$changeStreamSplitLargeEvent (aggregation stage)（聚合阶段）

The example scenario in this section shows the use of $changeStreamSplitLargeEvent with a new collection named myCollection.本节中的示例场景显示了如何将$changeStreamSplitRateEvent与名为myCollection的新集合一起使用。

Create myCollection and insert one document with just under 16 MB of data:创建myCollection并插入一个数据量不到16 MB的文档：

db.myCollection.insertOne(
   { _id: 0, largeField: "a".repeat( 16 * 1024 * 1024 - 1024 ) }
)

largeField contains the repeated letter a.largeField包含重复的字母a。

Enable changeStreamPreAndPostImages for myCollection, which allows a change stream to retrieve a document as it was before an update (pre-image) and after an update (post-image):为myCollection启用changeStreamPreAndPostImages，这允许更改流检索更新前（前映像）和更新后（后映像）的文档：

db.runCommand( {
   collMod: "myCollection",
   changeStreamPreAndPostImages: { enabled: true }
} )

Create a change stream cursor to monitor changes to myCollection using db.collection.watch():使用db.collection.watch()创建一个更改流游标来监视对myCollection的更改：

myChangeStreamCursor = db.myCollection.watch(
   [ { $changeStreamSplitLargeEvent: {} } ],
   { fullDocument: "required", fullDocumentBeforeChange: "required" }
)

For the change stream event:对于变更流事件：

• fullDocument: "required" includes the document post-image.包括文档后映像。
• fullDocumentBeforeChange: "required" includes the document pre-image.包括文档前映像。

For details, see $changeStream.有关详细信息，请参阅$changeStream。

Update the document in myCollection, which also produces a change stream event with the document pre- and post-images:更新myCollection中的文档，这也会生成一个包含文档前后映像的更改流事件：

db.myCollection.updateOne(
   { _id: 0 },
   { $set: { largeField: "b".repeat( 16 * 1024 * 1024 - 1024 ) } }
)

largeField now contains the repeated letter b.现在包含重复的字母b。

Retrieve the fragments from myChangeStreamCursor using the next() method and store the fragments in objects named firstFragment, secondFragment, and thirdFragment:使用next()方法从myChangeStreamCursor检索片段，并将片段存储在名为firstFragment、secondFragment和thirdFragment的对象中：

const firstFragment = myChangeStreamCursor.next()
const secondFragment = myChangeStreamCursor.next()
const thirdFragment = myChangeStreamCursor.next()

Show firstFragment.splitEvent:显示firstFragment.splitEvent：

firstFragment.splitEvent

Output with the fragment details:输出片段详细信息：

splitEvent: { fragment: 1, of: 3 }

Similarly, secondFragment.splitEvent and thirdFragment.splitEvent return:同样，secondFragment.splitEvent和thirdFragment.splitEvent返回：

splitEvent: { fragment: 2, of: 3 }
splitEvent: { fragment: 3, of: 3 }

To examine the object keys for firstFragment:要检查firstFragment的对象键，请执行以下操作：

Object.keys( firstFragment )

Output:输出：

[
   '_id',
   'splitEvent',
   'wallTime',
   'clusterTime',
   'operationType',
   'documentKey',
   'ns',
   'fullDocument'
]

To examine the size in bytes for firstFragment.fullDocument:要检查firstFragment.fullDocument的字节大小：

bsonsize( firstFragment.fullDocument )

Output:输出：

16776223

secondFragment contains the fullDocumentBeforeChange pre-image, which is approximately 16 MB in size. The following example shows the object keys for secondFragment:secondFragment包含完整的DocumentBeforeChange前映像，大小约为16 MB。以下示例显示了secondFragment的对象键：

Object.keys( secondFragment )

Output:输出：

[ '_id', 'splitEvent', 'fullDocumentBeforeChange' ]

thirdFragment contains the updateDescription field, which is approximately 16 MB in size. The following example shows the object keys for thirdFragment:thirdFragment包含updateDescription字段，其大小约为16MB。以下示例显示了thirdFragment的对象键：

Object.keys( thirdFragment )

Output:输出：

[ '_id', 'splitEvent', 'updateDescription' ]

The C# examples on this page use the sample_mflix database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB .NET/C# Driver documentation.本页上的C#示例使用Atlas示例数据集中的sample_mflix数据库。要了解如何创建免费的MongoDB Atlas集群并加载示例数据集，请参阅MongoDB .NET/C#驱动程序文档中的入门。

The following Movie class models the documents in the sample_mflix.movies collection:以下Movie类对sample_mflix.movies集合中的文档进行建模：

public class Movie
{
    public ObjectId Id { get; set; }

    public int Runtime { get; set; }
    
    public string Title { get; set; }

    public string Rated { get; set; }

    public List<string> Genres { get; set; }

    public string Plot { get; set; }
    
    public ImdbData Imdb { get; set; }

    public int Year { get; set; }

    public int Index { get; set; }
    
    public string[] Comments { get; set; }
   
    [BsonElement("lastupdated")]
    public DateTime LastUpdated { get; set; }
}

var camelCaseConvention = new ConventionPack { new CamelCaseElementNameConvention() };
ConventionRegistry.Register("CamelCase", camelCaseConvention, type => true);

To use the MongoDB .NET/C# driver to add a $changeStreamSplitLargeEvent stage to an aggregation pipeline, call the ChangeStreamSplitLargeEvent() method on a PipelineDefinition object.要使用MongoDB .NET/C#驱动程序将$changeStreamSplitAllegeEvent阶段添加到聚合管道中，请在PipelineDefinition对象上调用ChangeStreamSplitLargeEvent()方法。

The following example creates a pipeline stage that splits events exceeding 16 MB into fragments and returns them sequentially in a change stream cursor:以下示例创建了一个管道阶段，该阶段将超过16 MB的事件拆分为片段，并在更改流游标中顺序返回它们：

var pipeline = new EmptyPipelineDefinition<ChangeStreamDocument<Movie>>()
    .ChangeStreamSplitLargeEvent();

You can use both the watch() method and the aggregate() method to execute a $changeStreamSplitLargeEvent operation. 您可以使用watch()方法和aggregate()方法来执行$changeStreamSplitargeEvent操作。$changeStreamSplitLargeEvent returns a ChangeStreamCursor when you pass the aggregation pipeline to the watch() method on a MongoDB Collection object. 当您将聚合管道传递给MongoDB Collection对象上的watch()方法时，$changeStreamSplitRateEvent返回一个ChangeStreamCursor。$changeStreamSplitLargeEvent returns an AggregationCursor when you pass the aggregation pipeline to the aggregate() method.当您将聚合管道传递给aggregate()方法时，$changeStreamSplitRateEvent返回AggregationCursor。

The following example splits events exceeding 16 MB into fragments and returns them sequentially in a ChangeStreamCursor:以下示例将超过16 MB的事件拆分为片段，并在ChangeStreamCursor中按顺序返回它们：

const pipeline = [{ changeStreamSplitLargeEvent: {} }];

const changeStream = collection.watch(pipeline);
return changeStream;