Open Change Streams

Overview

In this guide, you can learn how to use a change stream to monitor real-time changes to your database. A change stream is a MongoDB Server feature that allows your application to subscribe to data changes on a single collection, database, or deployment.

You can specify a set of aggregation operators to filter and transform the data your application receives. When connecting to a MongoDB deployment v6.0 or later, you can also configure the events to include the document data before and after the change.

Learn how to open and configure your change streams in the following sections:

• Open a Change Stream

• Apply Aggregation Operators to your Change Stream

• Include Pre-images and Post-images

Open a Change Stream

You can open a change stream to subscribe to specific types of data changes and produce change events in your application.

To open a change stream, call the watch() method on an instance of a MongoCollection, MongoDatabase, or MongoClient.

The object on which you call the watch() method on determines the scope of events that the change stream listens for.

If you call watch() on a MongoCollection, the change stream monitors a collection.

If you call watch() on a MongoDatabase, the change stream monitors all collections in that database.

If you call watch() on a MongoClient, the change stream monitors all changes in the connected MongoDB deployment.

MongoCollection<Document> collection = database.getCollection("myColl");
ChangeStreamIterable<Document> changeStream = collection.watch();
changeStream.forEach(event ->
        System.out.println("Received a change: " + event));

Received a change: ChangeStreamDocument{
   operationType=insert,
   resumeToken={"_data": "..."},
   namespace=myDb.myColl,
   ...
}

Apply Aggregation Operators to your Change Stream

You can pass an aggregation pipeline as a parameter to the watch() method to specify which change events the change stream receives.

To learn which aggregation operators your MongoDB Server version supports, see Modify Change Stream Output.

MongoCollection<Document> collection = database.getCollection("myColl");
List<Bson> pipeline = Arrays.asList(
        Aggregates.match(Filters.in("operationType", Arrays.asList("insert", "update"))));
ChangeStreamIterable<Document> changeStream = collection.watch(pipeline);
changeStream.forEach(event ->
        System.out.println("Received a change to the collection: " + event));

Received a change: ChangeStreamDocument{
   operationType=update,
   resumeToken={"_data": "..."},
   namespace=myDb.myColl,
   ...
}

Split Large Change Stream Events

Starting in MongoDB 7.0, you can use the $changeStreamSplitLargeEvent aggregation stage to split events that exceed 16 MB into smaller fragments.

Use $changeStreamSplitLargeEvent only when strictly necessary. For example, use $changeStreamSplitLargeEvent if your application requires full document pre- or post-images, and generates events that exceed 16 MB.

The $changeStreamSplitLargeEvent stage returns the fragments sequentially. You can access the fragments by using a change stream cursor. Each fragment includes a SplitEvent object containing the following fields:

The following example modifies your change stream by using the $changeStreamSplitLargeEvent aggregation stage to split large events:

ChangeStreamIterable<Document> changeStream = collection.watch(
      Arrays.asList(Document.parse("{ $changeStreamSplitLargeEvent: {} }")));

You can call the getSplitEvent() method on your change stream cursor to access the SplitEvent as shown in the following example:

MongoChangeStreamCursor<ChangeStreamDocument<Document>> cursor = changeStream.cursor();
SplitEvent event = cursor.tryNext().getSplitEvent();

For more information about the $changeStreamSplitLargeEvent aggregation stage, see the $changeStreamSplitLargeEvent server documentation.

Include Pre-images and Post-images

You can configure the change event to contain or omit the following data:

• The pre-image, a document that represents the version of the document before the operation, if it exists

• The post-image, a document that represents the version of the document after the operation, if it exists

To receive change stream events that include a pre-image or post-image, you must perform the following actions:

• Enable pre-images and post-images for the collection on your MongoDB deployment.

  Tip

  To learn how to enable pre- and post-images on your deployment, see Change Streams with Document Pre- and Post-Images in the Server manual.

  To learn how to instruct the driver to create a collection with pre-images and post-images enabled, see the Create a Collection with Pre-Image and Post-Images Enabled section.

• Configure your change stream to retrieve either or both the pre-images and post-images.

  Tip

  To configure your change stream to record the pre-image in change events, see the Pre-image Configuration Example.

  To configure your change stream to record the post-image in change events, see the Post-image Configuration Example.

CreateCollectionOptions collectionOptions = new CreateCollectionOptions();
collectionOptions.changeStreamPreAndPostImagesOptions(new ChangeStreamPreAndPostImagesOptions(true));
database.createCollection("myColl", collectionOptions);

MongoCollection<Document> collection = database.getCollection("myColl");
ChangeStreamIterable<Document> changeStream = collection.watch()
        .fullDocumentBeforeChange(FullDocumentBeforeChange.REQUIRED);
changeStream.forEach(event ->
        System.out.println("Received a change: " + event));

Received a change: ChangeStreamDocument{
   operationType=update,
   resumeToken={"_data": "..."},
   namespace=myDb.myColl,
   destinationNamespace=null,
   fullDocument=null,
   fullDocumentBeforeChange=Document{{_id=..., amount=150, ...}},
   ...
}

MongoCollection<Document> collection = database.getCollection("myColl");
ChangeStreamIterable<Document> changeStream = collection.watch()
        .fullDocument(FullDocument.WHEN_AVAILABLE);
changeStream.forEach(event ->
        System.out.println("Received a change: " + event));

Received a change: ChangeStreamDocument{
   operationType=update,
   resumeToken={"_data": "..."},
   namespace=myDb.myColl,
   destinationNamespace=null,
   fullDocument=Document{{_id=..., color=purple, ...}},
   updatedFields={"color": purple},
   ...
}