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Unsharded Collections

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Unsharded Collections

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Sharding

Unsharded Collections

Unshard a Collection

You can unshard a sharded collection with the unshardCollection command. When you unshard a collection, the collection cannot be partitioned across multiple shards and the shard key is removed.

By default, when you unshard a collection, MongoDB moves the collection's data to the shard with the least amount of data. Alternatively, you can specify which shard to place the data on.

About this Task

Compatibility

You can perform this task on deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Note

This task is not available on the MongoDB Atlas Free or Flex Tiers.

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Restrictions

unshardCollection can only be run on sharded clusters.
unshardCollection can only operate on sharded collections.
unshardCollection can only operate on a single collection at a time.
unshardCollection has a 5 minute minimum duration.
You must rebuild MongoDB Search indexes after unshardCollection runs.
writeConcernMajorityJournalDefault must be true.
You cannot make topology changes, such as adding or removing shards or transitioning between embedded and dedicated config servers, until unshardCollection completes.
You cannot run the following operations on the collection that is being unsharded while unshardCollection is in progress:
You cannot run the following operations on the cluster while unshardCollection is in progress:
Index builds that occur while unshardCollection is in progress might silently fail.
- Do not create indexes while unshardCollection is in progress.
- Do not call unshardCollection if there are ongoing index builds.
To avoid error, MongoDB automatically drops the zones in your collection when you run unshardCollection.
If the collection you're unsharding uses MongoDB Search, the search index becomes unavailable when the unsharding operation completes. You need to manually rebuild the search index once the unsharding operation completes.
If the collection you're unsharding is archived in Atlas Online Archives, the online archive files are marked as Orphaned once the unsharding operation completes. You can create another online archive for the same database, collection, and fields as the orphaned archive as long as there is no other archive for that same combination in the Active state.

Access Control

If your deployment has access control enabled, the enableSharding role grants you access to run the unshardCollection command.

Before you Begin

Before you unshard your collection, ensure that you meet the following requirements:

Your application can tolerate a period of two seconds where the affected collection blocks writes. During the time period where writes are blocked, your application experiences an increase in latency.
Your database meets these resource requirements:
- Ensure the shard you are moving the collection to has enough storage space for the collection and its indexes. The destination shard requires at least ( Collection storage size + Index Size ) * 2 bytes available.
- Ensure that your I/O capacity is below 50%.
- Ensure that your CPU load is below 80%.

Steps

(Optional) List shard names

If you want to put the data from your sharded collection on a specific shard, you need the target shard's name.

To see the list of shard names in your cluster, use the listShards command:

db.adminCommand( { listShards: 1 } )

The shards._id field lists the name of each shard.

Unshard the collection

To unshard a collection, run the unshardCollection command. The following example unshards a collection called us_accounts in the sales database:

db.adminCommand( {
   unshardCollection: "sales.us_accounts",
   toShard: "shard1"
} )

After the unshard operation completes, the data in the us_accounts collection is on shard1. If you omit the toShard field, the data is placed on the shard with the least amount of data.

Confirm that the collection is unsharded

To confirm that the collection is unsharded, use the $shardedDataDistribution stage and try to match on the unsharded namespace:

db.aggregate( [
   { $shardedDataDistribution: { } },
   { $match: {  "ns": "sales.us_accounts" } }
] )

If the aggregation doesn't return any data, the collection is unsharded.

Learn More

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