Database Manual / Change Streams / Change Events

shardCollection Event

Summary

shardCollection

New in version 6.0.

A shardCollection event occurs when a collection is sharded.

Description

FieldTypeDescription

_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.

lsid

document

The identifier for the session associated with the transaction.

Only present if the operation is part of a multi-document transaction.

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.
numInitialChunks

long

Number of chunks created on each shard during a shardCollection operation.

New in version 6.0.

operationDescription.
presplitHashedZones

boolean

Indicates whether the shard chunks were pre-split according to zones when the collection becamed sharded.

New in version 6.0.

operationDescription.
shardKey

document

The shard key for the collection where the change occurred.

New in version 6.0.

operationDescription.
unique

boolean

This has a value of true if the collection was sharded with a unique shard key.

New in version 6.0.

operationType

string

The type of operation that the change notification reports.

Returns a value of shardCollection for these change events.

txnNumber

NumberLong

Together with the lsid, a number that helps uniquely identify a transction.

Only present if the operation is part of a multi-document transaction.

wallTime

ISODate

The server date and time of the database operation. wallTime differs from clusterTime in that clusterTime is a timestamp taken from the oplog entry associated with the database operation event.

New in version 6.0.

Example

The following example shows a shardCollection event:

{
   "_id": { <ResumeToken> },
   "operationType": "shardCollection",
   "clusterTime": Timestamp({ t: 1654894852, i: 52 }),
   "collectionUUID": UUID("98046a1a-b649-4e5b-9c75-67594221ce19"),
   "wallTime": ISODate("2022-06-10T21:00:52.854Z"),
   "ns": {
      "db": "test",
      "coll": "authors"
   },
   "operationDescription": {
      "shardKey": { "age": "hashed" },
      "unique": false,
      "numInitialChunks": Long("0"),
      "presplitHashedZones": false
   }
}