event-bus/distributed/src/main/java/org/apache/james/events/GroupRegistration.java

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package org.apache.james.events;

import static org.apache.james.backends.rabbitmq.Constants.AUTO_DELETE;
import static org.apache.james.backends.rabbitmq.Constants.DURABLE;
import static org.apache.james.backends.rabbitmq.Constants.EXCLUSIVE;
import static org.apache.james.backends.rabbitmq.Constants.REQUEUE;

import java.util.Objects;
import java.util.Optional;
import java.util.function.Predicate;

import org.apache.james.backends.rabbitmq.RabbitMQConfiguration;
import org.apache.james.backends.rabbitmq.ReactorRabbitMQChannelPool;
import org.apache.james.backends.rabbitmq.ReceiverProvider;
import org.apache.james.util.MDCBuilder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.common.base.Preconditions;

import reactor.core.Disposable;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;
import reactor.rabbitmq.AcknowledgableDelivery;
import reactor.rabbitmq.ConsumeOptions;
import reactor.rabbitmq.QueueSpecification;
import reactor.rabbitmq.Receiver;
import reactor.rabbitmq.Sender;
import reactor.util.retry.Retry;

class GroupRegistration implements Registration {

    static class WorkQueueName {
        private final String prefix;
        private final Group group;

        WorkQueueName(String prefix, Group group) {
            this.prefix = prefix;
            Preconditions.checkNotNull(group, "Group must be specified");
            this.group = group;
        }

        String asString() {
            return prefix + "-workQueue-" + group.asString();
        }
    }

    private static final Logger LOGGER = LoggerFactory.getLogger(GroupRegistration.class);
    static final String RETRY_COUNT = "retry-count";
    static final int DEFAULT_RETRY_COUNT = 0;

    private final NamingStrategy namingStrategy;
    private final ReactorRabbitMQChannelPool channelPool;
    private final EventListener.ReactiveEventListener listener;
    private final WorkQueueName queueName;
    private final Runnable unregisterGroup;
    private final EventSerializer eventSerializer;
    private final GroupConsumerRetry retryHandler;
    private final WaitDelayGenerator delayGenerator;
    private final Group group;
    private final RetryBackoffConfiguration retryBackoff;
    private final ListenerExecutor listenerExecutor;
    private final RabbitMQConfiguration configuration;
    private Optional<Disposable> receiverSubscriber;
    private final ReceiverProvider receiverProvider;

    GroupRegistration(NamingStrategy namingStrategy, ReactorRabbitMQChannelPool channelPool, Sender sender, ReceiverProvider receiverProvider, EventSerializer eventSerializer,
                      EventListener.ReactiveEventListener listener, Group group, RetryBackoffConfiguration retryBackoff,
                      EventDeadLetters eventDeadLetters,
                      Runnable unregisterGroup, ListenerExecutor listenerExecutor, RabbitMQConfiguration configuration) {
        this.namingStrategy = namingStrategy;
        this.channelPool = channelPool;
        this.eventSerializer = eventSerializer;
        this.listener = listener;
        this.configuration = configuration;
        this.queueName = namingStrategy.workQueue(group);
        this.receiverProvider = receiverProvider;
        this.retryBackoff = retryBackoff;
        this.listenerExecutor = listenerExecutor;
        this.receiverSubscriber = Optional.empty();
        this.unregisterGroup = unregisterGroup;
        this.retryHandler = new GroupConsumerRetry(namingStrategy, sender, group, retryBackoff, eventDeadLetters, eventSerializer);
        this.delayGenerator = WaitDelayGenerator.of(retryBackoff);
        this.group = group;
    }

    GroupRegistration start() {
        receiverSubscriber = Optional
            .of(createGroupWorkQueue()
                .then(retryHandler.createRetryExchange(queueName))
                .then(Mono.fromCallable(this::consumeWorkQueue))
                .retryWhen(Retry.backoff(retryBackoff.getMaxRetries(), retryBackoff.getFirstBackoff()).jitter(retryBackoff.getJitterFactor()).scheduler(Schedulers.elastic()))
                .block());
        return this;
    }

    void restart() {
        Optional<Disposable> previousSubscriber = this.receiverSubscriber;
        receiverSubscriber = Optional.of(consumeWorkQueue());
        previousSubscriber
            .filter(Predicate.not(Disposable::isDisposed))
            .ifPresent(Disposable::dispose);
    }

    private Mono<Void> createGroupWorkQueue() {
        return channelPool.createWorkQueue(
            QueueSpecification.queue(queueName.asString())
                .durable(DURABLE)
                .exclusive(!EXCLUSIVE)
                .autoDelete(!AUTO_DELETE)
                .arguments(configuration.workQueueArgumentsBuilder(!AUTO_DELETE)
                    .deadLetter(namingStrategy.deadLetterExchange())
                    .build()));
    }

    private Disposable consumeWorkQueue() {
        return Flux.using(
                receiverProvider::createReceiver,
                receiver -> receiver.consumeManualAck(queueName.asString(), new ConsumeOptions().qos(EventBus.EXECUTION_RATE)),
                Receiver::close)
            .publishOn(Schedulers.parallel())
            .filter(delivery -> Objects.nonNull(delivery.getBody()))
            .flatMap(this::deliver, EventBus.EXECUTION_RATE)
            .subscribeOn(Schedulers.elastic())
            .subscribe();
    }

    private Mono<Void> deliver(AcknowledgableDelivery acknowledgableDelivery) {
        byte[] eventAsBytes = acknowledgableDelivery.getBody();
        int currentRetryCount = getRetryCount(acknowledgableDelivery);

        return deserializeEvent(eventAsBytes)
            .flatMap(event -> delayGenerator.delayIfHaveTo(currentRetryCount)
                .flatMap(any -> runListenerReliably(currentRetryCount, event))
                .then(Mono.<Void>fromRunnable(acknowledgableDelivery::ack).subscribeOn(Schedulers.elastic())))
            .onErrorResume(e -> {
                LOGGER.error("Unable to process delivery for group {}", group, e);
                return Mono.fromRunnable(() -> acknowledgableDelivery.nack(!REQUEUE))
                    .subscribeOn(Schedulers.elastic())
                    .then();
            });
    }

    public Mono<Void> runListenerReliably(int currentRetryCount, Event event) {
        return runListener(event)
            .onErrorResume(throwable -> retryHandler.handleRetry(event, currentRetryCount, throwable));
    }

    private Mono<Event> deserializeEvent(byte[] eventAsBytes) {
        return Mono.fromCallable(() -> eventSerializer.fromBytes(eventAsBytes))
            .subscribeOn(Schedulers.parallel());
    }

    Mono<Void> reDeliver(Event event) {
        return retryHandler.retryOrStoreToDeadLetter(event, DEFAULT_RETRY_COUNT);
    }

    private Mono<Void> runListener(Event event) {
        return listenerExecutor.execute(
            listener,
            MDCBuilder.create()
                .addToContext(EventBus.StructuredLoggingFields.GROUP, group.asString()),
            event);
    }

    private int getRetryCount(AcknowledgableDelivery acknowledgableDelivery) {
        return Optional.ofNullable(acknowledgableDelivery.getProperties().getHeaders())
            .flatMap(headers -> Optional.ofNullable(headers.get(RETRY_COUNT)))
            .filter(Integer.class::isInstance)
            .map(Integer.class::cast)
            .orElse(DEFAULT_RETRY_COUNT);
    }

    @Override
    public void unregister() {
        receiverSubscriber.filter(Predicate.not(Disposable::isDisposed))
            .ifPresent(Disposable::dispose);
        unregisterGroup.run();
    }
}