聊聊hystrix的BucketedCounterStream

/**
 * Abstract class that imposes a bucketing structure and provides streams of buckets
 *
 * @param <Event> type of raw data that needs to get summarized into a bucket
 * @param <Bucket> type of data contained in each bucket
 * @param <Output> type of data emitted to stream subscribers (often is the same as A but does not have to be)
 */
public abstract class BucketedCounterStream<Event extends HystrixEvent, Bucket, Output> {
    protected final int numBuckets;
    protected final Observable<Bucket> bucketedStream;
    protected final AtomicReference<Subscription> subscription = new AtomicReference<Subscription>(null);

    private final Func1<Observable<Event>, Observable<Bucket>> reduceBucketToSummary;

    private final BehaviorSubject<Output> counterSubject = BehaviorSubject.create(getEmptyOutputValue());

    protected BucketedCounterStream(final HystrixEventStream<Event> inputEventStream, final int numBuckets, final int bucketSizeInMs,
                                    final Func2<Bucket, Event, Bucket> appendRawEventToBucket) {
        this.numBuckets = numBuckets;
        this.reduceBucketToSummary = new Func1<Observable<Event>, Observable<Bucket>>() {
            @Override
            public Observable<Bucket> call(Observable<Event> eventBucket) {
                return eventBucket.reduce(getEmptyBucketSummary(), appendRawEventToBucket);
            }
        };

        final List<Bucket> emptyEventCountsToStart = new ArrayList<Bucket>();
        for (int i = 0; i < numBuckets; i++) {
            emptyEventCountsToStart.add(getEmptyBucketSummary());
        }

        this.bucketedStream = Observable.defer(new Func0<Observable<Bucket>>() {
            @Override
            public Observable<Bucket> call() {
                return inputEventStream
                        .observe()
                        .window(bucketSizeInMs, TimeUnit.MILLISECONDS) //bucket it by the counter window so we can emit to the next operator in time chunks, not on every OnNext
                        .flatMap(reduceBucketToSummary)                //for a given bucket, turn it into a long array containing counts of event types
                        .startWith(emptyEventCountsToStart);           //start it with empty arrays to make consumer logic as generic as possible (windows are always full)
            }
        });
    }

    abstract Bucket getEmptyBucketSummary();

    abstract Output getEmptyOutputValue();

    /**
     * Return the stream of buckets
     * @return stream of buckets
     */
    public abstract Observable<Output> observe();

    public void startCachingStreamValuesIfUnstarted() {
        if (subscription.get() == null) {
            //the stream is not yet started
            Subscription candidateSubscription = observe().subscribe(counterSubject);
            if (subscription.compareAndSet(null, candidateSubscription)) {
                //won the race to set the subscription
            } else {
                //lost the race to set the subscription, so we need to cancel this one
                candidateSubscription.unsubscribe();
            }
        }
    }

    /**
     * Synchronous call to retrieve the last calculated bucket without waiting for any emissions
     * @return last calculated bucket
     */
    public Output getLatest() {
        startCachingStreamValuesIfUnstarted();
        if (counterSubject.hasValue()) {
            return counterSubject.getValue();
        } else {
            return getEmptyOutputValue();
        }
    }

    public void unsubscribe() {
        Subscription s = subscription.get();
        if (s != null) {
            s.unsubscribe();
            subscription.compareAndSet(s, null);
        }
    }
}

/**
 * Refinement of {@link BucketedCounterStream} which reduces numBuckets at a time.
 *
 * @param <Event> type of raw data that needs to get summarized into a bucket
 * @param <Bucket> type of data contained in each bucket
 * @param <Output> type of data emitted to stream subscribers (often is the same as A but does not have to be)
 */
public abstract class BucketedRollingCounterStream<Event extends HystrixEvent, Bucket, Output> extends BucketedCounterStream<Event, Bucket, Output> {
    private Observable<Output> sourceStream;
    private final AtomicBoolean isSourceCurrentlySubscribed = new AtomicBoolean(false);

    protected BucketedRollingCounterStream(HystrixEventStream<Event> stream, final int numBuckets, int bucketSizeInMs,
                                           final Func2<Bucket, Event, Bucket> appendRawEventToBucket,
                                           final Func2<Output, Bucket, Output> reduceBucket) {
        super(stream, numBuckets, bucketSizeInMs, appendRawEventToBucket);
        Func1<Observable<Bucket>, Observable<Output>> reduceWindowToSummary = new Func1<Observable<Bucket>, Observable<Output>>() {
            @Override
            public Observable<Output> call(Observable<Bucket> window) {
                return window.scan(getEmptyOutputValue(), reduceBucket).skip(numBuckets);
            }
        };
        this.sourceStream = bucketedStream      //stream broken up into buckets
                .window(numBuckets, 1)          //emit overlapping windows of buckets
                .flatMap(reduceWindowToSummary) //convert a window of bucket-summaries into a single summary
                .doOnSubscribe(new Action0() {
                    @Override
                    public void call() {
                        isSourceCurrentlySubscribed.set(true);
                    }
                })
                .doOnUnsubscribe(new Action0() {
                    @Override
                    public void call() {
                        isSourceCurrentlySubscribed.set(false);
                    }
                })
                .share()                        //multiple subscribers should get same data
                .onBackpressureDrop();          //if there are slow consumers, data should not buffer
    }

    @Override
    public Observable<Output> observe() {
        return sourceStream;
    }

    /* package-private */ boolean isSourceCurrentlySubscribed() {
        return isSourceCurrentlySubscribed.get();
    }
}

/**
 * Refinement of {@link BucketedCounterStream} which accumulates counters infinitely in the bucket-reduction step
 *
 * @param <Event> type of raw data that needs to get summarized into a bucket
 * @param <Bucket> type of data contained in each bucket
 * @param <Output> type of data emitted to stream subscribers (often is the same as A but does not have to be)
 */
public abstract class BucketedCumulativeCounterStream<Event extends HystrixEvent, Bucket, Output> extends BucketedCounterStream<Event, Bucket, Output> {
    private Observable<Output> sourceStream;
    private final AtomicBoolean isSourceCurrentlySubscribed = new AtomicBoolean(false);

    protected BucketedCumulativeCounterStream(HystrixEventStream<Event> stream, int numBuckets, int bucketSizeInMs,
                                              Func2<Bucket, Event, Bucket> reduceCommandCompletion,
                                              Func2<Output, Bucket, Output> reduceBucket) {
        super(stream, numBuckets, bucketSizeInMs, reduceCommandCompletion);

        this.sourceStream = bucketedStream
                .scan(getEmptyOutputValue(), reduceBucket)
                .skip(numBuckets)
                .doOnSubscribe(new Action0() {
                    @Override
                    public void call() {
                        isSourceCurrentlySubscribed.set(true);
                    }
                })
                .doOnUnsubscribe(new Action0() {
                    @Override
                    public void call() {
                        isSourceCurrentlySubscribed.set(false);
                    }
                })
                .share()                        //multiple subscribers should get same data
                .onBackpressureDrop();          //if there are slow consumers, data should not buffer
    }

    @Override
    public Observable<Output> observe() {
        return sourceStream;
    }
}