reshardCollection (database command数据库命令)

Initialization Phase初始化阶段

During the initialization phase, the resharding coordinator determines the new data distribution for the sharded collection.在初始化阶段，重新分片协调器确定分片集合的新数据分布。

Clone Phase克隆阶段

During the clone phase:在克隆阶段：

• Each recipient shard creates a temporary, empty sharded collection with the same collection options as the donor sharded collection. This new collection is the target for where recipient shards write the new data. The recipient shards do not create any index except the _id index until the index phase.每个接收者分片都会创建一个临时的空分片集合，其集合选项与捐赠者分片集合相同。这个新集合是收件人分片写入新数据的目标。在索引阶段之前，接收方分片不会创建除_id索引之外的任何索引。
• Each recipient shard clones collection data from the donor shard, including all documents that the recipient shard owns under the new shard key.每个接收者分片克隆来自捐赠者分片的集合数据，包括接收者分片在新分片键下拥有的所有文档。

Index Phase指数阶段

During the index phase, each recipient shard builds the necessary new indexes. These include all existing indexes on the sharded collection and an index compatible with the new shard key pattern if such an index doesn't already exist on the sharded collection.在索引阶段，每个接收者分片都会构建必要的新索引。这些包括分片集合上的所有现有索引，以及与新分片键模式兼容的索引（如果分片集合中不存在这样的索引）。

Apply and Catch-up Phase申请和追赶阶段

During the apply and catch-up phase:在申请和追赶阶段：

• Each recipient shard begins applying oplog entries that were written to the the corresponding donor shard after the recipient cloned the data.每个接收者分片开始应用oplog条目，这些条目是在接收者克隆数据后写入相应的捐赠者分片的。
• When the estimate for the time remaining to complete the resharding operation is under 500 ms, the donor shard blocks writes on the source collection.当完成重新分片操作的剩余时间估计在500毫秒以下时，供体分片块会在源集合上写入。

Commit Phase提交阶段

During the commit phase:在提交阶段：

• The resharding coordinator waits for all shards to reach strict consistency, then commits the resharding operation.重新分片协调器等待所有分片达到严格一致性，然后提交重新分片操作。
• The resharding coordinator instructs each donor and recipient shard primary, independently, to rename the temporary sharded collection. The temporary collection becomes the new resharded collection.重新分片协调器指示每个捐赠者和接受者分片主独立重命名临时分片集合。临时集合将成为新的重新集合。
• Each donor shard drops the old sharded collection.每个捐赠者的分片都会丢弃旧的分片集合。

Reshard a Collection重新分片集合

The following example reshards the sales.orders collection with the new shard key { order_id: 1 }:以下示例使用新的分片键{ order_id: 1 }对sales.orders集合进行重新排序：

db.adminCommand({
  reshardCollection: "sales.orders",
  key: { order_id: 1 }
})

Output:输出：

{
  ok: 1,
  '$clusterTime': {
    clusterTime: Timestamp(1, 1624887954),
    signature: {
      hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0),
      keyId: 0
    }
  },
  operationTime: Timestamp(1, 1624887947)
}

Redistribute Data to New Shards将数据重新分配到新分片

Starting in MongoDB 8.0, you can reshard a collection on the same key, which can be used to redistribute data onto new shards.从MongoDB 8.0开始，您可以在同一键上重新分片一个集合，这可用于将数据重新分发到新的分片上。

After adding a shard to the cluster, you use the reshardCollection command with the forceRedistribution option to redistribute data across the cluster:在将分片添加到集群后，您可以使用带有forceRedistribution选项的reshardCollection命令在集群中重新分发数据：

db.adminCommand({
    reshardCollection: "accounts.invoices",
    key: { store_id: "hashed" },
    forceRedistribution: true
})

Redistribute Data to Different Zones将数据重新分配到不同的区域

Starting in MongoDB 8.0, you can use the reshardCollection command to move data into new zones without changing the shard key.从MongoDB 8.0开始，您可以使用reshardCollection命令将数据移动到新区域，而无需更改分片键。

The following command redistributes data for the accounts.sales collection using the same shard key, moving data to the shards associated with zones zone04 and zone05:以下命令使用相同的分片键重新分发accounts.sales集合的数据，将数据移动到与区域zone04和zone05关联的分片：

db.adminCommand({
    reshardCollection: "accounts.sales",
    key: { region_id: "hashed" },
    forceRedistribution: true,
    zones: [
        {
            zone: "zone04",
            min: { region_id: MinKey() },
            max: { region_id: 10 }
        },
        {
            zone: "zone05",
            min: { region_id: 10 },
            max: { region_id: MaxKey() }
        }
    ]
})