CavalliumDBEngine/src/main/java/it/cavallium/dbengine/database/disk/LLLocalDictionary.java

package it.cavallium.dbengine.database.disk;

import it.cavallium.dbengine.database.BoundedGroupedRocksFluxIterable;
import it.cavallium.dbengine.database.BoundedRocksFluxIterable;
import it.cavallium.dbengine.database.LLDictionary;
import it.cavallium.dbengine.database.LLDictionaryResultType;
import it.cavallium.dbengine.database.LLRange;
import it.cavallium.dbengine.database.LLSnapshot;
import it.cavallium.dbengine.database.LLUtils;
import it.cavallium.dbengine.database.UpdateMode;
import it.unimi.dsi.fastutil.ints.IntArrayList;
import it.unimi.dsi.fastutil.objects.ObjectArrayList;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.locks.StampedLock;
import java.util.function.Function;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import org.rocksdb.ColumnFamilyHandle;
import org.rocksdb.FlushOptions;
import org.rocksdb.Holder;
import org.rocksdb.ReadOptions;
import org.rocksdb.RocksDB;
import org.rocksdb.RocksDBException;
import org.rocksdb.RocksIterator;
import org.rocksdb.Snapshot;
import org.rocksdb.WriteOptions;
import org.warp.commonutils.concurrency.atomicity.NotAtomic;
import org.warp.commonutils.locks.Striped;
import org.warp.commonutils.log.Logger;
import org.warp.commonutils.log.LoggerFactory;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Scheduler;

@NotAtomic
public class LLLocalDictionary implements LLDictionary {

	protected static final Logger logger = LoggerFactory.getLogger(LLLocalDictionary.class);
	private static final boolean USE_CURRENT_FASTSIZE_FOR_OLD_SNAPSHOTS = true;
	static final int RESERVED_WRITE_BATCH_SIZE = 2 * 1024 * 1024; // 2MiB
	static final long MAX_WRITE_BATCH_SIZE = 1024L * 1024L * 1024L; // 1GiB
	static final int CAPPED_WRITE_BATCH_CAP = 50000; // 50K operations
	static final int MULTI_GET_WINDOW = 500;
	static final WriteOptions BATCH_WRITE_OPTIONS = new WriteOptions().setLowPri(true);

	private static final int STRIPES = 512;
	private static final byte[] FIRST_KEY = new byte[]{};
	private static final byte[] NO_DATA = new byte[0];
	private static final ReadOptions EMPTY_READ_OPTIONS = new ReadOptions();
	private final RocksDB db;
	private final ColumnFamilyHandle cfh;
	private final String databaseName;
	private final Scheduler dbScheduler;
	private final Function<LLSnapshot, Snapshot> snapshotResolver;
	private final Striped<StampedLock> itemsLock = Striped.readWriteStampedLock(STRIPES);
	private final UpdateMode updateMode;

	public LLLocalDictionary(@NotNull RocksDB db,
			@NotNull ColumnFamilyHandle columnFamilyHandle,
			String databaseName,
			Scheduler dbScheduler,
			Function<LLSnapshot, Snapshot> snapshotResolver,
			UpdateMode updateMode) {
		Objects.requireNonNull(db);
		this.db = db;
		Objects.requireNonNull(columnFamilyHandle);
		this.cfh = columnFamilyHandle;
		this.databaseName = databaseName;
		this.dbScheduler = dbScheduler;
		this.snapshotResolver = snapshotResolver;
		this.updateMode = updateMode;
	}

	@Override
	public String getDatabaseName() {
		return databaseName;
	}

	private ReadOptions resolveSnapshot(LLSnapshot snapshot) {
		if (snapshot != null) {
			return getReadOptions(snapshotResolver.apply(snapshot));
		} else {
			return EMPTY_READ_OPTIONS;
		}
	}

	private ReadOptions getReadOptions(Snapshot snapshot) {
		if (snapshot != null) {
			return new ReadOptions().setSnapshot(snapshot);
		} else {
			return EMPTY_READ_OPTIONS;
		}
	}

	private int getLockIndex(byte[] key) {
		return Math.abs(Arrays.hashCode(key) % STRIPES);
	}

	private IntArrayList getLockIndices(List<byte[]> keys) {
		var list = new IntArrayList(keys.size());
		for (byte[] key : keys) {
			list.add(getLockIndex(key));
		}
		return list;
	}

	private IntArrayList getLockIndicesEntries(List<Entry<byte[], byte[]>> keys) {
		var list = new IntArrayList(keys.size());
		for (Entry<byte[], byte[]> key : keys) {
			list.add(getLockIndex(key.getKey()));
		}
		return list;
	}

	@Override
	public Mono<byte[]> get(@Nullable LLSnapshot snapshot, byte[] key) {
		return Mono
				.fromCallable(() -> {
					StampedLock lock;
					long stamp;
					if (updateMode == UpdateMode.ALLOW) {
						lock = itemsLock.getAt(getLockIndex(key));
						
						stamp = lock.readLock();
					} else {
						lock = null;
						stamp = 0;
					}
					try {
						logger.trace("Reading {}", key);
						Holder<byte[]> data = new Holder<>();
						if (db.keyMayExist(cfh, resolveSnapshot(snapshot), key, data)) {
							if (data.getValue() != null) {
								return data.getValue();
							} else {
								return db.get(cfh, resolveSnapshot(snapshot), key);
							}
						} else {
							return null;
						}
					} finally {
						if (updateMode == UpdateMode.ALLOW) {
							lock.unlockRead(stamp);
						}
					}
				})
				.onErrorMap(IOException::new)
				.subscribeOn(dbScheduler);
	}

	@Override
	public Mono<Boolean> isRangeEmpty(@Nullable LLSnapshot snapshot, LLRange range) {
		if (range.isSingle()) {
			return containsKey(snapshot, range.getSingle()).map(contains -> !contains);
		} else {
			return containsRange(snapshot, range).map(contains -> !contains);
		}
	}

	public Mono<Boolean> containsRange(@Nullable LLSnapshot snapshot, LLRange range) {
		return Mono
				.fromCallable(() -> {
					try (RocksIterator iter = db.newIterator(cfh, resolveSnapshot(snapshot))) {
						if (range.hasMin()) {
							iter.seek(range.getMin());
						} else {
							iter.seekToFirst();
						}
						if (!iter.isValid()) {
							return false;
						}

						if (range.hasMax()) {
							byte[] key1 = iter.key();
							return Arrays.compareUnsigned(key1, range.getMax()) <= 0;
						} else {
							return true;
						}
					}
				})
				.onErrorMap(IOException::new)
				.subscribeOn(dbScheduler);
	}

	private Mono<Boolean> containsKey(@Nullable LLSnapshot snapshot, byte[] key) {
		return Mono
				.fromCallable(() -> {
					StampedLock lock;
					long stamp;
					if (updateMode == UpdateMode.ALLOW) {
						lock = itemsLock.getAt(getLockIndex(key));
						
						stamp = lock.readLock();
					} else {
						lock = null;
						stamp = 0;
					}
					try {
						int size = RocksDB.NOT_FOUND;
						Holder<byte[]> data = new Holder<>();
						if (db.keyMayExist(cfh, resolveSnapshot(snapshot), key, data)) {
							if (data.getValue() != null) {
								size = data.getValue().length;
							} else {
								size = db.get(cfh, resolveSnapshot(snapshot), key, NO_DATA);
							}
						}
						return size != RocksDB.NOT_FOUND;
					} finally {
						if (updateMode == UpdateMode.ALLOW) {
							lock.unlockRead(stamp);
						}
					}
				})
				.onErrorMap(IOException::new)
				.subscribeOn(dbScheduler);
	}

	@Override
	public Mono<byte[]> put(byte[] key, byte[] value, LLDictionaryResultType resultType) {
		return getPrevValue(key, resultType)
				.concatWith(Mono
						.fromCallable(() -> {
							StampedLock lock;
							long stamp;
							if (updateMode == UpdateMode.ALLOW) {
								lock = itemsLock.getAt(getLockIndex(key));
								
								stamp = lock.writeLock();
							} else {
								lock = null;
								stamp = 0;
							}
							try {
								logger.trace("Writing {}: {}", key, value);
								db.put(cfh, key, value);
								return null;
							} finally {
								if (updateMode == UpdateMode.ALLOW) {
									lock.unlockWrite(stamp);
								}
							}
						})
						.onErrorMap(IOException::new)
						.subscribeOn(dbScheduler)
						.then(Mono.empty())
				).singleOrEmpty();
	}

	@Override
	public Mono<Boolean> update(byte[] key, Function<Optional<byte[]>, Optional<byte[]>> value) {
		return Mono
						.fromCallable(() -> {
							if (updateMode == UpdateMode.DISALLOW) throw new UnsupportedOperationException("update() is disallowed");
							StampedLock lock;
							long stamp;
							if (updateMode == UpdateMode.ALLOW) {
								lock = itemsLock.getAt(getLockIndex(key));
								
								stamp = lock.readLock();
							} else {
								lock = null;
								stamp = 0;
							}
							try {
								logger.trace("Reading {}", key);
								while (true) {
									boolean changed = false;
									Optional<byte[]> prevData;
									var prevDataHolder = new Holder<byte[]>();
									if (db.keyMayExist(cfh, key, prevDataHolder)) {
										if (prevDataHolder.getValue() != null) {
											prevData = Optional.ofNullable(prevDataHolder.getValue());
										} else {
											prevData = Optional.ofNullable(db.get(cfh, key));
										}
									} else {
										prevData = Optional.empty();
									}

									Optional<byte[]> newData = value.apply(prevData);
									if (prevData.isPresent() && newData.isEmpty()) {
										if (updateMode == UpdateMode.ALLOW) {
											var ws = lock.tryConvertToWriteLock(stamp);
											if (ws != 0) {
												stamp = ws;
											} else {
												lock.unlockRead(stamp);

												stamp = lock.writeLock();
												continue;
											}
										}
										logger.trace("Deleting {}", key);
										changed = true;
										db.delete(cfh, key);
									} else if (newData.isPresent()
											&& (prevData.isEmpty() || !Arrays.equals(prevData.get(), newData.get()))) {
										if (updateMode == UpdateMode.ALLOW) {
											var ws = lock.tryConvertToWriteLock(stamp);
											if (ws != 0) {
												stamp = ws;
											} else {
												lock.unlockRead(stamp);

												stamp = lock.writeLock();
												continue;
											}
										}
										logger.trace("Writing {}: {}", key, newData.get());
										changed = true;
										db.put(cfh, key, newData.get());
									}
									return changed;
								}
							} finally {
								if (updateMode == UpdateMode.ALLOW) {
									lock.unlock(stamp);
								}
							}
						})
						.onErrorMap(IOException::new)
						.subscribeOn(dbScheduler);
	}

	@Override
	public Mono<byte[]> remove(byte[] key, LLDictionaryResultType resultType) {
		return getPrevValue(key, resultType)
				.concatWith(Mono
						.fromCallable(() -> {
							StampedLock lock;
							long stamp;
							if (updateMode == UpdateMode.ALLOW) {
								lock = itemsLock.getAt(getLockIndex(key));
								
								stamp = lock.writeLock();
							} else {
								lock = null;
								stamp = 0;
							}
							try {
								db.delete(cfh, key);
								return null;
							} finally {
								if (updateMode == UpdateMode.ALLOW) {
									lock.unlockWrite(stamp);
								}
							}
						})
						.onErrorMap(IOException::new)
						.subscribeOn(dbScheduler)
						.then(Mono.empty())
				).singleOrEmpty();
	}

	private Mono<byte[]> getPrevValue(byte[] key, LLDictionaryResultType resultType) {
		return Mono.defer(() -> {
			switch (resultType) {
				case VALUE_CHANGED:
					return containsKey(null, key).single().map(LLUtils::booleanToResponse);
				case PREVIOUS_VALUE:
					return Mono
							.fromCallable(() -> {
								StampedLock lock;
								long stamp;
								if (updateMode == UpdateMode.ALLOW) {
									lock = itemsLock.getAt(getLockIndex(key));
									
									stamp = lock.readLock();
								} else {
									lock = null;
									stamp = 0;
								}
								try {
									logger.trace("Reading {}", key);
									var data = new Holder<byte[]>();
									if (db.keyMayExist(cfh, key, data)) {
										if (data.getValue() != null) {
											return data.getValue();
										} else {
											return db.get(cfh, key);
										}
									} else {
										return null;
									}
								} finally {
									if (updateMode == UpdateMode.ALLOW) {
										lock.unlockRead(stamp);
									}
								}
							})
							.onErrorMap(IOException::new)
							.subscribeOn(dbScheduler);
				case VOID:
					return Mono.empty();
				default:
					return Mono.error(new IllegalStateException("Unexpected value: " + resultType));
			}
		});
	}

	@Override
	public Flux<Entry<byte[], byte[]>> getMulti(@Nullable LLSnapshot snapshot, Flux<byte[]> keys) {
		return keys
				.window(MULTI_GET_WINDOW)
				.flatMap(keysWindowFlux -> keysWindowFlux.collectList()
						.flatMapMany(keysWindow -> Mono
								.fromCallable(() -> {
									Iterable<StampedLock> locks;
									ArrayList<Long> stamps;
									if (updateMode == UpdateMode.ALLOW) {
										locks = itemsLock.bulkGetAt(getLockIndices(keysWindow));
										stamps = new ArrayList<>();
										for (var lock : locks) {
											
											stamps.add(lock.readLock());
										}
									} else {
										locks = null;
										stamps = null;
									}
									try {
										var handlesArray = new ColumnFamilyHandle[keysWindow.size()];
										Arrays.fill(handlesArray, cfh);
										var handles = ObjectArrayList.wrap(handlesArray, handlesArray.length);
										var results = db.multiGetAsList(resolveSnapshot(snapshot), handles, keysWindow);
										var mappedResults = new ArrayList<Entry<byte[], byte[]>>(results.size());
										for (int i = 0; i < results.size(); i++) {
											var val = results.get(i);
											if (val != null) {
												results.set(i, null);
												mappedResults.add(Map.entry(keysWindow.get(i), val));
											}
										}
										return mappedResults;
									} finally {
										if (updateMode == UpdateMode.ALLOW) {
											int index = 0;
											for (var lock : locks) {
												lock.unlockRead(stamps.get(index));
												index++;
											}
										}
									}
								})
								.subscribeOn(dbScheduler)
								.flatMapMany(Flux::fromIterable)
						)
				)
				.onErrorMap(IOException::new);
	}

	@Override
	public Flux<Entry<byte[], byte[]>> putMulti(Flux<Entry<byte[], byte[]>> entries, boolean getOldValues) {
		return entries
				.window(Math.min(MULTI_GET_WINDOW, CAPPED_WRITE_BATCH_CAP))
				.flatMap(Flux::collectList)
				.flatMap(entriesWindow -> this
						.getMulti(null, Flux.fromIterable(entriesWindow).map(Entry::getKey))
						.publishOn(dbScheduler)
						.concatWith(Mono.fromCallable(() -> {
							Iterable<StampedLock> locks;
							ArrayList<Long> stamps;
							if (updateMode == UpdateMode.ALLOW) {
								locks = itemsLock.bulkGetAt(getLockIndicesEntries(entriesWindow));
								stamps = new ArrayList<>();
								for (var lock : locks) {
									stamps.add(lock.writeLock());
								}
							} else {
								locks = null;
								stamps = null;
							}
							try {
								var batch = new CappedWriteBatch(db,
										CAPPED_WRITE_BATCH_CAP,
										RESERVED_WRITE_BATCH_SIZE,
										MAX_WRITE_BATCH_SIZE,
										BATCH_WRITE_OPTIONS
								);
								for (Entry<byte[], byte[]> entry : entriesWindow) {
									batch.put(entry.getKey(), entry.getValue());
								}
								batch.writeToDbAndClose();
								batch.close();
								return null;
							} finally {
								if (updateMode == UpdateMode.ALLOW) {
									int index = 0;
									for (var lock : locks) {
										lock.unlockWrite(stamps.get(index));
										index++;
									}
								}
							}
						})));
	}


	@NotNull
	private Mono<Entry<byte[], byte[]>> putEntryToWriteBatch(Entry<byte[], byte[]> newEntry, boolean getOldValues,
			CappedWriteBatch writeBatch) {
		return Mono.from(Mono
				.defer(() -> {
					if (getOldValues) {
						return get(null, newEntry.getKey());
					} else {
						return Mono.empty();
					}
				})
				.concatWith(Mono.<byte[]>fromCallable(() -> {
					synchronized (writeBatch) {
						writeBatch.put(cfh, newEntry.getKey(), newEntry.getValue());
					}
					return null;
				})
						.subscribeOn(dbScheduler))
				.map(oldValue -> Map.entry(newEntry.getKey(), oldValue)));
	}

	@Override
	public Flux<Entry<byte[], byte[]>> getRange(@Nullable LLSnapshot snapshot, LLRange range) {
		return Flux.defer(() -> {
			if (range.isSingle()) {
				return getRangeSingle(snapshot, range.getMin());
			} else {
				return getRangeMulti(snapshot, range);
			}
		});
	}

	@Override
	public Flux<List<Entry<byte[], byte[]>>> getRangeGrouped(@Nullable LLSnapshot snapshot,
			LLRange range,
			int prefixLength) {
		return Flux.defer(() -> {
			if (range.isSingle()) {
				return getRangeSingle(snapshot, range.getMin()).map(List::of);
			} else {
				return getRangeMultiGrouped(snapshot, range, prefixLength);
			}
		});
	}

	private Flux<Entry<byte[],byte[]>> getRangeSingle(LLSnapshot snapshot, byte[] key) {
		return this
				.get(snapshot, key)
				.map(value -> Map.entry(key, value))
				.flux();
	}

	private Flux<Entry<byte[],byte[]>> getRangeMulti(LLSnapshot snapshot, LLRange range) {
		return new BoundedRocksFluxIterable<Entry<byte[], byte[]>>(db, cfh, range) {

			@Override
			protected ReadOptions getReadOptions() {
				return resolveSnapshot(snapshot);
			}

			@Override
			protected Entry<byte[], byte[]> transformEntry(byte[] key) {
				return Map.entry(key, this.getValue());
			}
		}.generateNonblocking(dbScheduler, 128);
	}

	private Flux<List<Entry<byte[],byte[]>>> getRangeMultiGrouped(LLSnapshot snapshot, LLRange range, int prefixLength) {
		return new BoundedGroupedRocksFluxIterable<Entry<byte[], byte[]>>(db, cfh, range, prefixLength) {

			@Override
			protected ReadOptions getReadOptions() {
				return resolveSnapshot(snapshot);
			}

			@Override
			protected Entry<byte[], byte[]> transformEntry(byte[] key) {
				return Map.entry(key, this.getValue());
			}
		}.generateNonblocking(dbScheduler, 128);
	}

	@Override
	public Flux<byte[]> getRangeKeys(@Nullable LLSnapshot snapshot, LLRange range) {
		return Flux.defer(() -> {
			if (range.isSingle()) {
				return getRangeKeysSingle(snapshot, range.getMin());
			} else {
				return getRangeKeysMulti(snapshot, range);
			}
		});
	}

	@Override
	public Flux<List<byte[]>> getRangeKeysGrouped(@Nullable LLSnapshot snapshot, LLRange range, int prefixLength) {
		return new BoundedGroupedRocksFluxIterable<byte[]>(db, cfh, range, prefixLength) {

			@Override
			protected ReadOptions getReadOptions() {
				return resolveSnapshot(snapshot);
			}

			@Override
			protected byte[] transformEntry(byte[] key) {
				return key;
			}
		}.generateNonblocking(dbScheduler, 128);
	}

	private Flux<byte[]> getRangeKeysSingle(LLSnapshot snapshot, byte[] key) {
		return this
				.containsKey(snapshot, key)
				.filter(contains -> contains)
				.map(contains -> key)
				.flux();
	}

	private Flux<byte[]> getRangeKeysMulti(LLSnapshot snapshot, LLRange range) {
		return new BoundedRocksFluxIterable<byte[]>(db, cfh, range) {

			@Override
			protected ReadOptions getReadOptions() {
				return resolveSnapshot(snapshot);
			}

			@Override
			protected byte[] transformEntry(byte[] key) {
				return key;
			}
		}.generateNonblocking(dbScheduler, 128);
	}

	//todo: replace implementation with a simple Flux.push
	@Override
	public Flux<Entry<byte[], byte[]>> setRange(LLRange range,
			Flux<Entry<byte[], byte[]>> entries,
			boolean getOldValues) {
		return Flux.defer(() -> {
			if (range.isAll()) {
				return clear().thenMany(Flux.empty());
			} else {
				return Mono
						.fromCallable(() -> new CappedWriteBatch(db,
								CAPPED_WRITE_BATCH_CAP,
								RESERVED_WRITE_BATCH_SIZE,
								MAX_WRITE_BATCH_SIZE,
								BATCH_WRITE_OPTIONS
						))
						.subscribeOn(dbScheduler)
						.flatMapMany(writeBatch -> Mono
								.fromCallable(() -> {
									synchronized (writeBatch) {
										if (range.hasMin() && range.hasMax()) {
											writeBatch.deleteRange(cfh, range.getMin(), range.getMax());
											writeBatch.delete(cfh, range.getMax());
										} else if (range.hasMax()) {
											writeBatch.deleteRange(cfh, FIRST_KEY, range.getMax());
											writeBatch.delete(cfh, range.getMax());
										} else {
											try (var it = db.newIterator(cfh, getReadOptions(null))) {
												it.seekToLast();
												if (it.isValid()) {
													writeBatch.deleteRange(cfh, range.getMin(), it.key());
													writeBatch.delete(cfh, it.key());
												}
											}
										}
									}
									return null;
								})
								.subscribeOn(dbScheduler)
								.thenMany(entries)
								.flatMap(newEntry -> putEntryToWriteBatch(newEntry, getOldValues, writeBatch))
								.concatWith(Mono.<Entry<byte[], byte[]>>fromCallable(() -> {
									synchronized (writeBatch) {
										writeBatch.writeToDbAndClose();
										writeBatch.close();
									}
									return null;
								}).subscribeOn(dbScheduler))
								.doFinally(signalType -> {
									synchronized (writeBatch) {
										writeBatch.close();
									}
								}))
						.onErrorMap(IOException::new);
			}
		});
	}

	public Mono<Void> clear() {
		return Mono
				.<Void>fromCallable(() -> {
					try (RocksIterator iter = db.newIterator(cfh); CappedWriteBatch writeBatch = new CappedWriteBatch(db,
							CAPPED_WRITE_BATCH_CAP,
							RESERVED_WRITE_BATCH_SIZE,
							MAX_WRITE_BATCH_SIZE,
							BATCH_WRITE_OPTIONS
					)) {

						iter.seekToFirst();

						while (iter.isValid()) {
							writeBatch.delete(cfh, iter.key());

							iter.next();
						}

						writeBatch.writeToDbAndClose();

						// Compact range
						db.compactRange(cfh);

						db.flush(new FlushOptions().setWaitForFlush(true).setAllowWriteStall(true), cfh);
						db.flushWal(true);
					}
					return null;
				})
				.onErrorMap(IOException::new)
				.subscribeOn(dbScheduler);

	}

	@Override
	public Mono<Long> sizeRange(@Nullable LLSnapshot snapshot, LLRange range, boolean fast) {
		return Mono
				.defer(() -> {
					if (range.isAll()) {
						return Mono
								.fromCallable(() -> fast ? fastSizeAll(snapshot) : exactSizeAll(snapshot))
								.onErrorMap(IOException::new)
								.subscribeOn(dbScheduler);
					} else {
						return Mono
								.fromCallable(() -> {
									try (var iter = db.newIterator(cfh, resolveSnapshot(snapshot))) {
										if (range.hasMin()) {
											iter.seek(range.getMin());
										} else {
											iter.seekToFirst();
										}
										long i = 0;
										while (iter.isValid()) {
											if (range.hasMax()) {
												byte[] key1 = iter.key();
												if (Arrays.compareUnsigned(key1, range.getMax()) > 0) {
													break;
												}
											}

											iter.next();
											i++;
										}
										return i;
									}
								})
								.onErrorMap(IOException::new)
								.subscribeOn(dbScheduler);
					}
				});
	}

	@Override
	public Mono<Entry<byte[], byte[]>> getOne(@Nullable LLSnapshot snapshot, LLRange range) {
		return Mono
				.fromCallable(() -> {
					try (var rocksIterator = db.newIterator(cfh, resolveSnapshot(snapshot))) {
						if (range.hasMin()) {
							rocksIterator.seek(range.getMin());
						} else {
							rocksIterator.seekToFirst();
						}
						byte[] key;
						if (rocksIterator.isValid()) {
							key = rocksIterator.key();
							if (range.hasMax() && Arrays.compareUnsigned(key, range.getMax()) > 0) {
								return null;
							}
							return Map.entry(key, rocksIterator.value());
						} else {
							return null;
						}
					}
				})
				.subscribeOn(dbScheduler);
	}

	@Override
	public Mono<byte[]> getOneKey(@Nullable LLSnapshot snapshot, LLRange range) {
		return Mono
				.fromCallable(() -> {
					try (var rocksIterator = db.newIterator(cfh, resolveSnapshot(snapshot))) {
						if (range.hasMin()) {
							rocksIterator.seek(range.getMin());
						} else {
							rocksIterator.seekToFirst();
						}
						byte[] key;
						if (rocksIterator.isValid()) {
							key = rocksIterator.key();
							if (range.hasMax() && Arrays.compareUnsigned(key, range.getMax()) > 0) {
								return null;
							}
							return key;
						} else {
							return null;
						}
					}
				})
				.subscribeOn(dbScheduler);
	}

	private long fastSizeAll(@Nullable LLSnapshot snapshot) {
		var rocksdbSnapshot = resolveSnapshot(snapshot);
		if (USE_CURRENT_FASTSIZE_FOR_OLD_SNAPSHOTS || rocksdbSnapshot.snapshot() == null) {
			try {
				return db.getLongProperty(cfh, "rocksdb.estimate-num-keys");
			} catch (RocksDBException e) {
				e.printStackTrace();
				return 0;
			}
		} else {
			long count = 0;
			try (RocksIterator iter = db.newIterator(cfh, rocksdbSnapshot)) {
				iter.seekToFirst();
				// If it's a fast size of a snapshot, count only up to 1000 elements
				while (iter.isValid() && count < 1000) {
					count++;
					iter.next();
				}
				return count;
			}
		}
	}

	private long exactSizeAll(@Nullable LLSnapshot snapshot) {
		long count = 0;
		try (RocksIterator iter = db.newIterator(cfh, resolveSnapshot(snapshot))) {
			iter.seekToFirst();
			while (iter.isValid()) {
				count++;
				iter.next();
			}
			return count;
		}
	}

	@Override
	public Mono<Entry<byte[], byte[]>> removeOne(LLRange range) {
		return Mono
				.fromCallable(() -> {
					try (RocksIterator iter = db.newIterator(cfh)) {
						if (range.hasMin()) {
							iter.seek(range.getMin());
						} else {
							iter.seekToFirst();
						}
						if (!iter.isValid()) {
							return null;
						}
						if (range.hasMax() && Arrays.compareUnsigned(iter.key(), range.getMax()) > 0) {
							return null;
						}
						byte[] key = iter.key();
						byte[] value = iter.value();
						db.delete(cfh, key);
						return Map.entry(key, value);
					}
				})
				.onErrorMap(IOException::new)
				.subscribeOn(dbScheduler);
	}
}