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背景：

kafka的文件系统数据源可以支持精准一次的一致性,本文就从源码看下如何TextInputFormat如何支持状态的精准一致性

TextInputFormat源码解析

首先flink会把输入的文件进行切分，分成多个数据块的形式，每个数据源算子任务会被分配以读取其中的数据块,但是不是所有的文件都能进行分块，判断文件是否可以进行分块的代码如下：

protected boolean testForUnsplittable(FileStatus pathFile) {if (getInflaterInputStreamFactory(pathFile.getPath()) != null) {unsplittable = true;return true;}return false;
}private InflaterInputStreamFactory<?> getInflaterInputStreamFactory(Path path) {String fileExtension = extractFileExtension(path.getName());if (fileExtension != null) {return getInflaterInputStreamFactory(fileExtension);} else {return null;}
}

后缀名称是.gz,.bzip2等的文件都没法切分,如果可以切分，切分的具体代码如下所示：

while (samplesTaken < numSamples && fileNum < allFiles.size()) {// make a split for the sample and use it to read a recordFileStatus file = allFiles.get(fileNum);
// 根据偏移量进行切分FileInputSplit split = new FileInputSplit(0, file.getPath(), offset, file.getLen() - offset, null);// we open the split, read one line, and take its lengthtry {open(split);if (readLine()) {totalNumBytes += this.currLen + this.delimiter.length;samplesTaken++;}} finally {// close the file stream, do not release the bufferssuper.close();}
// 偏移量迁移offset += stepSize;// skip to the next file, if necessarywhile (fileNum < allFiles.size()&& offset >= (file = allFiles.get(fileNum)).getLen()) {offset -= file.getLen();fileNum++;}
}

再来看一下TextInputFormat如何支持checkpoint操作，保存文件的偏移量的代码：

@Override
public void snapshotState(StateSnapshotContext context) throws Exception {super.snapshotState(context);checkState(checkpointedState != null, "The operator state has not been properly initialized.");int subtaskIdx = getRuntimeContext().getIndexOfThisSubtask();// 算子列表状态checkpointedState.clear();// 获取文件的当前读取的偏移List<T> readerState = getReaderState();try {for (T split : readerState) {//保存到检查点路径中checkpointedState.add(split);}} catch (Exception e) {checkpointedState.clear();throw new Exception("Could not add timestamped file input splits to to operator "+ "state backend of operator "+ getOperatorName()+ '.',e);}if (LOG.isDebugEnabled()) {LOG.debug("{} (taskIdx={}) checkpointed {} splits: {}.",getClass().getSimpleName(),subtaskIdx,readerState.size(),readerState);}
}

从检查点中恢复状态的代码如下：

public void initializeState(StateInitializationContext context) throws Exception {super.initializeState(context);checkState(checkpointedState == null, "The reader state has already been initialized.");// 初始化算子操作状态checkpointedState =context.getOperatorStateStore().getListState(new ListStateDescriptor<>("splits", new JavaSerializer<>()));int subtaskIdx = getRuntimeContext().getIndexOfThisSubtask();LOG.info("Restoring state for the {} (taskIdx={}).", getClass().getSimpleName(), subtaskIdx);splits = splits == null ? new PriorityQueue<>() : splits;for (T split : checkpointedState.get()) {//从检查点状态中恢复各个切分的分块splits.add(split);}
}