1. 介绍
在发现有些commit出错时，可使用Hudi提供的rollback回滚至指定的commit，这样可防止出现错误的结果，并且当一次commit失败时，也会进行rollback操作，保证一次commit的原子性。
2. 分析
rollback（回滚）的入口在 HoodieWriteClient#rollback，其依赖 HoodieWriteClient#rollbackInternal方法完成实际的回滚，其核心代码如下
protected void rollbackInternal(String commitToRollback) { // 生成新的rollback时间 final String startRollbackTime = HoodieActiveTimeline.createNewInstantTime(); try { HoodieTable<T> table = HoodieTable.getHoodieTable( createMetaClient(true), config, jsc); // 找出第一个与rollback commit相等的instant Option<HoodieInstant> rollbackInstantOpt = Option.fromJavaOptional(table.getActiveTimeline().getCommitsTimeline().getInstants() .filter(instant -> HoodieActiveTimeline.EQUAL.test(instant.getTimestamp(), commitToRollback)) .findFirst()); // 存在 if (rollbackInstantOpt.isPresent()) { // 进行回滚 List<HoodieRollbackStat> stats = doRollbackAndGetStats(rollbackInstantOpt.get()); // 结束回滚 finishRollback(context, stats, Collections.singletonList(commitToRollback), startRollbackTime); } } catch (IOException e) { throw new HoodieRollbackException("Failed to rollback " + config.getBasePath() + " commits " + commitToRollback, e); } }
首先过滤出commit/delta_commit中是否存在待回滚instant的时间，如果存在，则进行回滚，回滚的核心方法为 doRollbackAndGetStats，该方法在前一篇讲解savepoint时已经分析过，该方法会调用 HoodieTable#rollback完成实际回滚动作，下面着重分析 HoodieTable#rollback方法，对于MOR和COW不同类型有不同实现，下面一一进行分析。
2.1 HoodieCopyOnWriteTable#rollback
对于COW类型而言， rollback核心代码如下
public List<HoodieRollbackStat> rollback(JavaSparkContext jsc, HoodieInstant instant, boolean deleteInstants) throws IOException { long startTime = System.currentTimeMillis(); List<HoodieRollbackStat> stats = new ArrayList<>(); HoodieActiveTimeline activeTimeline = this.getActiveTimeline(); if (instant.isCompleted()) { // instant状态为completed // 转变至inflight状态 instant = activeTimeline.revertToInflight(instant); } if (!instant.isRequested()) { // 不为requested状态 String commit = instant.getTimestamp(); // 生成回滚的请求 List<RollbackRequest> rollbackRequests = generateRollbackRequests(instant); // 进行回滚 stats = new RollbackExecutor(metaClient, config).performRollback(jsc, instant, rollbackRequests); } // 删除inflight和requested状态的instant deleteInflightAndRequestedInstant(deleteInstants, activeTimeline, instant); return stats; }
可以看到，进行回滚总体分为四步：1. 对于处理completed状态的instant，首先会将其转变至inflight状态，而对于不处于requested状态的instant（compaction会存在requested状态）；2. 生成回滚请求；3. 进行回滚；4. 删除instant。
2.1.1 转变instant状态
对于处于completed状态的instant，将其转变至 inflight状态，其核心代码如下
public HoodieInstant revertToInflight(HoodieInstant instant) { // 获取inflight状态的instant HoodieInstant inflight = HoodieTimeline.getInflightInstant(instant, metaClient.getTableType()); // 转变至inflight，即文件名会变为.inflight revertCompleteToInflight(instant, inflight); return inflight; }
对于状态转变体现在文件名后缀的变化，即会变为 .inflght状态。
2.1.2 生成回滚请求
回滚请求由 generateRollbackRequests方法生成，其核心代码如下
private List<RollbackRequest> generateRollbackRequests(HoodieInstant instantToRollback) throws IOException { // 获取所有的分区路径，对每个分区路径生成DELETE_DATA_AND_LOG_FILES类型的RollbackRequest return FSUtils.getAllPartitionPaths(this.metaClient.getFs(), this.getMetaClient().getBasePath(), config.shouldAssumeDatePartitioning()).stream().map(partitionPath -> RollbackRequest.createRollbackRequestWithDeleteDataAndLogFilesAction(partitionPath, instantToRollback)) .collect(Collectors.toList()); }
会根据不同的分区路径生成不同的RollbackRequest，该方法会生成会生成DELETEDATAANDLOGFILES类型，指定分区路径的RollbackRequest。
2.1.3 进行回滚
通过 RollbackExecutor#performRollback进行回滚，其核心代码如下
public List<HoodieRollbackStat> performRollback(JavaSparkContext jsc, HoodieInstant instantToRollback, List<RollbackRequest> rollbackRequests) { SerializablePathFilter filter = (path) -> { if (path.toString().contains(".parquet")) { // 获取parquet文件提交时间 String fileCommitTime = FSUtils.getCommitTime(path.getName()); // 是否等于指定回滚的时间 return instantToRollback.getTimestamp().equals(fileCommitTime); } else if (path.toString().contains(".log")) { // 获取log文件提交时间 String fileCommitTime = FSUtils.getBaseCommitTimeFromLogPath(path); // 是否等于指定回滚的时间 return instantToRollback.getTimestamp().equals(fileCommitTime); } return false; }; int sparkPartitions = Math.max(Math.min(rollbackRequests.size(), config.getRollbackParallelism()), 1); return jsc.parallelize(rollbackRequests, sparkPartitions).mapToPair(rollbackRequest -> { final Map<FileStatus, Boolean> filesToDeletedStatus = new HashMap<>(); switch (rollbackRequest.getRollbackAction()) { // rollback类型 case DELETE_DATA_FILES_ONLY: { // 仅仅删除数据文件 // 根据分区路径来删除该路径下文件 deleteCleanedFiles(metaClient, config, filesToDeletedStatus, instantToRollback.getTimestamp(), rollbackRequest.getPartitionPath()); return new Tuple2<>(rollbackRequest.getPartitionPath(), HoodieRollbackStat.newBuilder().withPartitionPath(rollbackRequest.getPartitionPath()) .withDeletedFileResults(filesToDeletedStatus).build()); } case DELETE_DATA_AND_LOG_FILES: { // 删除数据文件和日志文件 // 根据分区路径来删除该路径下文件 deleteCleanedFiles(metaClient, config, filesToDeletedStatus, rollbackRequest.getPartitionPath(), filter); return new Tuple2<>(rollbackRequest.getPartitionPath(), HoodieRollbackStat.newBuilder().withPartitionPath(rollbackRequest.getPartitionPath()) .withDeletedFileResults(filesToDeletedStatus).build()); } case APPEND_ROLLBACK_BLOCK: { // 添加ROLLBACK块 Writer writer = null; try { writer = HoodieLogFormat.newWriterBuilder() .onParentPath(FSUtils.getPartitionPath(metaClient.getBasePath(), rollbackRequest.getPartitionPath())) .withFileId(rollbackRequest.getFileId().get()) .overBaseCommit(rollbackRequest.getLatestBaseInstant().get()).withFs(metaClient.getFs()) .withFileExtension(HoodieLogFile.DELTA_EXTENSION).build(); // 生成元数据，如生成控制块（CommandBlock） Map<HeaderMetadataType, String> header = generateHeader(instantToRollback.getTimestamp()); writer = writer.appendBlock(new HoodieCommandBlock(header)); } catch (IOException | InterruptedException io) { throw new HoodieRollbackException("Failed to rollback for instant " + instantToRollback, io); } finally { try { if (writer != null) { writer.close(); } } catch (IOException io) { throw new UncheckedIOException(io); } } Map<FileStatus, Long> filesToNumBlocksRollback = new HashMap<>(); filesToNumBlocksRollback.put(metaClient.getFs().getFileStatus(Preconditions.checkNotNull(writer).getLogFile().getPath()), 1L); return new Tuple2<>(rollbackRequest.getPartitionPath(), HoodieRollbackStat.newBuilder().withPartitionPath(rollbackRequest.getPartitionPath()) .withRollbackBlockAppendResults(filesToNumBlocksRollback).build()); } default: throw new IllegalStateException("Unknown Rollback action " + rollbackRequest); } }).reduceByKey(this::mergeRollbackStat).map(Tuple2::_2).collect(); }
对于DELETEDATAFILES_ONLY类型的rollback，会调用 deleteCleanedFiles来删除数据文件，其核心代码如下
private Map<FileStatus, Boolean> deleteCleanedFiles(HoodieTableMetaClient metaClient, HoodieWriteConfig config, Map<FileStatus, Boolean> results, String commit, String partitionPath) throws IOException { FileSystem fs = metaClient.getFs(); PathFilter filter = (path) -> { if (path.toString().contains(".parquet")) { // 数据文件 String fileCommitTime = FSUtils.getCommitTime(path.getName()); // 与rollback时间相等 return commit.equals(fileCommitTime); } return false; }; // 过滤出与rollback时间相等的所有parquet文件 FileStatus[] toBeDeleted = fs.listStatus(FSUtils.getPartitionPath(config.getBasePath(), partitionPath), filter); for (FileStatus file : toBeDeleted) { // 逐一删除 boolean success = fs.delete(file.getPath(), false); results.put(file, success); } return results; }
首先会过滤指定分区下所有与rollback时间相等的parquet文件，然后逐一删除。
对于DELETEDATAANDLOGFILES类型的rollback，会调用同名的 deleteCleanedFiles来删除文件，其核心代码如下
private Map<FileStatus, Boolean> deleteCleanedFiles(HoodieTableMetaClient metaClient, HoodieWriteConfig config, Map<FileStatus, Boolean> results, String partitionPath, PathFilter filter) throws IOException { FileSystem fs = metaClient.getFs(); // 过滤出与rollback时间相等的所有parquet和log文件 FileStatus[] toBeDeleted = fs.listStatus(FSUtils.getPartitionPath(config.getBasePath(), partitionPath), filter); for (FileStatus file : toBeDeleted) { // 逐一删除 boolean success = fs.delete(file.getPath(), false); results.put(file, success); } return results; }
首先会过滤指定分区下所有与rollback时间相等的parquet/log文件，然后逐一删除。
对于APPENDROLLBACKBLOCK类型的rollback，会生成日志文件控制块并写入指定的文件中，在读取时，将不会读取该控制块的前一个块。
2.1.4 删除instant
在完成回滚后，还需要调用 deleteInflightAndRequestedInstant来删除instant，其核心代码如下
protected void deleteInflightAndRequestedInstant(boolean deleteInstant, HoodieActiveTimeline activeTimeline, HoodieInstant instantToBeDeleted) { // 删除marker下的目录 deleteMarkerDir(instantToBeDeleted.getTimestamp()); if (deleteInstant) { // 删除instant // 删除处于pending状态的instant activeTimeline.deletePending(instantToBeDeleted); if (instantToBeDeleted.isInflight() && !metaClient.getTimelineLayoutVersion().isNullVersion()) { // 删除处于requested状态的instant instantToBeDeleted = new HoodieInstant(State.REQUESTED, instantToBeDeleted.getAction(), instantToBeDeleted.getTimestamp()); activeTimeline.deletePending(instantToBeDeleted); } } }
删除instant主要是删除处于inflight和requested状态的在元数据目录下的文件。
2.2 HoodieMergeOnReadTable#rollback
对于MOR而言， rollback核心代码如下
public List<HoodieRollbackStat> rollback(JavaSparkContext jsc, HoodieInstant instant, boolean deleteInstants) throws IOException { long startTime = System.currentTimeMillis(); // rollback时间 String commit = instant.getTimestamp(); if (instant.isCompleted()) { // instant状态为completed // // 转变至inflight状态 instant = this.getActiveTimeline().revertToInflight(instant); } List<HoodieRollbackStat> allRollbackStats = new ArrayList<>(); // 不为requested状态 if (!instant.isRequested()) { // 生成rollback请求 List<RollbackRequest> rollbackRequests = generateRollbackRequests(jsc, instant); // 进行回滚 allRollbackStats = new RollbackExecutor(metaClient, config).performRollback(jsc, instant, rollbackRequests); } // 删除inflight和requested状态的instant deleteInflightAndRequestedInstant(deleteInstants, this.getActiveTimeline(), instant); return allRollbackStats; }
可以看到其流程与COW相同，不再赘述。
3. 总结
对于rollback而言，其主要分为四步：转变instant状态；2. 生成回滚请求；3. 进行回滚；4. 删除instant。而回滚时会分为三种情况，对于 DELETE_DATA_FILES_ONLY和 DELETE_DATA_AND_LOG_FILES类型的rollback，会直接删除对应commit的数据文件和日志文件，而对于 APPEND_ROLLBACK_BLOCK类型，则会写入控制块至文件中，在读取时不读取其前一个块。
