Database Manual / Change Streams / Change Events

reshardCollection Event

Summary

reshardCollection

New in version 6.1: (Also available in 6.0.14)

A reshardCollection event occurs when:

The shard key for a collection and the distribution of your data is changed, and
The change stream has showExpandedEvents set to true.

Description

Field	Type	Description
`_id`	Document	A BSON object which serves as an identifier for the change stream event. This value is used as the `resumeToken` for the `resumeAfter` parameter when resuming a change stream. The `_id` object has the following form: `{ "_data" : <BinData\|hex string> }` The `_data` type depends on the MongoDB versions and, in some cases, the feature compatibility version (FCV) at the time of the change stream's opening or resumption. See Resume Tokens for the full list of `_data` types. For an example of resuming a change stream by `resumeToken`, see Resume a Change Stream.
`clusterTime`	Timestamp	`clusterTime` is the timestamp from the oplog entry associated with the event. Due to oplog size limits, multi-document transactions may create multiple oplog entries. In a transaction, change stream events staged in a given oplog entry share the same `clusterTime`. Events with the same `clusterTime` may not all relate to the same transaction. Some events don't relate to a transaction at all. Starting in MongoDB 8.0, this may be true for events on any deployment. In previous versions, this behavior was possible only for events on a sharded cluster. To identify events for a single transaction, you can use the combination of `lsid` and `txnNumber` in the change stream event document. Changed in version 8.0.
`collectionUUID`	UUID	UUID identifying the collection where the change occurred. New in version 6.0.
`ns`	Document	The namespace (database and or collection) affected by the event.
`ns.coll`	String	The name of the collection where the event occurred.
`ns.db`	String	The name of the database where the event occurred.
`operationDescription`	Document	Additional information on the change operation. This document and its subfields only appears when the change stream uses expanded events. New in version 6.0.
`operationDescription.` `reshardUUID`	UUID	UUID that identifies the resharding operation. New in version 6.1.
`operationDescription.` \| `shardKey`	Document	The shard key for the collection where the change occurred. New in version 6.0.
`operationDescription.` \| `oldShardKey`	Document	The shard key for the collection that changed. New in version 6.1.
`operationDescription.` \| `unique`	Boolean	This has a value of true if the collection was sharded with a unique shard key. New in version 6.0.
`operationDescription.` \| `numInitialChunks`	NumberLong	Number of chunks created on each shard during a `shardCollection` operation. New in version 6.0. Starting in MongoDB 8.2, resharding operations ignore the `numInitialChunks` setting when the shard key contains a hashed prefix. Instead, MongoDB deterministically splits the hashed key space among recipients, using the same approach as initial chunk creation for empty hashed collections.
`operationDescription.` `collation`	Document	Collation document used for the shard key index. New in version 6.1.
`operationDescription.` `zones`	Array	The zones added for the new shard key. New in version 6.1.

Example

The following example shows a reshardCollection event:

{
   "_id": { <ResumeToken> },
   "operationType": "reshardCollection",
   "collectionUUID": 0,
   "ns": {"db": "reshard_collection_event", "coll": "coll"},
   "operationDescription": {
     "reshardUUID": 0,
     "shardKey": {"newKey": 1},
     "oldShardKey": {"_id": 1},
     "unique": false,
     "numInitialChunks": Long(1),
     "collation": {"locale": "simple"},
     "zones": [
         {"zone": "zone1", "min": {"newKey": {"$minKey": 1}}, "max": {"newKey": {"$maxKey": 1}}}
     ]
   }
}

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