Thread safe

pom.xml

@@ -6,7 +6,7 @@
 
     <groupId>it.cavallium</groupId>
     <artifactId>strangedb-core</artifactId>
-    <version>1.5.5</version>
+    <version>1.5.7</version>
 
     <name>strangedb-core</name>
     <url>https://git.ignuranza.net/andreacavalli/strangedb-core</url>

src/main/java/it/cavallium/strangedb/database/DatabaseCore.java

@@ -4,6 +4,7 @@ import it.cavallium.strangedb.database.blocks.DatabaseBlocksIO;
 import it.cavallium.strangedb.database.blocks.DatabaseBlocksMetadata;
 import it.cavallium.strangedb.database.references.DatabaseReferencesIO;
 import it.cavallium.strangedb.database.references.DatabaseReferencesMetadata;
+import it.cavallium.strangedb.database.references.ReferenceInfo;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
@@ -11,16 +12,18 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
 
+import static it.cavallium.strangedb.database.references.DatabaseReferencesMetadata.NONEXISTENT_REFERENCE_INFO;
+
 public class DatabaseCore implements IDatabase {
 
 	private final DatabaseFileIO fileIO;
 	private final DatabaseBlocksIO blocksIO;
-	private final DatabaseBlocksMetadata blocksMetadata;
+	protected final DatabaseBlocksMetadata blocksMetadata;
 	protected final DatabaseReferencesIO referencesIO;
 	protected final DatabaseReferencesMetadata referencesMetadata;
-	private final Path dataFile;
-	private final Path blocksMetaFile;
-	private final Path referencesMetaFile;
+	protected final Path dataFile;
+	protected final Path blocksMetaFile;
+	protected final Path referencesMetaFile;
 	protected volatile boolean closed;
 
 	public DatabaseCore(Path dataFile, Path blocksMetaFile, Path referencesMetaFile) throws IOException {
@@ -82,17 +85,24 @@ public class DatabaseCore implements IDatabase {
 		long referencesCount = databaseToClean.referencesMetadata.getFirstFreeReference();
 		long blocksCount = databaseToClean.blocksMetadata.getTotalBlocksCount();
 		long writtenReferences = 0;
+		long writtenBlocks = 0;
 
 		for (int referenceID = 0; referenceID < referencesCount; referenceID++) {
 			try {
-				ByteBuffer buffer = databaseToClean.referencesIO.readFromReference(referenceID);
-				newDatabase.referencesIO.writeToReference(referenceID, buffer.limit(), buffer);
-				writtenReferences++;
+				ReferenceInfo ref = databaseToClean.referencesMetadata.getCleanReference(referenceID);
+				if (!NONEXISTENT_REFERENCE_INFO.equals(ref)) {
+					ByteBuffer buffer = databaseToClean.referencesIO.readFromReference(referenceID);
+					newDatabase.referencesIO.writeToReference(referenceID, ref.getCleanerId(), buffer.limit(), buffer);
+					writtenReferences++;
+					if (buffer.limit() > 0) {
+						writtenBlocks++;
+					}
+				}
 			} catch (IOException ex) {
 				System.out.println("Error while reading reference " + referenceID + ". References written: " + writtenReferences);
 			}
 		}
-		System.out.println("References written: " + writtenReferences + ". Removed " + (blocksCount - writtenReferences) + " blocks. Removed " + (referencesCount - writtenReferences) + " references.");
+		System.out.println("[Core Cleaner] References written: " + writtenReferences + ". Removed " + (blocksCount - writtenBlocks) + " blocks. Removed " + (referencesCount - writtenReferences) + " references.");
 		databaseToClean.close();
 		newDatabase.close();
 		Files.deleteIfExists(newDataFile);

src/main/java/it/cavallium/strangedb/database/DatabaseFileIO.java

@@ -3,59 +3,70 @@ package it.cavallium.strangedb.database;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
-import java.nio.channels.SeekableByteChannel;
-import java.nio.file.Files;
+import java.nio.channels.AsynchronousFileChannel;
 import java.nio.file.Path;
 import java.nio.file.StandardOpenOption;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicLong;
 
 public class DatabaseFileIO implements IFileIO {
 
-	private final SeekableByteChannel dataFileChannel;
-	private final Object dataAccessLock = new Object();
-	private long firstFreeIndex;
+	private final AsynchronousFileChannel dataFileChannel;
+	private final AtomicLong firstFreeIndex;
+	private volatile boolean closed = false;
 
 	public DatabaseFileIO(Path dataFile) throws IOException {
-		synchronized (dataAccessLock) {
-			dataFileChannel = Files.newByteChannel(dataFile, StandardOpenOption.READ, StandardOpenOption.WRITE);
-			firstFreeIndex = dataFileChannel.size();
-		}
+		dataFileChannel = AsynchronousFileChannel.open(dataFile, StandardOpenOption.READ, StandardOpenOption.WRITE);
+		firstFreeIndex = new AtomicLong(dataFileChannel.size());
 	}
 
 	@Override
 	public ByteBuffer readAt(long index, int length) throws IOException {
+		if (closed) throw new IOException("Database closed!");
 		ByteBuffer dataBuffer = ByteBuffer.allocate(length);
-		dataFileChannel.position(index).read(dataBuffer);
+		try {
+			dataFileChannel.read(dataBuffer, index).get();
+		} catch (InterruptedException e) {
+			throw new IOException(e);
+		} catch (ExecutionException e) {
+			throw new IOException(e.getCause());
+		}
 		dataBuffer.flip();
 		return dataBuffer;
 	}
 
 	@Override
-	public void writeAt(long index, int length, ByteBuffer data) throws IOException {
-		synchronized (dataAccessLock) {
-			if (data.position() != 0) {
-				throw new IOException("You didn't flip the ByteBuffer!");
-			}
-			if (firstFreeIndex < index + length) {
-				firstFreeIndex = index + length;
-			}
-			dataFileChannel.position(index).write(data);
+	public int writeAt(long index, int length, ByteBuffer data) throws IOException {
+		if (closed) throw new IOException("Database closed!");
+		return writeAt_(index, length, data);
+	}
+
+	private int writeAt_(long index, int length, ByteBuffer data) throws IOException {
+		if (data.position() != 0) {
+			throw new IOException("You didn't flip the ByteBuffer!");
+		}
+		firstFreeIndex.updateAndGet((firstFreeIndex) -> firstFreeIndex < index + length ? index + length : firstFreeIndex);
+		try {
+			return dataFileChannel.write(data, index).get();
+		} catch (InterruptedException e) {
+			throw new IOException(e);
+		} catch (ExecutionException e) {
+			throw new IOException(e.getCause());
 		}
 	}
 
 	@Override
 	public long writeAtEnd(int length, ByteBuffer data) throws IOException {
-		synchronized (dataAccessLock) {
-			long index = firstFreeIndex;
-			firstFreeIndex += length;
-			writeAt(index, length, data);
-			return index;
-		}
+		if (closed) throw new IOException("Database closed!");
+		long index = firstFreeIndex.getAndAdd(length);
+		writeAt_(index, length, data);
+		return index;
 	}
 
 	@Override
 	public void close() throws IOException {
-		synchronized (dataAccessLock) {
-			dataFileChannel.close();
-		}
+		if (closed) throw new IOException("Database already closed!");
+		closed = true;
+		dataFileChannel.close();
 	}
 }

src/main/java/it/cavallium/strangedb/database/IDatabase.java

@@ -4,6 +4,8 @@ import java.io.IOException;
 
 public interface IDatabase {
 
+	int DISK_BLOCK_SIZE = 4096;
+
 	void close() throws IOException;
 
 	boolean isClosed();

src/main/java/it/cavallium/strangedb/database/IFileIO.java

@@ -18,7 +18,7 @@ public interface IFileIO {
 	 * @param length length
 	 * @param data bytes
 	 */
-	void writeAt(long index, int length, ByteBuffer data) throws IOException;
+	int writeAt(long index, int length, ByteBuffer data) throws IOException;
 
 	/**
 	 * Write *length* bytes in position *index*

src/main/java/it/cavallium/strangedb/database/IReferencesIO.java

@@ -21,10 +21,11 @@ public interface IReferencesIO {
 	/**
 	 * Write some data to the reference
 	 * @param reference reference
+	 * @param cleanerId cleaner id
 	 * @param size data size
 	 * @param data bytes
 	 */
-	void writeToReference(long reference, int size, ByteBuffer data) throws IOException;
+	void writeToReference(long reference, byte cleanerId, int size, ByteBuffer data) throws IOException;
 
 	/**
 	 * Read data from the reference

src/main/java/it/cavallium/strangedb/database/IReferencesMetadata.java

@@ -1,5 +1,7 @@
 package it.cavallium.strangedb.database;
 
+import it.cavallium.strangedb.database.references.ReferenceInfo;
+
 import java.io.IOException;
 
 public interface IReferencesMetadata {
@@ -8,21 +10,36 @@ public interface IReferencesMetadata {
 	 * @param reference reference
 	 * @return block id
 	 */
-	long getReference(long reference) throws IOException;
+	long getReferenceBlockId(long reference) throws IOException;
+
+	/**
+	 * Get reference info
+	 * @param reference reference
+	 * @return reference info
+	 */
+	ReferenceInfo getCleanReference(long reference) throws IOException;
 
 	/**
 	 * Allocate a block for a new reference
+	 *
 	 * @param blockId block id
 	 * @return reference
 	 */
 	long newReference(long blockId) throws IOException;
 
 	/**
-	 * Change reference size
+	 * Change reference
 	 * @param reference reference
+	 * @param cleanerId cleaner id
 	 * @param blockId block id
 	 */
-	void editReference(long reference, long blockId) throws IOException;
+	void editReference(long reference, byte cleanerId, long blockId) throws IOException;
+
+	/**
+	 * Delete reference
+	 * @param reference reference
+	 */
+	void deleteReference(long reference) throws IOException;
 
 	/**
 	 * Close file

src/main/java/it/cavallium/strangedb/database/blocks/DatabaseBlocksIO.java

@@ -6,6 +6,11 @@ import it.cavallium.strangedb.database.IBlocksMetadata;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.util.concurrent.CompletionException;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 import static it.cavallium.strangedb.database.IBlocksMetadata.EMPTY_BLOCK_ID;
 

src/main/java/it/cavallium/strangedb/database/blocks/DatabaseBlocksMetadata.java

@@ -4,6 +4,7 @@ package it.cavallium.strangedb.database.blocks;
 import it.cavallium.strangedb.database.IBlocksMetadata;
 
 import java.io.IOException;
+import java.nio.BufferUnderflowException;
 import java.nio.ByteBuffer;
 import java.nio.channels.AsynchronousFileChannel;
 import java.nio.file.Path;
@@ -11,11 +12,14 @@ import java.nio.file.StandardOpenOption;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
 
+import static it.cavallium.strangedb.database.IDatabase.DISK_BLOCK_SIZE;
+
 public class DatabaseBlocksMetadata implements IBlocksMetadata {
 	public static final BlockInfo ERROR_BLOCK_INFO = new BlockInfo(-2, 0);
+	private static final int BLOCK_META_BYTES_COUNT = Long.BYTES + Integer.BYTES;
+	public static final int BLOCK_META_READS_AT_EVERY_READ = (DISK_BLOCK_SIZE - DISK_BLOCK_SIZE % BLOCK_META_BYTES_COUNT) / BLOCK_META_BYTES_COUNT;
 
 	private final AsynchronousFileChannel metaFileChannel;
-	private final int BLOCK_META_BYTES_COUNT = Long.BYTES + Integer.BYTES;
 	private final DatabaseBlocksMetadataCache cache;
 	private long firstFreeBlock;
 
@@ -34,19 +38,48 @@ public class DatabaseBlocksMetadata implements IBlocksMetadata {
 		if ((blockInfo = cache.get(blockId)) != ERROR_BLOCK_INFO) {
 			return blockInfo;
 		}
-		ByteBuffer buffer = ByteBuffer.allocate(BLOCK_META_BYTES_COUNT);
+		long position = blockId * BLOCK_META_BYTES_COUNT;
+		int size = BLOCK_META_READS_AT_EVERY_READ * BLOCK_META_BYTES_COUNT;
+		if (blockId + (size - 1) / BLOCK_META_BYTES_COUNT >= firstFreeBlock) {
+			size = (int) ((firstFreeBlock - blockId) * BLOCK_META_BYTES_COUNT);
+		}
+		int blocksCount = size / BLOCK_META_BYTES_COUNT;
+
+		ByteBuffer buffer = ByteBuffer.allocate(size);
 		try {
-			metaFileChannel.read(buffer, blockId * BLOCK_META_BYTES_COUNT).get();
+			metaFileChannel.read(buffer, position).get();
 		} catch (InterruptedException e) {
 			throw new IOException(e);
 		} catch (ExecutionException e) {
 			throw new IOException(e.getCause());
 		}
 		buffer.flip();
-		long index = buffer.getLong();
-		int size = buffer.getInt();
-		blockInfo = new BlockInfo(index, size);
-		cache.put(blockId, blockInfo);
+
+		if (blocksCount < 1) {
+			throw new IOException("Trying to read <1 blocks");
+		}
+		if (buffer.limit() % BLOCK_META_BYTES_COUNT != 0 || buffer.limit() < BLOCK_META_BYTES_COUNT) {
+			throw new IOException("The buffer is smaller than the data requested.");
+		} else if (buffer.limit() != size) {
+			size = buffer.limit();
+			blocksCount = size / BLOCK_META_BYTES_COUNT;
+		}
+
+		long[] allBlockIds = new long[blocksCount];
+		BlockInfo[] allBlockInfo = new BlockInfo[blocksCount];
+
+		for (int delta = 0; delta < blocksCount; delta++) {
+			long blockToLoad = blockId + delta;
+			long blockIndex = buffer.getLong();
+			int blockSize = buffer.getInt();
+			BlockInfo currentBlockInfo = new BlockInfo(blockIndex, blockSize);
+			allBlockIds[delta] = blockToLoad;
+			allBlockInfo[delta] = currentBlockInfo;
+			if (blockToLoad == blockId) {
+				blockInfo = currentBlockInfo;
+			}
+		}
+		cache.putAll(allBlockIds, allBlockInfo);
 		return blockInfo;
 	}
 

src/main/java/it/cavallium/strangedb/database/blocks/DatabaseBlocksMetadataCache.java

@@ -1,25 +1,31 @@
 package it.cavallium.strangedb.database.blocks;
 
-import it.unimi.dsi.fastutil.longs.Long2ObjectLinkedOpenHashMap;
-import it.unimi.dsi.fastutil.longs.Long2ObjectMap;
+import it.unimi.dsi.fastutil.longs.*;
 import it.unimi.dsi.fastutil.objects.ObjectIterator;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.LinkedList;
 import java.util.List;
-import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Future;
+import java.util.concurrent.*;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+import static it.cavallium.strangedb.database.blocks.DatabaseBlocksMetadata.BLOCK_META_READS_AT_EVERY_READ;
 
 public class DatabaseBlocksMetadataCache {
 
-	private static final int GOOD_CACHE_SIZE = 70000;
-	private static final int MAX_CACHE_SIZE = 100000;
+	private static final int BASE_QUANTITY = (BLOCK_META_READS_AT_EVERY_READ < 500 ? BLOCK_META_READS_AT_EVERY_READ : 500);
+	private static final int GOOD_CACHE_SIZE = 140 * BASE_QUANTITY;
+	private static final int FLUSH_CACHE_SIZE = 300 * BLOCK_META_READS_AT_EVERY_READ;
+	private static final int MAX_CACHE_SIZE = 400 * BLOCK_META_READS_AT_EVERY_READ;
 
-	private final Long2ObjectMap<BlockInfo> blocks2Info = new Long2ObjectLinkedOpenHashMap<>(MAX_CACHE_SIZE);
-	private final Object readAccessLock = new Object();
-	private final Object writeAccessLock = new Object();
+	private final Long2ObjectMap<BlockInfo> blocks2Info = new Long2ObjectLinkedOpenHashMap<>(MAX_CACHE_SIZE, 0.5f);
+
+	private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(false);
 	private final DatabaseBlocksMetadataCacheFlusher flusher;
 	private volatile boolean closed;
+	private ExecutorService flushService = Executors.newSingleThreadExecutor();
+	private volatile boolean flushing;
 
 	public DatabaseBlocksMetadataCache(DatabaseBlocksMetadataCacheFlusher flusher) {
 		this.flusher = flusher;
@@ -27,75 +33,132 @@ public class DatabaseBlocksMetadataCache {
 
 	public BlockInfo get(long block) throws IOException {
 		if (closed) throw new IOException("Cache already closed!");
-		synchronized (readAccessLock) {
+		lock.readLock().lock();
+		try {
 			return blocks2Info.getOrDefault(block, DatabaseBlocksMetadata.ERROR_BLOCK_INFO);
+		} finally {
+			lock.readLock().unlock();
 		}
 	}
 
-	public void put(long block, BlockInfo blockInfo) throws IOException {
+	public void put(long block, BlockInfo blockInfo) {
 		if (closed) return;
-		synchronized (readAccessLock) {
-			synchronized (writeAccessLock) {
-				blocks2Info.put(block, blockInfo);
-			}
+		lock.writeLock().lock();
+		try {
+			blocks2Info.put(block, blockInfo);
+		} finally {
+			lock.writeLock().unlock();
+			flushAsync();
+		}
+	}
+
+	@SuppressWarnings("unchecked")
+	public void putAll(long[] blocks, BlockInfo[] blockInfos) {
+		if (closed) return;
+		lock.writeLock().lock();
+		try {
+			Long2ObjectMap blocksInfosToAdd = new Long2ObjectLinkedOpenHashMap<>(blocks, blockInfos, 0.5f);
+			blocks2Info.putAll(blocksInfosToAdd);
+		} finally {
+			lock.writeLock().unlock();
+			flushAsync();
+		}
+	}
+
+	private void flushAsync() {
+		if (blocks2Info.size() >= FLUSH_CACHE_SIZE && !flushing) {
+			flushing = true;
+			flushService.execute(() -> {
+				try {
+					flush();
+				} catch (IOException e) {
+					throw new RejectedExecutionException(e);
+				}
+			});
 		}
-		this.flush();
 	}
 
 	private void flush() throws IOException {
-		if (closed) return;
+		if (closed) {
+			flushing = false;
+			return;
+		}
 		int blocks2InfoSize = blocks2Info.size();
-		if (blocks2InfoSize > MAX_CACHE_SIZE) {
-			synchronized (writeAccessLock) {
+		if (blocks2InfoSize >= FLUSH_CACHE_SIZE) {
+			lock.writeLock().lock();
+			try {
 				ObjectIterator<Long2ObjectMap.Entry<BlockInfo>> entriesIterator = blocks2Info.long2ObjectEntrySet().iterator();
-				List<Future<?>> entriesToFlush = new LinkedList<>();
-				while (blocks2InfoSize > GOOD_CACHE_SIZE) {
+
+				List<Future<?>> entriesToFlush = new ArrayList<>();
+				while (blocks2InfoSize >= GOOD_CACHE_SIZE) {
 					Long2ObjectMap.Entry<BlockInfo> entry = entriesIterator.next();
 					BlockInfo blockInfo = entry.getValue();
 					entriesToFlush.add(flusher.flush(entry.getLongKey(), blockInfo.getIndex(), blockInfo.getSize()));
 					entriesIterator.remove();
+
 					if (entriesToFlush.size() >= 1000) {
-						executeAsyncFlush(entriesToFlush);
+						awaitFlushWriteEnd(entriesToFlush);
 					}
 					blocks2InfoSize--;
 				}
-				executeAsyncFlush(entriesToFlush);
+				awaitFlushWriteEnd(entriesToFlush);
+			} finally {
+				flushing = false;
+				lock.writeLock().unlock();
 			}
+		} else {
+			flushing = false;
 		}
 	}
 
 	public void close() throws IOException {
-		synchronized (readAccessLock) {
-			synchronized (writeAccessLock) {
-				if (!closed) {
-					closed = true;
-					int blocks2InfoSize = blocks2Info.size();
-					ObjectIterator<Long2ObjectMap.Entry<BlockInfo>> entriesIterator = blocks2Info.long2ObjectEntrySet().iterator();
-					List<Future<?>> entriesToFlush = new LinkedList<>();
-					while (blocks2InfoSize > 0) {
-						Long2ObjectMap.Entry<BlockInfo> entry = entriesIterator.next();
-						BlockInfo blockInfo = entry.getValue();
-						entriesToFlush.add(flusher.flush(entry.getLongKey(), blockInfo.getIndex(), blockInfo.getSize()));
-						entriesIterator.remove();
-						if (entriesToFlush.size() >= 1000) {
-							executeAsyncFlush(entriesToFlush);
-						}
-						blocks2InfoSize--;
+		if (!closed) {
+			closed = true;
+			stopFlushService();
+			lock.writeLock().lock();
+			try {
+				int blocks2InfoSize = blocks2Info.size();
+				ObjectIterator<Long2ObjectMap.Entry<BlockInfo>> entriesIterator = blocks2Info.long2ObjectEntrySet().iterator();
+
+				List<Future<?>> entriesToFlush = new LinkedList<>();
+				while (blocks2InfoSize > 0) {
+					Long2ObjectMap.Entry<BlockInfo> entry = entriesIterator.next();
+					BlockInfo blockInfo = entry.getValue();
+					entriesToFlush.add(flusher.flush(entry.getLongKey(), blockInfo.getIndex(), blockInfo.getSize()));
+					entriesIterator.remove();
+
+					if (entriesToFlush.size() >= 1000) {
+						awaitFlushWriteEnd(entriesToFlush);
 					}
-					executeAsyncFlush(entriesToFlush);
+					blocks2InfoSize--;
 				}
+				awaitFlushWriteEnd(entriesToFlush);
+			} finally {
+				lock.writeLock().unlock();
 			}
 		}
 	}
 
-	private void executeAsyncFlush(List<Future<?>> entriesToFlush) throws IOException {
+	private void stopFlushService() {
+		flushService.shutdown();
+		try { flushService.awaitTermination(180, TimeUnit.SECONDS); } catch (InterruptedException e) {}
+		if (!flushService.isTerminated()) {
+			flushService.shutdownNow();
+		}
+	}
+
+	private void awaitFlushWriteEnd(List<Future<?>> entriesToFlush) throws IOException {
 		try {
-			for (Future<?> entryToFlush : entriesToFlush) {
-				entryToFlush.get();
-			}
-		} catch (InterruptedException e) {
-			throw new IOException(e);
-		} catch (ExecutionException e) {
+			entriesToFlush.parallelStream().forEach((entry) -> {
+				try {
+					entry.get();
+				} catch (InterruptedException e) {
+					throw new CompletionException(e);
+				} catch (ExecutionException e) {
+					throw new CompletionException(e.getCause());
+				}
+			});
+		} catch (CompletionException e) {
 			throw new IOException(e.getCause());
 		} finally {
 			entriesToFlush.clear();

src/main/java/it/cavallium/strangedb/database/references/DatabaseReferencesIO.java

@@ -5,6 +5,8 @@ import it.cavallium.strangedb.database.blocks.DatabaseBlocksIO;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 import static it.cavallium.strangedb.database.IBlocksMetadata.EMPTY_BLOCK_ID;
 
@@ -12,6 +14,7 @@ public class DatabaseReferencesIO implements IReferencesIO {
 
 	private final DatabaseBlocksIO blocksIO;
 	private final DatabaseReferencesMetadata referencesMetadata;
+	private ReentrantReadWriteLock lock = new ReentrantReadWriteLock(false);
 
 	public DatabaseReferencesIO(DatabaseBlocksIO blocksIO, DatabaseReferencesMetadata referencesMetadata) {
 		this.blocksIO = blocksIO;
@@ -30,14 +33,24 @@ public class DatabaseReferencesIO implements IReferencesIO {
 	}
 
 	@Override
-	public void writeToReference(long reference, int size, ByteBuffer data) throws IOException {
-		long blockId = (size == 0) ? EMPTY_BLOCK_ID : blocksIO.newBlock(size, data);
-		referencesMetadata.editReference(reference, blockId);
+	public void writeToReference(long reference, byte cleanerId, int size, ByteBuffer data) throws IOException {
+		lock.writeLock().lock();
+		try {
+			long blockId = (size == 0) ? EMPTY_BLOCK_ID : blocksIO.newBlock(size, data);
+			referencesMetadata.editReference(reference, cleanerId, blockId);
+		} finally {
+			lock.writeLock().unlock();
+		}
 	}
 
 	@Override
 	public ByteBuffer readFromReference(long reference) throws IOException {
-		long blockId = referencesMetadata.getReference(reference);
-		return blocksIO.readBlock(blockId);
+		lock.readLock().lock();
+		try {
+			long blockId = referencesMetadata.getReferenceBlockId(reference);
+			return blocksIO.readBlock(blockId);
+		} finally {
+			lock.readLock().unlock();
+		}
 	}
 }

src/main/java/it/cavallium/strangedb/database/references/DatabaseReferencesMetadata.java

@@ -7,79 +7,203 @@ import java.nio.ByteBuffer;
 import java.nio.channels.AsynchronousFileChannel;
 import java.nio.file.Path;
 import java.nio.file.StandardOpenOption;
+import java.util.Arrays;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 import static it.cavallium.strangedb.database.IBlocksMetadata.EMPTY_BLOCK_ID;
 import static it.cavallium.strangedb.database.IBlocksMetadata.ERROR_BLOCK_ID;
+import static it.cavallium.strangedb.database.IDatabase.DISK_BLOCK_SIZE;
 
 public class DatabaseReferencesMetadata implements IReferencesMetadata {
+	public static final byte ERRORED_CLEANER = (byte) -1;
+	public static final byte BLANK_DATA_CLEANER = (byte) -2;
+	public static final ReferenceInfo NONEXISTENT_REFERENCE_INFO = new ReferenceInfo(ERRORED_CLEANER, ERROR_BLOCK_ID);
+	private static final int REF_META_BYTES_COUNT = Long.BYTES + Integer.BYTES;
+	public static final int REF_META_READS_AT_EVERY_READ = (DISK_BLOCK_SIZE - DISK_BLOCK_SIZE % REF_META_BYTES_COUNT) / REF_META_BYTES_COUNT;
+
 	private final AsynchronousFileChannel metaFileChannel;
-	private final int REF_META_BYTES_COUNT = Long.BYTES;
 	private final DatabaseReferencesMetadataCache cache;
 	private long firstFreeReference;
+	private long lastWrittenReference;
+	private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(false);
 
 	public DatabaseReferencesMetadata(Path refMetaFile) throws IOException {
 		metaFileChannel = AsynchronousFileChannel.open(refMetaFile, StandardOpenOption.READ, StandardOpenOption.WRITE);
 		firstFreeReference = metaFileChannel.size() / REF_META_BYTES_COUNT;
+		lastWrittenReference = firstFreeReference - 1;
 		this.cache = new DatabaseReferencesMetadataCache(this::writeReferenceToDisk);
 	}
 
 	@Override
-	public long getReference(long reference) throws IOException {
+	public long getReferenceBlockId(long reference) throws IOException {
+		lock.readLock().lock();
+		try {
+			return getReferenceBlockId_(reference);
+		} finally {
+			lock.readLock().unlock();
+		}
+	}
+
+	private long getReferenceBlockId_(long reference) throws IOException {
 		if (reference >= firstFreeReference) {
 			return EMPTY_BLOCK_ID;
 		}
 		long block;
-		if ((block = cache.get(reference)) != ERROR_BLOCK_ID) {
+		if ((block = cache.getBlock(reference)) != ERROR_BLOCK_ID) {
 			return block;
 		}
-		ByteBuffer buffer = ByteBuffer.allocate(REF_META_BYTES_COUNT);
+		long position = reference * REF_META_BYTES_COUNT;
+		int size = REF_META_READS_AT_EVERY_READ * REF_META_BYTES_COUNT;
+		if (reference + (size - 1) / REF_META_BYTES_COUNT >= firstFreeReference) {
+			size = (int) ((firstFreeReference - reference) * REF_META_BYTES_COUNT);
+		}
+		int referencesCount = size / REF_META_BYTES_COUNT;
+
+		ByteBuffer buffer = ByteBuffer.allocate(size);
 		try {
-			metaFileChannel.read(buffer, reference * REF_META_BYTES_COUNT).get();
+			metaFileChannel.read(buffer, position).get();
 		} catch (InterruptedException e) {
 			throw new IOException(e);
 		} catch (ExecutionException e) {
 			throw new IOException(e.getCause());
 		}
 		buffer.flip();
-		block = buffer.getLong();
-		if (buffer.limit() != 0 && block != 0xFFFFFFFFFFFFFFFFL) {
-			cache.put(reference, block);
-			return block;
-		} else {
-			cache.put(reference, EMPTY_BLOCK_ID);
-			return EMPTY_BLOCK_ID;
+
+		if (referencesCount < 1) {
+			throw new IOException("Trying to read <1 references");
+		}
+		if (buffer.limit() % REF_META_BYTES_COUNT != 0 || buffer.limit() < REF_META_BYTES_COUNT) {
+			throw new IOException("The buffer is smaller than the data requested.");
+		} else if (buffer.limit() != size) {
+			size = buffer.limit();
+			referencesCount = size / REF_META_BYTES_COUNT;
+		}
+
+		long[] allReferences = new long[referencesCount];
+		byte[] allCleaners = new byte[referencesCount];
+		long[] allBlocks = new long[referencesCount];
+
+		block = EMPTY_BLOCK_ID;
+		for (int delta = 0; delta < referencesCount; delta++) {
+			long referenceToLoad = reference + delta;
+			long currentBlock = buffer.getLong();
+			byte cleanerId = (byte) buffer.getInt();
+			if (buffer.limit() != 0 && currentBlock != 0xFFFFFFFFFFFFFFFFL) {
+				allReferences[delta] = referenceToLoad;
+				allCleaners[delta] = cleanerId;
+				allBlocks[delta] = currentBlock;
+				if (referenceToLoad == reference) {
+					block = currentBlock;
+				}
+			} else {
+				allReferences[delta] = referenceToLoad;
+				allCleaners[delta] = cleanerId;
+				allBlocks[delta] = EMPTY_BLOCK_ID;
+				if (referenceToLoad == reference) {
+					block = EMPTY_BLOCK_ID;
+				}
+			}
+		}
+
+		for (int delta = 0; delta < referencesCount; delta++) {
+			if (allCleaners[delta] == 0) {
+				System.out.println("ro");
+			}
+		}
+		cache.putAll(allReferences, allCleaners, allBlocks);
+		return block;
+	}
+
+	/**
+	 * This method is <b>SLOW</b>! Use this only for the cleaner
+	 * @param reference reference
+	 * @return
+	 * @throws IOException
+	 */
+	@Deprecated
+	@Override
+	public ReferenceInfo getCleanReference(long reference) throws IOException {
+		lock.readLock().lock();
+		try {
+			getReferenceBlockId_(reference);
+			return cache.get(reference);
+		} finally {
+			lock.readLock().unlock();
 		}
 	}
 
 	@Override
 	public long newReference(long blockId) throws IOException {
-		long newReference = firstFreeReference++;
-		cache.put(newReference, blockId);
-		return newReference;
+		lock.writeLock().lock();
+		try {
+			long newReference = firstFreeReference++;
+			cache.put(newReference, BLANK_DATA_CLEANER, blockId);
+			return newReference;
+		} finally {
+			lock.writeLock().unlock();
+		}
 	}
 
 	@Override
-	public void editReference(long reference, long blockId) throws IOException {
-		cache.put(reference, blockId);
+	public void editReference(long reference, byte cleanerId, long blockId) throws IOException {
+		lock.writeLock().lock();
+		try {
+			cache.put(reference, cleanerId, blockId);
+		} finally {
+			lock.writeLock().unlock();
+		}
+	}
+
+	@Override
+	public void deleteReference(long reference) throws IOException {
+		lock.writeLock().lock();
+		try {
+			cache.put(reference, NONEXISTENT_REFERENCE_INFO.getCleanerId(), NONEXISTENT_REFERENCE_INFO.getBlockId());
+		} finally {
+			lock.writeLock().unlock();
+		}
 	}
 
 	@Override
 	public void close() throws IOException {
-		cache.close();
-		metaFileChannel.close();
+		lock.writeLock().lock();
+		try {
+			cache.close();
+			metaFileChannel.close();
+		} finally {
+			lock.writeLock().unlock();
+		}
 	}
 
 	@Override
 	public long getFirstFreeReference() {
-		return firstFreeReference;
+		synchronized (lock.readLock()) {
+			return firstFreeReference;
+		}
 	}
 
-	private Future<Integer> writeReferenceToDisk(long reference, long blockId) {
+	private Future<Integer> writeReferenceToDisk(long reference, byte cleanerId, long blockId) {
+		if (cleanerId == ERRORED_CLEANER) {
+			return CompletableFuture.failedFuture(new IOException("Passing a cleaner with the id of ERRORED_CLIENT"));
+		}
 		ByteBuffer data = ByteBuffer.allocate(REF_META_BYTES_COUNT);
 		data.putLong(blockId);
+		data.putInt(cleanerId & 0xFF);
 		data.flip();
+		while (lastWrittenReference < reference - 1) {
+			ByteBuffer emptyData = ByteBuffer.allocate(REF_META_BYTES_COUNT);
+			emptyData.putLong(ERROR_BLOCK_ID);
+			emptyData.putInt(ERRORED_CLEANER & 0xFF);
+			emptyData.flip();
+			metaFileChannel.write(emptyData, ++lastWrittenReference * REF_META_BYTES_COUNT);
+		}
+		if (reference > lastWrittenReference) {
+			lastWrittenReference = reference;
+		}
 		return metaFileChannel.write(data, reference * REF_META_BYTES_COUNT);
 	}
 

src/main/java/it/cavallium/strangedb/database/references/DatabaseReferencesMetadataCache.java

@@ -4,108 +4,192 @@ import it.unimi.dsi.fastutil.longs.*;
 import it.unimi.dsi.fastutil.objects.ObjectIterator;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.LinkedList;
 import java.util.List;
-import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Future;
+import java.util.concurrent.*;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 import static it.cavallium.strangedb.database.IBlocksMetadata.ERROR_BLOCK_ID;
+import static it.cavallium.strangedb.database.blocks.DatabaseBlocksMetadata.BLOCK_META_READS_AT_EVERY_READ;
+import static it.cavallium.strangedb.database.references.DatabaseReferencesMetadata.*;
 
 public class DatabaseReferencesMetadataCache {
 
-	private static final int GOOD_CACHE_SIZE = 70000;
-	private static final int MAX_CACHE_SIZE = 100000;
+	private static final int BASE_QUANTITY = (REF_META_READS_AT_EVERY_READ < 500 ? REF_META_READS_AT_EVERY_READ : 500);
+	private static final int GOOD_CACHE_SIZE = 140 * BASE_QUANTITY;
+	private static final int FLUSH_CACHE_SIZE = 300 * BLOCK_META_READS_AT_EVERY_READ;
+	private static final int MAX_CACHE_SIZE = 400 * BASE_QUANTITY;
 
-	private final Long2LongMap references2Blocks = new Long2LongLinkedOpenHashMap(MAX_CACHE_SIZE);
-	private final Object readAccessLock = new Object();
-	private final Object writeAccessLock = new Object();
+	private final Long2LongMap references2Blocks = new Long2LongLinkedOpenHashMap(MAX_CACHE_SIZE, 0.5f);
+	private final Long2ByteMap referencesCleaners = new Long2ByteLinkedOpenHashMap(MAX_CACHE_SIZE, 0.5f);
+	private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(false);
 	private final DatabaseReferencesMetadataCacheFlusher flusher;
 	private volatile boolean closed;
+	private ExecutorService flushService = Executors.newSingleThreadExecutor();
+	private volatile boolean flushing;
 
 	public DatabaseReferencesMetadataCache(DatabaseReferencesMetadataCacheFlusher flusher) {
 		this.flusher = flusher;
 	}
 
-	public long get(long reference) throws IOException {
-		synchronized (readAccessLock) {
-			if (closed) throw new IOException("Cache already closed!");
+	public long getBlock(long reference) throws IOException {
+		if (closed) throw new IOException("Cache already closed!");
+		lock.readLock().lock();
+		try {
 			return references2Blocks.getOrDefault(reference, ERROR_BLOCK_ID);
+		} finally {
+			lock.readLock().unlock();
 		}
 	}
 
-	public void put(long reference, long blockId) throws IOException {
-		synchronized (readAccessLock) {
-			synchronized (writeAccessLock) {
-				if (closed) return;
-				references2Blocks.put(reference, blockId);
+	public ReferenceInfo get(long reference) throws IOException {
+		if (closed) throw new IOException("Cache already closed!");
+		lock.readLock().lock();
+		try {
+			long blockId = references2Blocks.getOrDefault(reference, ERROR_BLOCK_ID);
+			if (blockId == ERROR_BLOCK_ID) {
+				return NONEXISTENT_REFERENCE_INFO;
 			}
+			byte cleanerId = referencesCleaners.get(reference);
+			return new ReferenceInfo(cleanerId, blockId);
+		} finally {
+			lock.readLock().unlock();
+		}
+	}
+
+	public void put(long reference, byte cleanerId, long blockId) throws IOException {
+		if (closed) throw new IOException("Cache already closed!");
+		lock.writeLock().lock();
+		try {
+			if (cleanerId == 0) {
+				throw new IOException("Null cleaner id");
+			}
+			references2Blocks.put(reference, blockId);
+			referencesCleaners.put(reference, cleanerId);
+		} finally {
+			lock.writeLock().unlock();
+			flushAsync();
+		}
+	}
+
+	public void putAll(long[] references, byte[] cleanerIds, long[] blockIds) throws IOException {
+		if (closed) throw new IOException("Cache already closed!");
+		lock.writeLock().lock();
+		try {
+			for (int i = 0; i < references.length; i++) {
+				if (cleanerIds[i] == 0) {
+					throw new IOException("Null cleaner id");
+				}
+			}
+			Long2LongMap referencesBlocksToAdd = new Long2LongLinkedOpenHashMap(references, blockIds, 0.5f);
+			Long2ByteMap referencesCleanersToAdd = new Long2ByteLinkedOpenHashMap(references, cleanerIds, 0.5f);
+			references2Blocks.putAll(referencesBlocksToAdd);
+			referencesCleaners.putAll(referencesCleanersToAdd);
+		} finally {
+			lock.writeLock().unlock();
+			flushAsync();
+		}
+	}
+
+	private void flushAsync() {
+		if (references2Blocks.size() >= FLUSH_CACHE_SIZE && !flushing) {
+			flushing = true;
+			flushService.execute(() -> {
+				try {
+					flush();
+				} catch (IOException e) {
+					throw new RejectedExecutionException(e);
+				}
+			});
 		}
-		this.flush();
 	}
 
 	private void flush() throws IOException {
-		synchronized (readAccessLock) {
-			synchronized (writeAccessLock) {
-				if (closed) return;
-				int references2BlocksSize = references2Blocks.size();
-				if (references2BlocksSize > MAX_CACHE_SIZE) {
-					synchronized (writeAccessLock) {
-						ObjectIterator<Long2LongMap.Entry> entriesIterator = references2Blocks.long2LongEntrySet().iterator();
-						List<Future<?>> entriesToFlush = new LinkedList<>();
-						while (references2BlocksSize > GOOD_CACHE_SIZE) {
-							Long2LongMap.Entry entry = entriesIterator.next();
-							entriesToFlush.add(flusher.flush(entry.getLongKey(), entry.getLongValue()));
-							entriesIterator.remove();
-							if (entriesToFlush.size() >= 1000) {
-								executeAsyncFlush(entriesToFlush);
-							}
-							references2BlocksSize--;
-						}
-						executeAsyncFlush(entriesToFlush);
+		if (closed) {
+			flushing = false;
+			return;
+		}
+		int references2BlocksSize = references2Blocks.size();
+		if (references2BlocksSize >= FLUSH_CACHE_SIZE) {
+			lock.writeLock().lock();
+			try {
+				ObjectIterator<Long2LongMap.Entry> entriesIterator = references2Blocks.long2LongEntrySet().iterator();
+				ObjectIterator<Long2ByteMap.Entry> cleanersIterator = referencesCleaners.long2ByteEntrySet().iterator();
+				List<Future<?>> entriesToFlush = new ArrayList<>();
+				while (references2BlocksSize >= GOOD_CACHE_SIZE) {
+					Long2LongMap.Entry entry = entriesIterator.next();
+					Long2ByteMap.Entry cleaner = cleanersIterator.next();
+					entriesToFlush.add(flusher.flush(entry.getLongKey(), cleaner.getByteValue(), entry.getLongValue()));
+					entriesIterator.remove();
+					cleanersIterator.remove();
+					if (entriesToFlush.size() >= 1000) {
+						awaitFlushWriteEnd(entriesToFlush);
 					}
+					references2BlocksSize--;
 				}
+				awaitFlushWriteEnd(entriesToFlush);
+			} finally {
+				flushing = false;
+				lock.writeLock().unlock();
 			}
+		} else {
+			flushing = false;
 		}
 	}
 
 	public void close() throws IOException {
-		synchronized (readAccessLock) {
-			synchronized (writeAccessLock) {
-				if (!closed) {
-					closed = true;
-					int references2BlocksSize = references2Blocks.size();
-					ObjectIterator<Long2LongMap.Entry> entriesIterator = references2Blocks.long2LongEntrySet().iterator();
-					List<Future<?>> entriesToFlush = new LinkedList<>();
-					while (references2BlocksSize > 0) {
-						Long2LongMap.Entry entry = entriesIterator.next();
-						entriesToFlush.add(flusher.flush(entry.getLongKey(), entry.getLongValue()));
-						entriesIterator.remove();
-						if (entriesToFlush.size() >= 1000) {
-							executeAsyncFlush(entriesToFlush);
-						}
-						references2BlocksSize--;
+		if (!closed) {
+			closed = true;
+			stopFlushService();
+			lock.writeLock().lock();
+			try {
+				int references2BlocksSize = references2Blocks.size();
+				ObjectIterator<Long2LongMap.Entry> entriesIterator = references2Blocks.long2LongEntrySet().iterator();
+				ObjectIterator<Long2ByteMap.Entry> cleanersIterator = referencesCleaners.long2ByteEntrySet().iterator();
+				List<Future<?>> entriesToFlush = new LinkedList<>();
+				while (references2BlocksSize > 0) {
+					Long2LongMap.Entry entry = entriesIterator.next();
+					Long2ByteMap.Entry cleaner = cleanersIterator.next();
+					entriesToFlush.add(flusher.flush(entry.getLongKey(), cleaner.getByteValue(), entry.getLongValue()));
+					entriesIterator.remove();
+					cleanersIterator.remove();
+
+					if (entriesToFlush.size() >= 1000) {
+						awaitFlushWriteEnd(entriesToFlush);
 					}
-					executeAsyncFlush(entriesToFlush);
+					references2BlocksSize--;
 				}
+				awaitFlushWriteEnd(entriesToFlush);
+			} finally {
+				lock.writeLock().unlock();
 			}
 		}
 	}
 
-	private void executeAsyncFlush(List<Future<?>> entriesToFlush) throws IOException {
-		synchronized (readAccessLock) {
-			synchronized (writeAccessLock) {
+	private void stopFlushService() {
+		flushService.shutdown();
+		try { flushService.awaitTermination(30, TimeUnit.SECONDS); } catch (InterruptedException e) {}
+		if (!flushService.isTerminated()) {
+			flushService.shutdownNow();
+		}
+	}
+
+	private void awaitFlushWriteEnd(List<Future<?>> entriesToFlush) throws IOException {
+		try {
+			entriesToFlush.parallelStream().forEach((entry) -> {
 				try {
-					for (Future<?> entryToFlush : entriesToFlush) {
-						entryToFlush.get();
-					}
+					entry.get();
 				} catch (InterruptedException e) {
-					throw new IOException(e);
+					throw new CompletionException(e);
 				} catch (ExecutionException e) {
-					throw new IOException(e.getCause());
-				} finally {
-					entriesToFlush.clear();
+					throw new CompletionException(e.getCause());
 				}
-			}
+			});
+		} catch (CompletionException e) {
+			throw new IOException(e.getCause());
+		} finally {
+			entriesToFlush.clear();
 		}
 	}
 }

src/main/java/it/cavallium/strangedb/database/references/DatabaseReferencesMetadataCacheFlusher.java

@@ -4,5 +4,5 @@ import java.io.IOException;
 import java.util.concurrent.Future;
 
 public interface DatabaseReferencesMetadataCacheFlusher {
-	Future<Integer> flush(long key, long value) throws IOException;
+	Future<Integer> flush(long reference, byte cleanerId, long block) throws IOException;
 }

src/main/java/it/cavallium/strangedb/database/references/ReferenceInfo.java

@@ -0,0 +1,44 @@
+package it.cavallium.strangedb.database.references;
+
+import java.util.Objects;
+import java.util.StringJoiner;
+
+public class ReferenceInfo {
+	private final byte cleanerId;
+	private final long blockId;
+
+	public ReferenceInfo(byte cleanerId, long blockId) {
+		this.cleanerId = cleanerId;
+		this.blockId = blockId;
+	}
+
+	public byte getCleanerId() {
+		return cleanerId;
+	}
+
+	public long getBlockId() {
+		return blockId;
+	}
+
+	@Override
+	public String toString() {
+		return new StringJoiner(", ", ReferenceInfo.class.getSimpleName() + "[", "]")
+						.add("cleanerId=" + cleanerId)
+						.add("blockId=" + blockId)
+						.toString();
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o) return true;
+		if (o == null || getClass() != o.getClass()) return false;
+		ReferenceInfo that = (ReferenceInfo) o;
+		return cleanerId == that.cleanerId &&
+						blockId == that.blockId;
+	}
+
+	@Override
+	public int hashCode() {
+		return Objects.hash(cleanerId, blockId);
+	}
+}