Observables

The MongoDB Async Driver is fully callback-based and makes extensive use of SingleResultCallback<T> to achieve this. The SingleResultCallback<T> interface requires the implementation of a single method onResult(T result, Throwable t) which is called once the operation has completed or errored. The result parameter contains the result of the operation if successful. If the operation failed for any reason then the t contains the Throwable reason for the failure. This pattern allows the users application logic to be deferred until the underlying network IO to MongoDB has been completed.

The callback pattern is extremely flexible but can quickly become unwieldy if the application logic requires a chain of operations. Nesting of callbacks can make code harder to read and give the appearance of making the codebase more complex that it actually is. To help with this we also have released two observable based asynchronous drivers:

1. MongoDB Reactive Streams Driver
2. MongoDB RxJava Driver

These observable drivers follow similar patterns that split the logic into onNext, onError and onComplete(d) methods. These methods split out the logic used by SingleResultCallback<T>.onResult(T result, Throwable t) into individual components that can make the code easier to reason with.

The MongoDB Async Driver provides a factory and interfaces that do the heavy lifting of converting callback-based operations into an observable operations. There are three interfaces Observable, Subscription and Observer. The Observables helpers convert any callback-based operations into observable operations.

Observable

The Observable represents a MongoDB operation which emits its results to the Observer based on demand requested by the Subscription to the Observable.

Subscription

A Subscription represents a one-to-one lifecycle of an Observer subscribing to an Observable. A Subscription to an Observable can only be used by a single Observer. The purpose of a Subscription is to control demand and to allow unsubscribing from the Observable.

Observer

An Observer provides the mechanism for receiving push-based notifications from the Observable. Demand for these events is signalled by its Subscription. On subscription to an Observable the Observer will be passed the Subscription via the onSubscribe(Subscription subscription). Demand for results is signaled via the Subscription and any results are passed to the onNext(TResult result) method. If there is an error for any reason the onError(Throwable e) will be called and no more events passed to the Observer. Alternatively, when the Observer has consumed all the results from the Observable the onComplete() method will be called.

Wrapping a MongoIterable

With the Observables factory creating an Observable from a MongoIterable is simple.

In the following example we iterate and print out all json forms of documents in a collection:

    Observables.observe(collection.find()).subscribe(new Observer<Document>(){
        @Override
        void onSubscribe(final Subscription subscription) {
            System.out.println("Subscribed and requesting all documents");
            subscription.request(Long.MAX_VALUE);
        }

        @Override
        void onNext(final Document document) {
            System.out.println(document.toJson());
        }

        @Override
        void onError(final Throwable e) {
            System.out.println("There was an error: " + e.getMessage());
        }

        @Override
        void onComplete() {
            System.out.println("Finished iterating all documents");
        }
    });

Wrapping a callback-based method

Creating an Observable from any callback-based methods requires the wrapping of the method inside a Block. This allows the execution of the operation to be delayed, until demand is request by the Subscription. The method must use the supplied callback to convert the results into an Observable.

In the following example we print out the count of the number of documents in a collection:

    Block<SingleResultCallback<Long>> operation = new Block<SingleResultCallback<Long>>() {
        @Override
        void apply(final SingleResultCallback<Long> callback) {
            collection.count(callback);
        }
    };

    // Or in Java 8 syntax:
    operation = (Block<SingleResultCallback<Long>>) collection::count;

    Observables.observe(operation).subscribe(new Observer<Long>(){
        @Override
        void onSubscribe(final Subscription subscription) {
            System.out.println("Subscribed and requesting the count");
            subscription.request(1);
        }

        @Override
        void onNext(final Long count) {
            System.out.println("The collection has " + count + " documents");
        }

        @Override
        void onError(final Throwable e) {
            System.out.println("There was an error: " + e.getMessage());
        }

        @Override
        void onComplete() {
            System.out.println("Finished");
        }
    });

Back Pressure

In the following example, the Subscription is used to control demand when iterating an Observable. This is similar in concept to the MongoIterable.forEach method but allows demand-driven iteration:

 Observables.observe(collection.find()).subscribe(new Observer<Document>(){
        private long batchSize = 10;
        private long seen = 0;
        private Subscription subscription;

         @Override
         void onSubscribe(final Subscription subscription) {
             this.subscription = subscription;
             subscription.request(batchSize);
         }

         @Override
         void onNext(final Document document) {
             System.out.println(document.toJson());
             seen += 1;
             if (seen == batchSize) {
                seen = 0;
                subscription.request(batchSize);
             }
         }

         @Override
         void onError(final Throwable e) {
             System.out.println("There was an error: " + e.getMessage());
         }

         @Override
         void onComplete() {
             System.out.println("Finished iterating all documents");
         }
     });