Reshard a Collection重新分片集合

Requirements要求

Before you reshard your collection, ensure that you meet the following requirements:在重新分片之前，请确保您满足以下要求：

• Your application can tolerate a period of two seconds where the collection that is being resharded blocks writes. 您的应用程序可以容忍两秒钟的时间，在这段时间内，正在被重新分片的集合将阻止写入。During the time period where writes are blocked your application experiences an increase in latency. 在写入被阻止的时间段内，应用程序的延迟会增加。If your workload cannot tolerate this requirement, consider refining your shard key instead.如果您的工作负载不能满足此要求，请考虑再精细化您的分片键。
• Your database meets these resource requirements:您的数据库满足以下资源要求：
  • Available storage space: Ensure that your available storage space is at least 1.2x the size of the collection that you want to reshard. 可用存储空间：确保您的可用存储空间至少是要重新分片的集合大小的1.2倍。For example, if the size of the collection you want to reshard is 1 TB, you should have at least 1.2 TB of free storage when starting the sharding operation.例如，如果要重新分发的集合的大小为1 TB，则在启动分片操作时应至少有1.2 TB的可用存储空间。
  • I/O: Ensure that your I/O capacity is below 50%.I/O：确保您的I/O容量低于50%。
  • CPU load: Ensure your CPU load is below 80%.CPU负载：确保您的CPU负载低于80%。
  Important

  These requirements are not enforced by the database. A failure to allocate enough resources can result in:这些要求不是由数据库强制执行的。未能分配足够的资源可能导致：

  • the database running out of space and shutting down数据库空间不足，正在关闭
  • decreased performance性能下降
  • the resharding operation taking longer than expected重新分拣操作的时间比预期的要长

  If your application has time periods with less traffic, reshard your collection during that time if possible.如果您的应用程序有流量较少的时间段，请尽可能在该时间段内重新分片您的集合。

• You must perform one of these tasks:您必须执行以下任务之一：
  • rewrite your application's queries to use both the current shard key and the new shard key重写应用程序的查询以同时使用当前的分片键和新的分片键
  • stop your application and then:停止您的应用程序，然后：
    • rewrite your application's queries to use the new shard key重写应用程序的查询以使用新的分片键
    • wait until the resharding of the collection completes (to monitor the resharding process, use a $currentOp pipeline stage)等待集合的重新分片完成（要监视重新分片过程，请使用$currentOp管道阶段）
    • deploy your rewritten application部署重写的应用程序

  The following queries return an error if the query filter does not include both the current shard key or a unique field (like _id):如果查询筛选器不包括当前分片键或唯一字段（如_id），则以下查询将返回错误：

  • deleteOne()
  • findAndModify()
  • findOneAndDelete()
  • findOneAndReplace()
  • findOneAndUpdate()
  • replaceOne()
  • updateOne()

  For optimal performance, we recommend that you also rewrite other queries to include the new shard key.为了获得最佳性能，我们建议您还重写其他查询，以包含新的分片键。

  Once the resharding operation completes, you can remove the old shard key from the queries.一旦重新分片操作完成，就可以从查询中删除旧的分片键。

• No index builds are in progress. 没有正在进行的索引生成。Use db.currentOp() to check for any running index builds:使用db.currentOp()检查是否有任何正在运行的索引生成：

   db.adminCommand(
      {
        currentOp: true,
        $or: [
          { op: "command", "command.createIndexes": { $exists: true }  },
          { op: "none", "msg" : /^Index Build/ }
        ]
      }
  )

  In the result document, if the inprog field value is an empty array, there are no index builds in progress:在结果文档中，如果inprog字段值为空数组，则没有正在进行的索引构建：

  {
     inprog: [],
     ok: 1,
     '$clusterTime': { ... },
     operationTime: <timestamp>
  }

Limitations局限性

• Only one collection can be resharded at a time.一次只能重新分片一个集合。
• writeConcernMajorityJournalDefault must be true.必须为true。
• Resharding a collection that has a uniqueness constraint is not supported.不支持重新分片具有唯一性约束的集合。
• The new shard key cannot have a uniqueness constraint.新的分片键不能具有唯一性约束。
• The following commands and corresponding shell methods are not supported on the collection that is being resharded while the resharding operation is in progress:在进行重新分片操作时，正在重新分片的集合不支持以下命令和相应的shell方法：
  • collMod
  • convertToCapped
  • createIndexes
  • createIndex()
  • drop
  • drop()
  • dropIndexes
  • dropIndex()
  • renameCollection
  • renameCollection()
• The following commands and methods are not supported on the cluster while the resharding operation is in progress:在进行重新分片操作时，群集上不支持以下命令和方法：
  • addShard
  • removeShard
  • db.createCollection()
  • dropDatabase
  Warning

  Using any of the preceding commands during a resharding operation causes the resharding operation to fail.在重新分片操作期间使用任何前面的命令都会导致重新分片操作失败。

• If the collection to be resharded uses Atlas Search, the search index will become unavailable when the resharding operation completes. 如果要重新分片的集合使用Atlas Search，则当重新分片操作完成时，搜索索引将不可用。You need to manually rebuild the search index once the resharding operation completes.重新分片操作完成后，您需要手动重新生成搜索索引。
• You can't reshard a sharded time series collection.无法重新分片已分片的时间序列集合

Resharding Process重新分片过程

In a collection resharding operation, a shard can be a:在集合重新分片操作中，分片可以是：

• donor, which currently stores chunks for the sharded collection.，当前为分片集合存储块。
• recipient, which stores new chunks for the sharded collection based on the shard keys and zones.，它基于分片键和区域为分片集合存储新的块。

A shard can be donor and a recipient at the same time. 一个分片可以同时是捐赠者和接受者。The set of donor shards is identical to the recipient shards, unless you use zones.供体分片集与受体分片集相同，除非使用区域。

The config server primary is always the resharding coordinator and starts each phase of the resharding operation.配置服务器主服务器始终是重新分片协调器，并启动重新分片操作的每个阶段。

db.adminCommand({
  reshardCollection: "<database>.<collection>",
  key: <shardkey>
})

db.getSiblingDB("admin").aggregate([
  { $currentOp: { allUsers: true, localOps: false } },
  {
    $match: {
      type: "op",
      "originatingCommand.reshardCollection": "<database>.<collection>"
    }
  }
])

[
  {
    shard: '<shard>',
    type: 'op',
    desc: 'ReshardingRecipientService | ReshardingDonorService | ReshardingCoordinatorService <reshardingUUID>',
    op: 'command',
    ns: '<database>.<collection>',
    originatingCommand: {
      reshardCollection: '<database>.<collection>',
      key: <shardkey>,
      unique: <boolean>,
      collation: { locale: 'simple' }
    },
    totalOperationTimeElapsedSecs: <number>,
    remainingOperationTimeEstimatedSecs: <number>,
    ...
  },
  ...
]

{
  ok: 1,
  '$clusterTime': {
    clusterTime: <timestamp>,
    signature: {
      hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0),
      keyId: <number>
    }
  },
  operationTime: <timestamp>
}

sh.status()

databases
[
  {
     database: {
     _id: '<database>',
     primary: '<shard>',
     partitioned: false,
     version: {
       uuid: <uuid>,
       timestamp: <timestamp>,
       lastMod: <number>
     }
   },
   collections: {
     '<database>.<collection>': {
       shardKey: <shardkey>,
       unique: <boolean>,
       balancing: <boolean>,
       chunks: [],
       tags: []
     }
   }
  }
  ...
]

db.adminCommand({
  commitReshardCollection: "<database>.<collection>"
})

db.adminCommand({
  abortReshardCollection: "<database>.<collection>"
})

Behavior行为

Error Case错误案例