String uri = "mongodb://localhost:27000,localhost:27001,localhost:27002/admin?replicaSet=repl";

		final MongoClient client = MongoClients.create(uri);

		/**
		 * Create collections. CRUD operations in transactions must be on existing collections.
		 */
		client.getDatabase("mydb1").getCollection("foo").withWriteConcern(MAJORITY).insertOne(new Document("abc", 0));
		client.getDatabase("mydb2").getCollection("bar").withWriteConcern(MAJORITY).insertOne(new Document("xyz", 0));

		final ClientSession clientSession = client.startSession();

		TransactionOptions txnOptions = TransactionOptions.builder()
				.readPreference(primary())
				.readConcern(LOCAL)
				.writeConcern(MAJORITY)
				.build();

		TransactionBody<?> txnBody = new TransactionBody<String>() {

			@Override
			public String execute() {

				MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");
		        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");

		        coll1.insertOne(clientSession, new Document("abc", 1));
		        coll2.insertOne(clientSession, new Document("xyz", 999));

				return "SUCCESS";
			}
		};

		try {
			clientSession.withTransaction(txnBody, txnOptions);
		} finally {
			clientSession.close();
		}