event-bus/in-vm/src/main/java/org/apache/james/events/InVMEventBus.java

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package org.apache.james.events;

import java.util.Collection;
import java.util.Optional;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import javax.inject.Inject;

import org.apache.james.events.delivery.EventDelivery;

import com.google.common.collect.HashMultimap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Multimap;
import com.google.common.collect.Multimaps;

import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

public class InVMEventBus implements EventBus {

    public static EventBusName IN_VN_EVENTBUS_NAME = new EventBusName("InVMEventBus");

    private final Multimap<RegistrationKey, EventListener.ReactiveEventListener> registrations;
    private final ConcurrentHashMap<Group, EventListener.ReactiveEventListener> groups;
    private final EventDelivery eventDelivery;
    private final RetryBackoffConfiguration retryBackoff;
    private final EventDeadLetters eventDeadLetters;

    @Inject
    public InVMEventBus(EventDelivery eventDelivery, RetryBackoffConfiguration retryBackoff, EventDeadLetters eventDeadLetters) {
        this.eventDelivery = eventDelivery;
        this.retryBackoff = retryBackoff;
        this.eventDeadLetters = eventDeadLetters;
        this.registrations = Multimaps.synchronizedSetMultimap(HashMultimap.create());
        this.groups = new ConcurrentHashMap<>();
    }

    @Override
    public Mono<Registration> register(EventListener.ReactiveEventListener listener, RegistrationKey key) {
        registrations.put(key, listener);
        return Mono.just(() -> Mono.fromRunnable(() -> registrations.remove(key, listener)));
    }

    @Override
    public Registration register(EventListener.ReactiveEventListener listener, Group group) {
        EventListener previous = groups.putIfAbsent(group, listener);
        if (previous == null) {
            return () -> Mono.fromRunnable(() -> groups.remove(group, listener));
        }
        throw new GroupAlreadyRegistered(group);
    }

    @Override
    public Mono<Void> dispatch(Event event, Set<RegistrationKey> keys) {
        if (!event.isNoop()) {
            return Flux.merge(groupDeliveries(event), keyDeliveries(event, keys))
                .then()
                .onErrorResume(throwable -> Mono.empty());
        }
        return Mono.empty();
    }

    @Override
    public Mono<Void> reDeliver(Group group, Event event) {
        if (!event.isNoop()) {
            return groupDelivery(event, retrieveListenerFromGroup(group), group);
        }
        return Mono.empty();
    }

    @Override
    public EventBusName eventBusName() {
        return IN_VN_EVENTBUS_NAME;
    }

    @Override
    public Collection<Group> listRegisteredGroups() {
        return groups.keySet();
    }

    private EventListener.ReactiveEventListener retrieveListenerFromGroup(Group group) {
        return Optional.ofNullable(groups.get(group))
            .orElseThrow(() -> new GroupRegistrationNotFound(group));
    }

    private Mono<Void> keyDeliveries(Event event, Set<RegistrationKey> keys) {
        return Flux.fromIterable(registeredListenersByKeys(keys))
            .flatMap(listener -> eventDelivery.deliver(listener, event, EventDelivery.DeliveryOption.none()), EventBus.EXECUTION_RATE)
            .then();
    }

    private Mono<Void> groupDeliveries(Event event) {
        return Flux.fromIterable(groups.entrySet())
            .flatMap(entry -> groupDelivery(event, entry.getValue(), entry.getKey()), EventBus.EXECUTION_RATE)
            .then();
    }

    private Mono<Void> groupDelivery(Event event, EventListener.ReactiveEventListener listener, Group group) {
        return eventDelivery.deliver(
            listener,
            event,
            EventDelivery.DeliveryOption.of(
                EventDelivery.Retryer.BackoffRetryer.of(retryBackoff, listener),
                EventDelivery.PermanentFailureHandler.StoreToDeadLetters.of(group, eventDeadLetters)));
    }

    public Set<Group> registeredGroups() {
        return groups.keySet();
    }

    private Set<EventListener.ReactiveEventListener> registeredListenersByKeys(Set<RegistrationKey> keys) {
        return keys.stream()
            .flatMap(registrationKey -> registrations.get(registrationKey).stream())
            .collect(ImmutableSet.toImmutableSet());
    }
}