reshardCollection

Initialization Phase

During the initialization phase, the resharding coordinator determines the new data distribution for the sharded collection.

Index Phase

During the index phase:

• Each shard recipient creates a new, empty sharded collection with the same collection options as the existing sharded collection. This new sharded collection is the target for where recipient shards write the new data.

• Each shard recipient 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.

Clone, Apply, and Catch-up Phase

During the clone, apply, and catch-up phase:

• Each shard recipient clones an initial copy of the documents it would own under the new shard key.

• Each shard recipient begins applying oplog entries from operations that happened after the recipient cloned the data.

• When the estimate for the time remaining to complete the resharding operation is under two seconds, the resharding coordinator blocks writes for the collection.

  Note

  If desired, you can manually force the resharding operation to complete by issuing the commitReshardCollection command. This is useful if the current time estimate to complete the resharding operation is an acceptable duration for your collection to block writes. The commitReshardCollection command blocks writes early and forces the resharding operation to complete. During the time period where writes are blocked your application experiences an increase in latency.

Commit Phase

• Once the resharding process reaches the commit phase, it may no longer be aborted with abortReshardCollection.

• When all shards have reached strict consistency, the resharding coordinator commits the resharding operation and installs the new routing table.

• 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.

• Each donor shard drops the old sharded collection.

  Tip

  See also:

  Reshard a Collection

Reshard a Collection

The following example reshards the sales.orders collection with the new shard key { order_id: 1 }:

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

Output:

{
  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

Starting in MongoDB 8.0, you can reshard a collection on the same key, which can be used to redistribute data onto new shards.

After adding a shard to the cluster, you use the reshardCollection command with the forceRedistribution option to redistribute data across the cluster:

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

Redistribute Data to Different Zones

Starting in MongoDB 8.0, you can use the reshardCollection command to move data into new zones without changing the shard key.

The following command redistributes data for the accounts.sales collection using the same shard key, moving data from zones zone01 and zone02 to zone03 and zone04:

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() }
        }
    ]
})