How to do failure tolerance for Flink to sink data to hdfs as gzip compression? - gzip

Question

We want to write compressed data to HDFS by Flink's BucketingSink or StreamingFileSink. I have write my own Writer which works fine if no failure occurs. However when It encounters a failure and restart from checkpoint, It will generate valid-length file(hadoop < 2.7) or truncate the file. Unluckily gzips are binary files which have trailer at the end of file. Therefore simple truncation does not work in my case. Any ideas to enable exactly-once semantic for compression hdfs sink?
That's my writer's code:
public class HdfsCompressStringWriter extends StreamWriterBaseV2<JSONObject> {
private static final long serialVersionUID = 2L;
/**
* The {#code CompressFSDataOutputStream} for the current part file.
*/
private transient GZIPOutputStream compressionOutputStream;
public HdfsCompressStringWriter() {}
#Override
public void open(FileSystem fs, Path path) throws IOException {
super.open(fs, path);
this.setSyncOnFlush(true);
compressionOutputStream = new GZIPOutputStream(this.getStream(), true);
}
public void close() throws IOException {
if (compressionOutputStream != null) {
compressionOutputStream.close();
compressionOutputStream = null;
}
resetStream();
}
#Override
public void write(JSONObject element) throws IOException {
if (element == null || !element.containsKey("body")) {
return;
}
String content = element.getString("body") + "\n";
compressionOutputStream.write(content.getBytes());
compressionOutputStream.flush();
}
#Override
public Writer<JSONObject> duplicate() {
return new HdfsCompressStringWriter();
}
}

Answer 1

I would recommend to implement a BulkWriter for the StreamingFileSink which compresses the elements via a GZIPOutputStream. The code could look the following:
public static void main(String[] args) throws Exception {
StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
env.setParallelism(1);
env.enableCheckpointing(1000);
final DataStream<Integer> input = env.addSource(new InfinitySource());
final StreamingFileSink<Integer> streamingFileSink = StreamingFileSink.<Integer>forBulkFormat(new Path("output"), new GzipBulkWriterFactory<>()).build();
input.addSink(streamingFileSink);
env.execute();
}
private static class GzipBulkWriterFactory<T> implements BulkWriter.Factory<T> {
#Override
public BulkWriter<T> create(FSDataOutputStream fsDataOutputStream) throws IOException {
final GZIPOutputStream gzipOutputStream = new GZIPOutputStream(fsDataOutputStream, true);
return new GzipBulkWriter<>(new ObjectOutputStream(gzipOutputStream), gzipOutputStream);
}
}
private static class GzipBulkWriter<T> implements BulkWriter<T> {
private final GZIPOutputStream gzipOutputStream;
private final ObjectOutputStream objectOutputStream;
public GzipBulkWriter(ObjectOutputStream objectOutputStream, GZIPOutputStream gzipOutputStream) {
this.gzipOutputStream = gzipOutputStream;
this.objectOutputStream = objectOutputStream;
}
#Override
public void addElement(T t) throws IOException {
objectOutputStream.writeObject(t);
}
#Override
public void flush() throws IOException {
objectOutputStream.flush();
}
#Override
public void finish() throws IOException {
objectOutputStream.flush();
gzipOutputStream.finish();
}
}