Working with change streams in Java MongoDB

16 Oct 2023

MongoDB

Change streams in MongoDB allow you to subscribe to real-time data changes happening in your database. With change streams, you can monitor inserts, updates, and deletions on specific collections and receive notifications whenever changes occur. This feature is particularly useful when building applications that require real-time updates or data synchronization.

In this blog post, we will explore how to work with change streams in Java using the MongoDB Java driver.

Prerequisites

Before getting started, make sure you have the following:

Java Development Kit (JDK) installed
MongoDB Java driver added to your project (Maven, Gradle, or manual download)

Setting up a Change Stream

To set up a change stream, you need to establish a connection to your MongoDB database using the Java driver. Here’s an example:

import com.mongodb.ConnectionString;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoCursor;
import com.mongodb.client.MongoDatabase;
import org.bson.Document;

public class ChangeStreamExample {

    public static void main(String[] args) {
        String connectionString = "mongodb://localhost:27017";
        ConnectionString connString = new ConnectionString(connectionString);

        try (com.mongodb.client.MongoClient client = MongoClients.create(connString)) {

            MongoDatabase database = client.getDatabase("mydb");
            MongoCollection<Document> collection = database.getCollection("mycollection");
            MongoCursor<ChangeStreamDocument<Document>> cursor = collection.watch().iterator();

            while (cursor.hasNext()) {
                Document changeDocument = cursor.next().getFullDocument();
                System.out.println("Change document: " + changeDocument.toJson());
            }
        }
    }
}

In the example code, we first create a MongoDB client using the connection string and connect to the desired database and collection. We then obtain a change stream cursor by calling the watch() method on the collection. Finally, we iterate over the cursor to print the change documents as they occur.

Filtering Change Events

Change streams provide various options to filter change events based on your requirements. For example, you can filter events by the operation type (insert, update, delete), document fields, or events related to a specific document ID.

To filter change events, you can use the Aggregates.match() method with a query filter. Here’s an example that filters change events for documents with a specific field value:

import com.mongodb.client.model.Aggregates;
import com.mongodb.client.model.Filters;

// ...

MongoCursor<ChangeStreamDocument<Document>> cursor = collection.watch(
    Arrays.asList(Aggregates.match(Filters.eq("fieldName", "fieldValue")))
).iterator();

Error Handling and Resumability

When working with change streams, it’s important to handle errors and make your application resilient to failures. If an error occurs, you can catch exceptions such as MongoException to handle them gracefully.

Change streams also provide the ability to resume from the last seen event in case of driver or network failures. The MongoDB Java driver automatically handles resumability by storing a resume token for each processed event. When starting the change stream cursor again, you can pass the resume token to continue from the last processed event.

Conclusion

Working with change streams in Java using the MongoDB Java driver opens up a world of possibilities for real-time data processing and synchronization. By subscribing to data changes, you can build applications that react to changes in your MongoDB database instantly.

In this blog post, we covered the basics of setting up a change stream, filtering change events, and handling errors. To learn more about change streams and their capabilities, refer to the official MongoDB documentation.

Happy coding!

References:

MongoDB Java Driver Documentation: https://docs.mongodb.com/drivers/java/
MongoDB Change Streams Documentation: https://docs.mongodb.com/manual/changeStreams/

#Java #MongoDB