/plugin/src/main/java/com/atlassian/confluence/extra/userlister/UserListener.java

package com.atlassian.confluence.extra.userlister;

import com.atlassian.confluence.event.events.security.LoginEvent;
import com.atlassian.confluence.event.events.security.LogoutEvent;
import com.atlassian.event.api.AsynchronousPreferred;
import com.atlassian.event.api.EventListener;
import com.atlassian.event.api.EventListenerRegistrar;
import com.atlassian.plugin.spring.scanner.annotation.imports.ComponentImport;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy;

import static io.atlassian.util.concurrent.ThreadFactories.Type.DAEMON;
import static io.atlassian.util.concurrent.ThreadFactories.namedThreadFactory;
import static java.util.concurrent.TimeUnit.MILLISECONDS;

/**
 * A component which listens for {@link LoginEvent}s and {@link LogoutEvent}s published by Confluence, and updates
 * the record of currently logged-in users in a {@link UserListManager}.
 * <p>
 * Note that {@link LoginEvent} and {@link LogoutEvent} are both synchronous events, i.e. are delivered to this component
 * on the same thread that publishes them. This is important because {@link DefaultUserListManager} uses a Hazelcast data
 * structure to store the session records, and we don't want to block the HTTP thread by talking to another cluster node.
 * <p>
 * To avoid blocking this thread, we use an internal {@link ExecutorService} to asynchronously process the events, since
 * eventual consistency is fine. Ideally, the events would be {@link AsynchronousPreferred}, avoiding the need to do this.
 * <p>
 * Note also that the executor is configured defensively, with a single background thread and a fixed size queue. If events
 * are generated faster than we can process them, then it will start discarding the oldest events in the queue.
 */
@Component
public class UserListener implements InitializingBean, DisposableBean {

    private final EventListenerRegistrar eventListenerRegistrar;
    private final UserListManager userListManager;
    private final ExecutorService eventListenerExecutor;

    @Autowired
    public UserListener(final @ComponentImport EventListenerRegistrar eventListenerRegistrar,
                        final UserListManager userListManager) {
        this(eventListenerRegistrar, userListManager, createEventListenerExecutor(1000, 1));
    }

    UserListener(final EventListenerRegistrar eventListenerRegistrar,
                 final UserListManager userListManager,
                 final ExecutorService eventListenerExecutor) {
        this.eventListenerRegistrar = eventListenerRegistrar;
        this.userListManager = userListManager;
        this.eventListenerExecutor = eventListenerExecutor;
    }

    @EventListener
    public void handleLoginEvent(final LoginEvent loginEvent) {
        eventListenerExecutor.submit(() -> userListManager.registerLoggedInUser(loginEvent.getUsername(), loginEvent.getSessionId()));
    }

    @EventListener
    public void handleLogoutEvent(final LogoutEvent logoutEvent) {
        eventListenerExecutor.submit(() -> userListManager.unregisterLoggedInUser(logoutEvent.getUsername(), logoutEvent.getSessionId()));
    }

    @Override
    public void afterPropertiesSet() {
        eventListenerRegistrar.register(this);
    }

    @Override
    public void destroy() {
        eventListenerExecutor.shutdownNow();
        eventListenerRegistrar.unregister(this);
    }

    /**
     * Creates an executor with a queue of the given capacity and number of threads, and which
     * uses a {@link DiscardOldestPolicy} when the queue fills up.
     */
    private static ExecutorService createEventListenerExecutor(final int capacity, final int poolSize) {
        return new ThreadPoolExecutor(
                poolSize, poolSize,
                0L, MILLISECONDS,
                new LinkedBlockingQueue<>(capacity),
                namedThreadFactory(UserListener.class.getName(), DAEMON),
                new DiscardOldestPolicy());
    }
}