【Flink系列】- RocksDB增量模式checkpoint大小持续增长的问题及解决

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背景

Flink版本：1.13.5

一个使用FlinkSQL开发的生产线上任务，使用Tumble Window做聚和统计，并且配置table.exec.state.ttl为7200000，设置checkpoint周期为5分钟，使用rocksdb的增量模式。

正常情况下，任务运行一段时间以后，新增和过期的状态达到动态的平衡，随着RocksDB的compaction，checkpoint的大小会在小范围内上下起伏。

实际观察到，checkpoint大小持续缓慢增长，运行20天以后，从最初了100M左右，增长到了2G，checkpoint的时间也从1秒增加到了几十秒。

源码分析

我们看一下RocksIncrementalSnapshotStrategy.RocksDBIncrementalSnapshotOperation类中的get()方法：

public SnapshotResult<KeyedStateHandle> get(CloseableRegistry snapshotCloseableRegistry) throws Exception {boolean completed = false;SnapshotResult<StreamStateHandle> metaStateHandle = null;Map<StateHandleID, StreamStateHandle> sstFiles = new HashMap();HashMap miscFiles = new HashMap();boolean var15 = false;SnapshotResult var18;try {var15 = true;metaStateHandle = this.materializeMetaData(snapshotCloseableRegistry);Preconditions.checkNotNull(metaStateHandle, "Metadata was not properly created.");Preconditions.checkNotNull(metaStateHandle.getJobManagerOwnedSnapshot(), "Metadata for job manager was not properly created.");this.uploadSstFiles(sstFiles, miscFiles, snapshotCloseableRegistry);synchronized(RocksIncrementalSnapshotStrategy.this.materializedSstFiles) {RocksIncrementalSnapshotStrategy.this.materializedSstFiles.put(this.checkpointId, sstFiles.keySet());}IncrementalRemoteKeyedStateHandle jmIncrementalKeyedStateHandle = new IncrementalRemoteKeyedStateHandle(RocksIncrementalSnapshotStrategy.this.backendUID, RocksIncrementalSnapshotStrategy.this.keyGroupRange, this.checkpointId, sstFiles, miscFiles, (StreamStateHandle)metaStateHandle.getJobManagerOwnedSnapshot());DirectoryStateHandle directoryStateHandle = this.localBackupDirectory.completeSnapshotAndGetHandle();SnapshotResult snapshotResult;if (directoryStateHandle != null && metaStateHandle.getTaskLocalSnapshot() != null) {IncrementalLocalKeyedStateHandle localDirKeyedStateHandle = new IncrementalLocalKeyedStateHandle(RocksIncrementalSnapshotStrategy.this.backendUID, this.checkpointId, directoryStateHandle, RocksIncrementalSnapshotStrategy.this.keyGroupRange, (StreamStateHandle)metaStateHandle.getTaskLocalSnapshot(), sstFiles.keySet());snapshotResult = SnapshotResult.withLocalState(jmIncrementalKeyedStateHandle, localDirKeyedStateHandle);} else {snapshotResult = SnapshotResult.of(jmIncrementalKeyedStateHandle);}completed = true;var18 = snapshotResult;var15 = false;} finally {if (var15) {if (!completed) {List<StateObject> statesToDiscard = new ArrayList(1 + miscFiles.size() + sstFiles.size());statesToDiscard.add(metaStateHandle);statesToDiscard.addAll(miscFiles.values());statesToDiscard.addAll(sstFiles.values());this.cleanupIncompleteSnapshot(statesToDiscard);}}}

重点关注uploadSstFiles()方法的实现细节：

            Preconditions.checkState(this.localBackupDirectory.exists());Map<StateHandleID, Path> sstFilePaths = new HashMap();Map<StateHandleID, Path> miscFilePaths = new HashMap();Path[] files = this.localBackupDirectory.listDirectory();if (files != null) {this.createUploadFilePaths(files, sstFiles, sstFilePaths, miscFilePaths);sstFiles.putAll(RocksIncrementalSnapshotStrategy.this.stateUploader.uploadFilesToCheckpointFs(sstFilePaths, this.checkpointStreamFactory, snapshotCloseableRegistry));miscFiles.putAll(RocksIncrementalSnapshotStrategy.this.stateUploader.uploadFilesToCheckpointFs(miscFilePaths, this.checkpointStreamFactory, snapshotCloseableRegistry));}

进入到createUploadFilePaths()方法：

        private void createUploadFilePaths(Path[] files, Map<StateHandleID, StreamStateHandle> sstFiles, Map<StateHandleID, Path> sstFilePaths, Map<StateHandleID, Path> miscFilePaths) {Path[] var5 = files;int var6 = files.length;for(int var7 = 0; var7 < var6; ++var7) {Path filePath = var5[var7];String fileName = filePath.getFileName().toString();StateHandleID stateHandleID = new StateHandleID(fileName);if (!fileName.endsWith(".sst")) {miscFilePaths.put(stateHandleID, filePath);} else {boolean existsAlready = this.baseSstFiles != null && this.baseSstFiles.contains(stateHandleID);if (existsAlready) {sstFiles.put(stateHandleID, new PlaceholderStreamStateHandle());} else {sstFilePaths.put(stateHandleID, filePath);}}}}

这里是问题的关键，我们可以归纳出主要逻辑：

1. 扫描rocksdb本地存储目录下的所有文件，获取到所有的sst文件和misc文件（除sst文件外的其他所有文件）；

2. 将sst文件和历史checkpoint上传的sst文件做对比，将新增的sst文件路径记录下来；

3. 将misc文件的路径记录下来；

这里就是增量checkpoint的关键逻辑了，我们发现一点，增量的checkpoint只针对sst文件，对其他的misc文件是每次全量备份的，我们进到一个目录节点看一下有哪些文件被全量备份了：

[hadoop@fsp-hadoop-1 db]$ ll
总用量 8444
-rw-r--r-- 1 hadoop hadoop       0 3月  28 14:56 000058.log
-rw-r--r-- 1 hadoop hadoop 2065278 3月  31 10:17 025787.sst
-rw-r--r-- 1 hadoop hadoop 1945453 3月  31 10:18 025789.sst
-rw-r--r-- 1 hadoop hadoop   75420 3月  31 10:18 025790.sst
-rw-r--r-- 1 hadoop hadoop   33545 3月  31 10:18 025791.sst
-rw-r--r-- 1 hadoop hadoop   40177 3月  31 10:18 025792.sst
-rw-r--r-- 1 hadoop hadoop   33661 3月  31 10:18 025793.sst
-rw-r--r-- 1 hadoop hadoop   40494 3月  31 10:19 025794.sst
-rw-r--r-- 1 hadoop hadoop   33846 3月  31 10:19 025795.sst
-rw-r--r-- 1 hadoop hadoop      16 3月  30 19:46 CURRENT
-rw-r--r-- 1 hadoop hadoop      37 3月  28 14:56 IDENTITY
-rw-r--r-- 1 hadoop hadoop       0 3月  28 14:56 LOCK
-rw-rw-r-- 1 hadoop hadoop   38967 3月  28 14:56 LOG
-rw-r--r-- 1 hadoop hadoop 1399964 3月  31 10:19 MANIFEST-022789
-rw-r--r-- 1 hadoop hadoop   10407 3月  28 14:56 OPTIONS-000010
-rw-r--r-- 1 hadoop hadoop   13126 3月  28 14:56 OPTIONS-000012

1. CURRENT、IDENTIFY、LOCK、OPTIONS-*，这些文件基本是固定大小，不会有变化；

2. LOG文件，这个文件是rocksdb的日志文件，默认情况下，flink设置的rocksdb的日志输出级别是HEAD级别，几乎不会有日志输出，但是如果你配置了state.backend.rocksdb.log.level，比如说配置为了INFO_LEVEL，那么这个LOG文件会持续输出并且不会被清理；

3. MANIFEST-*，这是rocksdb的事务日志，在任务恢复重放过程中会用到，这个日志也会持续增长，达到阈值以后滚动生成新的并且清楚旧文件；