smiklosovic commented on code in PR #4399: URL: https://github.com/apache/cassandra/pull/4399#discussion_r2410973646
########## src/java/org/apache/cassandra/db/compression/ZstdDictionaryTrainer.java: ########## @@ -0,0 +1,334 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you under the Apache License, Version 2.0 (the + * "License"); you may not use this file except in compliance + * with the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ + +package org.apache.cassandra.db.compression; + +import java.nio.ByteBuffer; +import java.util.concurrent.ThreadLocalRandom; +import java.util.concurrent.atomic.AtomicLong; +import java.util.function.Consumer; + +import com.google.common.annotations.VisibleForTesting; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + +import com.github.luben.zstd.Zstd; +import com.github.luben.zstd.ZstdDictTrainer; +import org.apache.cassandra.config.DatabaseDescriptor; +import org.apache.cassandra.db.compression.CompressionDictionary.DictId; +import org.apache.cassandra.db.compression.CompressionDictionary.Kind; +import org.apache.cassandra.io.compress.IDictionaryCompressor; +import org.apache.cassandra.io.compress.ZstdDictionaryCompressor; +import org.apache.cassandra.schema.CompressionParams; + +/** + * Zstd implementation of dictionary trainer with lifecycle management. + */ +public class ZstdDictionaryTrainer implements ICompressionDictionaryTrainer +{ + private static final Logger logger = LoggerFactory.getLogger(ZstdDictionaryTrainer.class); + + private final String keyspaceName; + private final String tableName; + private final CompressionDictionaryTrainingConfig config; + private final AtomicLong totalSampleSize; + private final AtomicLong sampleCount; + private final int compressionLevel; // optimal if using the same level for training as when compressing. + + // Sampling rate can be updated during training + private volatile int samplingRate; + + // Minimum number of samples required by ZSTD library + private static final int MIN_SAMPLES_REQUIRED = 10; + + private volatile Consumer<CompressionDictionary> dictionaryTrainedListener; + // TODO: manage the samples in this class for auto-train (follow-up). The ZstdDictTrainer cannot be re-used for multiple training runs. + private ZstdDictTrainer zstdTrainer; + private volatile boolean closed = false; + private volatile TrainingStatus currentTrainingStatus; + + public ZstdDictionaryTrainer(String keyspaceName, String tableName, + CompressionDictionaryTrainingConfig config, + int compressionLevel) + { + this.keyspaceName = keyspaceName; + this.tableName = tableName; + this.config = config; + this.totalSampleSize = new AtomicLong(0); + this.sampleCount = new AtomicLong(0); + this.compressionLevel = compressionLevel; + this.samplingRate = config.samplingRate; + this.currentTrainingStatus = TrainingStatus.NOT_STARTED; + } + + @Override + public boolean shouldSample() + { + return zstdTrainer != null && ThreadLocalRandom.current().nextInt(samplingRate) == 0; + } + + @Override + public void addSample(ByteBuffer sample) + { + if (closed || sample == null || !sample.hasRemaining() || zstdTrainer == null) + return; + + byte[] sampleBytes = new byte[sample.remaining()]; + sample.duplicate().get(sampleBytes); + + if (zstdTrainer.addSample(sampleBytes)) + { + // Update the totalSampleSize and sampleCount if the sample is added + totalSampleSize.addAndGet(sampleBytes.length); + sampleCount.incrementAndGet(); + } + } + + @Override + public CompressionDictionary trainDictionary(boolean force) + { + boolean isReady = isReady(); + if (!force && !isReady) + { + currentTrainingStatus = TrainingStatus.FAILED; + throw new IllegalStateException("Trainer is not ready"); + } + + long currentSampleCount = sampleCount.get(); + if (currentSampleCount < MIN_SAMPLES_REQUIRED) // minimum samples should be required even if force training + { + currentTrainingStatus = TrainingStatus.FAILED; + String errorMsg = String.format("Insufficient samples for training: %d (minimum required: %d)", + currentSampleCount, MIN_SAMPLES_REQUIRED); + throw new IllegalStateException(errorMsg); + } + + currentTrainingStatus = TrainingStatus.TRAINING; + try + { + logger.debug("Training with sample count: {}, sample size: {}, isReady: {}", + currentSampleCount, totalSampleSize.get(), isReady); + byte[] dictBytes = zstdTrainer.trainSamples(); + long zstdDictId = Zstd.getDictIdFromDict(dictBytes); + DictId dictId = new DictId(Kind.ZSTD, makeDictionaryId(System.currentTimeMillis(), zstdDictId)); + currentTrainingStatus = TrainingStatus.COMPLETED; + logger.debug("New dictionary is trained with {}", dictId); + CompressionDictionary dictionary = new ZstdCompressionDictionary(dictId, dictBytes); + notifyDictionaryTrainedListener(dictionary); + return dictionary; + } + catch (Exception e) + { + currentTrainingStatus = TrainingStatus.FAILED; + throw new RuntimeException("Failed to train Zstd dictionary", e); + } + } + + @Override + public boolean isReady() + { + return currentTrainingStatus != TrainingStatus.TRAINING + && !closed + && zstdTrainer != null + && totalSampleSize.get() >= config.acceptableTotalSampleSize + && sampleCount.get() > MIN_SAMPLES_REQUIRED; + } + + @Override + public TrainingStatus getTrainingStatus() + { + return currentTrainingStatus; + } + + @Override + public boolean start(boolean manualTraining) + { + if (closed || !(manualTraining || shouldAutoStartTraining())) + return false; + + try + { + // reset on starting; a new zstdTrainer instance is created during reset + reset(); + logger.info("Started dictionary training for {}.{}", keyspaceName, tableName); + currentTrainingStatus = TrainingStatus.SAMPLING; + return true; + } + catch (Exception e) + { + logger.warn("Failed to create ZstdDictTrainer for {}.{}", keyspaceName, tableName, e); + currentTrainingStatus = TrainingStatus.FAILED; + } + return false; + } + + /** + * Determines if training should auto-start based on configuration. + */ + private boolean shouldAutoStartTraining() + { + return DatabaseDescriptor.getCompressionDictionaryTrainingAutoTrainEnabled(); + } + + @Override + public void reset() + { + if (closed) + { + return; + } + + currentTrainingStatus = TrainingStatus.NOT_STARTED; + synchronized (this) + { + totalSampleSize.set(0); + sampleCount.set(0); + zstdTrainer = new ZstdDictTrainer(config.maxTotalSampleSize, config.maxDictionarySize, compressionLevel); + } + } + + @Override + public Kind kind() + { + return Kind.ZSTD; + } + + @Override + public void setDictionaryTrainedListener(Consumer<CompressionDictionary> listener) + { + this.dictionaryTrainedListener = listener; + } + + @Override + public void updateSamplingRate(int newSamplingRate) + { + if (newSamplingRate <= 0) + { + throw new IllegalArgumentException("Sampling rate must be positive, got: " + newSamplingRate); + } + this.samplingRate = newSamplingRate; + logger.debug("Updated sampling rate to {} for {}.{}", newSamplingRate, keyspaceName, tableName); + } + + /** + * Notifies the registered listener that a dictionary has been trained. + * + * @param dictionary the newly trained dictionary + */ + private void notifyDictionaryTrainedListener(CompressionDictionary dictionary) + { + Consumer<CompressionDictionary> listener = this.dictionaryTrainedListener; + if (listener != null) + { + try + { + listener.accept(dictionary); + } + catch (Exception e) + { + logger.warn("Error notifying dictionary trained listener for {}.{}", keyspaceName, tableName, e); + } + } + } + + @Override + public boolean isCompatibleWith(CompressionParams newParams) + { + if (!newParams.isDictionaryCompressionEnabled()) + { + return false; + } + + IDictionaryCompressor newCompressor = (IDictionaryCompressor) newParams.getSstableCompressor(); + + // Check if the compressor type is compatible with this trainer + if (newCompressor.acceptableDictionaryKind() != Kind.ZSTD) + { + return false; + } + + ZstdDictionaryCompressor zstdDictionaryCompressor = (ZstdDictionaryCompressor) newCompressor; + // For Zstd compressors, check if compression level matches + return this.compressionLevel == zstdDictionaryCompressor.compressionLevel(); + } + + @Override + public void close() + { + if (closed) + return; + + closed = true; + currentTrainingStatus = TrainingStatus.NOT_STARTED; + + synchronized (this) + { + // Permanent shutdown: clear all state and prevent restart + totalSampleSize.set(0); + sampleCount.set(0); + zstdTrainer = null; + } + + logger.info("Permanently closed dictionary trainer for {}.{}", keyspaceName, tableName); + } + + /** + * Creates a monotonically increasing dictionary ID by combining timestamp and Zstd dictionary ID. + * + * The resulting dictionary ID has the following structure: + * - Upper 32 bits: timestamp in minutes (signed int) + * - Lower 32 bits: Zstd dictionary ID (unsigned int, passed as long due to Java limitations) + * + * This ensures dictionary IDs are monotonically increasing over time, which helps to identify + * the latest dictionary. + * + * The implementation assumes that dictionary training frequency is significantly larger than + * every minute, which a healthy system should do. In the scenario when multiple dictionaries + * are trained in the same minute (only possible using manual training), there should not be + * correctness concerns since the dictionary is attached to the SSTables, but leads to performance + * hit from having too many dictionary. Therefore, such scenario should be avoided at the best. + * + * @param currentTimeMillis the current time in milliseconds + * @param zstdDictId Zstd dictionary ID (unsigned 32-bit value represented as long) + * @return combined dictionary ID that is monotonically increasing over time + */ + static long makeDictionaryId(long currentTimeMillis, long zstdDictId) Review Comment: is this really ZSTD specific? seems like moving this to DictId or similar would be better so it can be used for other compressors too. I just dont see why is this ZSTD specific only. -- This is an automated message from the Apache Git Service. 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