Database Manual / Reference / mongosh Methods / Cursors

cursor.readPref() (mongosh method方法)

Definition定义

cursor.readPref(mode, tagSet)

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。

Append readPref() to a cursor to control how the client routes the query to members of the replica set.将readPref()附加到游标，以控制客户端如何将查询路由到副本集的成员。

Note

You must apply readPref() to the cursor before retrieving any documents from the database.在从数据库检索任何文档之前，必须对游标应用readPref()。

Parameters参数

Parameter参数Type类型Description描述
modestring字符串

One of the following read preference modes: primary, primaryPreferred, secondary, secondaryPreferred, or nearest以下读取首选项模式之一：primary、primaryPreferred、secondary、secondaryPreferred或nearest

tagSetarray of documents

Optional. 可选。A tag set used to target reads to members with the specified tag(s). 用于将读取目标定位到具有指定标记的成员的标记集。tagSet is not available if using primary.如果使用primary，则tagSet不可用。

For details, see Read Preference Tag Set Lists.有关详细信息，请参阅读取首选项标记集列表。

readPref() does not support the Read Preference maxStalenessSeconds option for read preference.不支持读取首选项的读取首选项maxStalenessSeconds选项。

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

Note

This command is supported in all MongoDB Atlas clusters. 所有MongoDB Atlas集群都支持此命令。For information on Atlas support for all commands, see Unsupported Commands.有关Atlas支持所有命令的信息，请参阅不支持的命令。

  • 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

Examples示例

Specify Read Preference Mode指定读取首选项模式

The following operation uses the read preference mode to target the read to a secondary member.以下操作使用读取首选项mode将读取目标定位到次要成员。

db.collection.find({ }).readPref( "secondary")

Specify Read Preference Tag Set指定读取首选项标记集

To target secondaries with specific tags, include both the mode and the tagSet array:要针对具有特定标签的secondary，请同时包含mode和tagSet数组：

db.collection.find({ }).readPref(
   "secondary",
   [
      { "datacenter": "B" },    // First, try matching by the datacenter tag首先，尝试按数据中心标签进行匹配
      { "region": "West"},      // If not found, then try matching by the region tag如果未找到，请尝试按区域标记进行匹配
      { }                       // If not found, then use the empty document to match all eligible members如果未找到，则使用空文档匹配所有符合条件的成员
   ]
)

During the secondary selection process, MongoDB tries to find secondary members with the datacenter: "B" tag first.在二次选择过程中，MongoDB试图找到数据中心的datacenter: "B"标签优先。

  • If found, MongoDB limits the eligible secondaries to those with the datacenter: "B" tag and ignores the remaining tags.如果找到，MongoDB将符合条件的secondary限制为具有数据中心的datacenter: "B"标签，并忽略其余标签。

  • If none are found, then, MongoDB tries to find secondary members with the "region": "West" tag.如果没有找到，那么MongoDB会尝试找到带有"region": "West"标签的secondary成员。

    • If found, MongoDB limits the eligible secondaries to those with the "region": "West" tag.如果找到，MongoDB将符合条件的二级服务器限制为带有"region": "West"标签的服务器。
    • If none are found, MongoDB uses any eligible secondaries.如果没有找到，MongoDB将使用任何符合条件的二级服务器。

See Order of Tag Matching for details.详见标签匹配顺序。

Tip

Configure Replica Set Tag Sets配置副本集标记集