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SirixDB is a hybrid on-disk and in-memory document oriented, versioned database system. It has a lightweight buffer manager, stores everything in a huge persistent and durable tree and allows efficient reconstruction of every revision. Furthermore, SirixDB implements change tracking, diffing and supports time travel queries.

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package io.sirix.access;

import io.sirix.api.ResourceSession;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.annotation.concurrent.ThreadSafe;
import javax.inject.Inject;
import javax.inject.Singleton;
import java.nio.file.Path;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Semaphore;

/**
 * A registry for write locks used for {@link ResourceSession resource managers}.
 *
 * Each {@link ResourceSession}, identified by its {@link Path resource path}, will be
 * assigned a unique write lock.
 *
 * @author Joao Sousa
 */
@Singleton
@ThreadSafe
public class WriteLocksRegistry {

    /**
     * Logger for {@link WriteLocksRegistry}.
     */
    private static final Logger logger = LoggerFactory.getLogger(WriteLocksRegistry.class);

    /** Central repository of all resource {@code <=>} write locks mappings. */
    private final Map locks;

    @Inject
    WriteLocksRegistry() {
        locks = new ConcurrentHashMap<>();
    }

    /**
     * Fetches the write lock for the provider resource, identified by its {@code resourcePath}.
     *
     * 
This method will create a new write lock if no lock exists for the provided resource.
     *
     * @param resourcePath The path that identifies the resource.
     * @return The lock to be used to perform write operations to the given resource.
     */
    public Semaphore getWriteLock(final Path resourcePath) {
        logger.trace("Getting lock for resource with path {}", resourcePath);
        return this.locks.computeIfAbsent(resourcePath, res -> new Semaphore(1));
    }

    /**
     * De-registers the lock for the provided resource, identified by its {@code resourcePath}.
     *
     * Note that this method does not prevent the de-registered lock from still being used.
     *
     * @param resourcePath The path that identifies the resource.
     */
    public void removeWriteLock(final Path resourcePath) {
        logger.trace("Removing lock for resource with path {}", resourcePath);
        this.locks.remove(resourcePath);
    }
}