http://blog.51cto.com/ultrasql/1789038 ，如需转载请自行联系原作者

[https_docs.mongodb.com_v2.6_reference_method_cursor.addOption]: https://docs.mongodb.com/v2.6/reference/method/cursor.addOption/
[https_docs.mongodb.com_manual_reference_method_cursor.tailable]: https://docs.mongodb.com/manual/reference/method/cursor.tailable/