Config Servers配置服务器

Writes to Config Servers写入配置服务器

The admin database contains the collections related to the authentication and authorization as well as the other system.* collections for internal use.admin数据库包含与身份验证和授权相关的集合以及其他供内部使用的system.*集合。

The config database contains the collections that contain the sharded cluster metadata. config数据库包含包含分片集群元数据的集合。MongoDB writes data to the config database when the metadata changes, such as after a chunk migration or a chunk split.当元数据发生变化时，例如在块迁移或块分割之后，MongoDB会将数据写入config数据库。

Users should avoid writing directly to the config database in the course of normal operation or maintenance.用户在正常操作或维护过程中应避免直接写入配置数据库。

When writing to the config servers, MongoDB uses a write concern of "majority".在写入配置服务器时，MongoDB使用"majority"的写入关注。

Reads from Config Servers从配置服务器读取

MongoDB reads from the admin database for authentication and authorization data and other internal uses.MongoDB从admin数据库中读取身份验证和授权数据以及其他内部用途。

MongoDB reads from the config database when a mongos starts or after a change in the metadata, such as after a chunk migration. Shards also read chunk metadata from the config servers.MongoDB在mongos启动时或元数据更改后（如块迁移后）从config数据库中读取。Shards还从配置服务器读取块元数据。

When reading from the replica set config servers, MongoDB uses a Read Concern level of "majority".从副本集配置服务器读取时，MongoDB使用"majority"的读取关注级别。

Metadata Views Must be Up-to-Date元数据视图必须是最新的

For an operation to succeed, the view of the metadata on the specific shard member must be up-to-date. The shard and the router issuing the request must have the same version of the chunks metadata.为了使操作成功，特定分片成员上的元数据视图必须是最新的。发出请求的分片和路由器必须具有相同版本的块元数据。

If the metadata is not up-to-date, the operation fails with the StaleConfig error and the metadata refresh process is triggered. Refreshing the metadata can introduce additional operational latency.如果元数据不是最新的，则操作失败，出现StaleConfig错误，并触发元数据刷新过程。刷新元数据可能会引入额外的操作延迟。

On a secondary, a metadata refresh can take a long time if there is significant replication lag. 在辅助服务器上，如果存在明显的复制延迟，元数据刷新可能需要很长时间。For secondary reads, set maxStalenessSeconds to minimize the impact of replication lag.对于二次读取，设置maxStalenessSeconds以最小化复制延迟的影响。

Commands命令

To configure a dedicated config server to run as a config shard, run the transitionFromDedicatedConfigServer command.要将专用配置服务器配置为作为配置分片运行，请运行transitionFromDedicatedConfigServer命令。

To configure a config shard to run as a dedicated config server, run the transitionToDedicatedConfigServer command.要将配置分片配置为作为专用配置服务器运行，请运行transitionToDedicatedConfigServer命令。

Users用户

A user created on a config shard has the same behavior as a user created on a dedicated config server.在配置分片上创建的用户与在专用配置服务器上创建的用户名具有相同的行为。

Confirm use of Config Shard确认配置分片的使用

You can confirm that a sharded cluster uses a config shard by using one of the following methods:您可以使用以下方法之一确认分片集群使用配置分片：

• Run the sh.isConfigShardEnabled() method in mongosh. 在mongosh中运行sh.isConfigShardEnabled()方法。If the sh.isConfigShardEnabled() output contains enabled: true, the cluster uses a config shard. If the output contains enabled: false, the cluster does not use a config shard.如果sh.isConfigShardEnabled()输出包含enabled:true，则集群使用配置分片。如果输出包含enabled:false，则集群不使用配置分片。
• Run the listShards command against the admin database while connected to a mongos and inspect the output for a document where _id is set to "config". 在连接到mongos时，对admin数据库运行listShards命令，并检查_id设置为“config”的文档的输出。If the listShards output does not contain a document where _id is set to "config", the cluster does not use a config shard.如果listShards输出不包含_id设置为“config”的文档，则集群不使用配置分片。

The following example runs the listShards command and tries to find a document where _id is set to "config".以下示例运行listShards命令，并尝试查找_id设置为“config”的文档。

db.adminCommand({ listShards: 1 })["shards"].find(element => element._id === "config")

In this example, the returned document has _id set to "config" which confirms that this cluster uses a config shard.在这个例子中，返回的文档将_id设置为“config”，这确认了该集群使用了配置分片。

{
  _id: "config",
  host: "configRepl/localhost:27018",
  state: 1,
  topologyTime: Timestamp({ t: 1732218671, i: 13 }),
  replSetConfigVersion: Long('-1')
}

Downgrade Feature Compatibility Version降级功能兼容性版本

If your cluster has a config shard and you need to downgrade the feature compatibility version to earlier than 8.0, connect to mongos and perform this procedure:如果集群有一个配置分片，你需要将功能兼容性版本降级到8.0之前，请连接到mongos并执行以下过程：

db.adminCommand( {
   transitionToDedicatedConfigServer: 1
} )

db.adminCommand(
   {
      moveCollection: "<database>.<collection>",
      toShard: "<new shard>",
   }
   )

db.adminCommand( {
   transitionToDedicatedConfigServer: 1
} )

db.adminCommand( { setFeatureCompatibilityVersion: "7.0" } )