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Change Events

update Event

Summary

update

An update event occurs when an operation updates a document in a collection.

Note

Disambiguation

To learn more about events that occur when collection options are modified, see the modify event.

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.

On sharded clusters, events with the same clusterTime may not all relate to the same transaction. Some events don't relate to a transaction at all.

To identify events for a single transaction, you can use the combination of lsid and txnNumber in the change stream event document.

collectionUUID

UUID

UUID identifying the collection where the change occurred.

New in version 6.0.

The collectionUUID field is only included if the change stream is created with the showExpandedEvents field set to true. For details, see showExpandedEvents.

documentKey

document

Document that contains the _id value of the document created or modified by the CRUD operation.

For sharded collections, this field also displays the full shard key for the document. The _id field is not repeated if it is already a part of the shard key.

fullDocument

document

The document created or modified by a CRUD operation.

This field only appears if you configured the change stream with fullDocument set to updateLookup. When you configure the change stream with updateLookup, the field represents the current majority-committed version of the document modified by the update operation. The document may differ from the changes described in updateDescription if any other majority-committed operations have modified the document between the original update operation and the full document lookup.

For more information, see Lookup Full Document for Update Operations.

Changed in version 6.0.

Starting in MongoDB 6.0, if you set the changeStreamPreAndPostImages option using db.createCollection(), create, or collMod, then the fullDocument field shows the document after it was inserted, replaced, or updated (the document post-image). fullDocument is always included for insert events.

fullDocumentBeforeChange

document

The document before changes were applied by the operation. That is, the document pre-image.

This field is available when you enable the changeStreamPreAndPostImages field for a collection using db.createCollection() method or the create or collMod commands.

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.

operationType

string

The type of operation that the change notification reports.

Returns a value of update for these change events.

updateDescription

document

A document describing the fields that were updated or removed by the update operation.

updateDescription.
removedFields

array

An array of fields that were removed by the update operation.

updateDescription.
truncatedArrays

array

An array of documents which record array truncations performed with pipeline-based updates using one or more of the following stages:

If the entire array is replaced, the truncations will be reported under updateDescription.updatedFields.

updateDescription.
truncatedArrays.
field

string

The name of the truncated field.

updateDescription.
truncatedArrays.
newSize

integer

The number of elements in the truncated array.

updateDescription.
updatedFields

document

A document whose keys correspond to the fields that were modified by the update operation. The value of each field corresponds to the new value of those fields, rather than the operation that resulted in the new value.

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.

Behavior

Document Pre- and Post-Images

Starting in MongoDB 6.0, you see a fullDocumentBeforeChange document with the fields before the document was changed (or deleted) if you perform these steps:

  1. Enable the new changeStreamPreAndPostImages field for a collection using db.createCollection(), create, or collMod.

  2. Set fullDocumentBeforeChange to "required" or "whenAvailable" in db.collection.watch().

Example fullDocumentBeforeChange document in the change stream output:

"fullDocumentBeforeChange" : {
 "_id" : ObjectId("599af247bb69cd89961c986d"),
 "userName" : "alice123",
 "name" : "Alice Smith"
}

For complete examples with the change stream output, see Change Streams with Document Pre- and Post-Images.

Pre- and post-images are not available for a change stream event if the images were:

  • Not enabled on the collection at the time of a document update or delete operation.

  • Removed after the pre- and post-image retention time set in expireAfterSeconds.

    • The following example sets expireAfterSeconds to 100 seconds on an entire cluster:

      use admin
      db.runCommand( {
       setClusterParameter:
       { changeStreamOptions: {
       preAndPostImages: { expireAfterSeconds: 100 }
       } }
      } )

    • The following example returns the current changeStreamOptions settings, including expireAfterSeconds:

      db.adminCommand( { getClusterParameter: "changeStreamOptions" } )

    • Setting expireAfterSeconds to off uses the default retention policy: pre- and post-images are retained until the corresponding change stream events are removed from the oplog.

    • If a change stream event is removed from the oplog, then the corresponding pre- and post-images are also deleted regardless of the expireAfterSeconds pre- and post-image retention time.

Additional considerations:

  • Enabling pre- and post-images consumes storage space and adds processing time. Only enable pre- and post-images if you need them.

  • Limit the change stream event size to less than 16 mebibytes. To limit the event size, you can:

    • Limit the document size to 8 megabytes. You can request pre- and post-images simultaneously in the change stream output if other change stream event fields like updateDescription are not large.

    • Request only post-images in the change stream output for documents up to 16 mebibytes if other change stream event fields like updateDescription are not large.

    • Request only pre-images in the change stream output for documents up to 16 mebibytes if:

      • document updates affect only a small fraction of the document structure or content, and

      • do not cause a replace change event. A replace event always includes the post-image.

  • To request a pre-image, you set fullDocumentBeforeChange to required or whenAvailable in db.collection.watch(). To request a post-image, you set fullDocument using the same method.

  • Pre-images are written to the config.system.preimages collection.

    • The config.system.preimages collection may become large. To limit the collection size, you can set expireAfterSeconds time for the pre-images as shown earlier.

    • Pre-images are removed asynchronously by a background process.

Important

Backward-Incompatible Feature

Starting in MongoDB 6.0, if you are using document pre- and post-images for change streams, you must disable changeStreamPreAndPostImages for each collection using the collMod command before you can downgrade to an earlier MongoDB version.

Tip

Example

The following example illustrates an update event:

{
 "_id": { <Resume Token> },
 "operationType": "update",
 "clusterTime": <Timestamp>,
 "wallTime": <ISODate>,
 "ns": {
 "db": "engineering",
 "coll": "users"
 },
 "documentKey": {
 "_id": ObjectId("58a4eb4a30c75625e00d2820")
 },
 "updateDescription": {
 "updatedFields": {
 "email": "alice@10gen.com"
 },
 "removedFields": ["phoneNumber"],
 "truncatedArrays": [ {
 "field" : "vacation_time",
 "newSize" : 36
 } ]
 }
}

The following example illustrates an update event for change streams opened with the fullDocument : updateLookup option:

{
 "_id": { <Resume Token> },
 "operationType": "update",
 "clusterTime": <Timestamp>,
 "wallTime": <ISODate>,
 "ns": {
 "db": "engineering",
 "coll": "users"
 },
 "documentKey": {
 "_id": ObjectId("58a4eb4a30c75625e00d2820")
 },
 "updateDescription": {
 "updatedFields": {
 "email": "alice@10gen.com"
 },
 "removedFields": ["phoneNumber"],
 "truncatedArrays": [ {
 "field" : "vacation_time",
 "newSize" : 36
 } ]
 },
 "fullDocument": {
 "_id": ObjectId("58a4eb4a30c75625e00d2820"),
 "name": "Alice",
 "userName": "alice123",
 "email": "alice@10gen.com",
 "team": "replication"
 }
}

The fullDocument document represents the most current majority-committed version of the updated document. The fullDocument document may vary from the document at the time of the update operation depending on the number of interleaving majority-committed operations that occur between the update operation and the document lookup.

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