Change Streams - Draft

MongoDB 3.6 introduces a new $changeStream aggregation pipeline operator.

Change streams provide a way to watch changes to documents in a collection. To improve the usability of this new stage, the MongoCollection API includes a new watch method. The ChangeStreamIterable sets up the change stream and automatically attempts to resume if it encounters a potentially recoverable error.

Prerequisites

  • The example below requires a restaurants collection in the test database. To create and populate the collection, follow the directions in github.

  • Include the following import statements:

import com.mongodb.Block;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoClient;
import com.mongodb.ConnectionString;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.changestream.FullDocument;
import com.mongodb.client.model.changestream.ChangeStreamDocument;
import org.bson.Document;
  • Include the following code which the examples in the tutorials will use to print the results of the change stream:
Block<ChangeStreamDocument<Document>> printBlock = new Block<>() {
    @Override
    public void apply(final ChangeStreamDocument<Document> changeStreamDocument) {
        System.out.println(changeStreamDocument);
    }
};

Connect to a MongoDB Deployment

Connect to a MongoDB deployment and declare and define a MongoDatabase and a MongoCollection instance.

For example, include the following code to connect to a replicaSet MongoDB deployment running on localhost on ports 27017, 27018 and 27019. It also defines database to refer to the test database and collection to refer to the restaurants collection.

MongoClient mongoClient = MongoClients.create(new ConnectionString("mongodb://localhost:27017,localhost:27018,localhost:27019"));
MongoDatabase database = mongoClient.getDatabase("test");
MongoCollection<Document> collection = database.getCollection("restaurants");

For additional information on connecting to MongoDB, see Connect to MongoDB.

Watch the collection

To create a change stream use the the MongoCollection.watch() method.

In the following example, the change stream prints out all changes it observes.

collection.watch().forEach(printBlock);

Filtering content

The watch method can also be passed a list of aggregation stages, that can modify the data returned by the $changeStream operator. Note: not all aggregation operators are supported. See the $changeStream documentation for more information.

In the following example the change stream prints out all changes it observes, for insert, update, replace and delete operations:

  • First it uses a $match stage to filter for documents where the operationType is either an insert, update, replace or delete.

  • Then, it sets the fullDocument to FullDocument.UPDATE_LOOKUP, so that the document after the update is included in the results.

collection.watch(asList(Aggregates.match(Filters.in("operationType", asList("insert", "update", "replace", "delete")))))
        .fullDocument(FullDocument.UPDATE_LOOKUP).forEach(printBlock);