Quick Tour Primer

The aim of this guide is to provide background about the Java driver and its asynchronous API before going onto looking at how to use the driver and MongoDB.


See the installation guide for instructions on how to install the MongoDB Reactive Streams Java Driver.

Reactive Streams

This library is an implementation of the reactive streams specification and the reactive stream API consists of the following components:

  1. Publisher
  2. Subscriber
  3. Subscription

A Publisher is a provider of a potentially unbounded number of sequenced elements, published according to the demand received from it’s Subscriber(s).

In response to a call to Publisher.subscribe(Subscriber) the possible invocation sequences for methods on the Subscriber are given by the following protocol:

onSubscribe onNext* (onError | onComplete)?

This means that onSubscribe is always signalled, followed by a possibly unbounded number of onNext signals (as requested by Subscriber) followed by an onError signal if there is a failure, or an onComplete signal when no more elements are available—all as long as the Subscription is not cancelled.

For more information about reactive streams go to: http://www.reactive-streams.org.

From Async Callbacks to Subscribers

The MongoDB Reactive Streams Java Driver is built upon the MongoDB Async driver which is callback driven. The API mirrors the Async driver API and any methods that cause network IO return a Publisher<T>, where T is the type of response for the operation.
The exception to that rule is for methods in the async driver that return a Void value in the callback. In reactive streams this is forbidden, so in those circumstances we return a Success on the operation.


All Publishers returned from the API are cold, meaning that no I/O happens until they are subscribed to and the subscription makes a request. So just creating a Publisher won’t cause any network IO. It’s not until Subscription.request() is called that the driver executes the operation.

Publishers in this implementation are unicast. Each Subscription to a Publisher relates to a single MongoDB operation and its ‘Subscriber’
will receive its own specific set of results.

Subscribers used in the Quick Tour

For the Quick Tour we have implemented a couple of different Subscribers and although this is an artificial scenario for reactive streams we do block on the results of one example before starting the next, so as to ensure the state of the database.

  1. ObservableSubscriber

    The base subscriber class is the ObservableSubscriber<T>, a Subscriber that stores the results of the Publisher<T>. It also contains an await() method so we can block for results to ensure the state of the database before going on to the next example.

  2. OperationSubscriber

    An implementation of the ObservableSubscriber that immediately calls Subscription.request when it’s subscribed to.

  3. PrintSubscriber

    An implementation of the OperationSubscriber that prints a message Subscriber.onComplete.

  4. PrintDocumentSubscriber

    An implementation of the OperationSubscriber that prints the json version of a Document when Subscriber.onNext(Document document) is called.

Blocking and non blocking examples

As our subscribers contain a latch that is only released when the onComplete method of the Subscriber is called, we can use that latch to block on by calling the await method. Below are two examples using our auto-requesting PrintDocumentSubscriber.
The first is non-blocking and the second blocks waiting for the Publisher to complete:

// Create a publisher
Publisher<Document> publisher = collection.find();

// Non blocking
publisher.subscribe(new PrintDocumentSubscriber());

Subscriber<Document> subscriber = new PrintDocumentSubscriber();
subscriber.await(); // Block for the publisher to complete

Publishers, Subscribers and Subscriptions

In general Publishers, Subscribers and Subscriptions are a low level API and it’s expected that users and libraries will build more expressive APIs upon them rather than solely use these interfaces. As a library solely implementing these interfaces, users will benefit from this growing ecosystem, which is a core design principle of reactive streams.