Reading/writing with Avro schemas AND Parquet format in SparkSQL

问题

I'm trying to write and read Parquet files from SparkSQL. For reasons of schema evolution, I would like to use Avro schemas with my writes and reads.

My understanding is that this is possible outside of Spark (or manually within Spark) using e.g. AvroParquetWriter and Avro's Generic API. However, I would like to use SparkSQL's write() and read() methods (which work with DataFrameWriter and DataFrameReader), and which integrate well with SparkSQL (I will be writing and reading Dataset's).

I can't for the life of me figure out how to do this, and am wondering if this is possible at all. The only options the SparkSQL parquet format seems to support are "compression" and "mergeSchema" -- i.e. no options for specifying an alternate schema format or alternate schema. In other words, it appears that there is no way to read/write Parquet files using Avro schemas using the SparkSQL API. But perhaps I'm just missing something?

To clarify, I also understand that this will basically just add the Avro schema to the Parquet metadata on write, and will add one more translation layer on read (Parquet format -> Avro schema -> SparkSQL internal format) but will specifically allow me to add default values for missing columns (which Avro schema supports but Parquet schema does not).

Also, I am not looking for a way to convert Avro to Parquet, or Parquet to Avro (rather a way to use them together), and I am not looking for a way to read/write plain Avro within SparkSQL (you can do this using databricks/spark-avro).

回答1:

I am doing something similar. I use avro schema to write into parquet file however, dont read it as avro. But the same technique should work on read as well. I am not sure if this is the best way to do it, but here it is anyways: I have AvroData.avsc which has the avro schema.

KafkaUtils.createDirectStream[String,Array[Byte],StringDecoder,DefaultDecoder,Tuple2[String, Array[Byte]]](ssc, kafkaProps, fromOffsets, messageHandler)


kafkaArr.foreachRDD  { (rdd,time) 
       => { val schema =  SchemaConverters.toSqlType(AvroData.getClassSchema).dataType.asInstanceOf[StructType] val ardd = rdd.mapPartitions{itr =>
              itr.map { r =>
try {
                    val cr = avroToListWithAudit(r._2, offsetSaved, loadDate, timeNow.toString)
                    Row.fromSeq(cr.toArray)
    } catch{
      case e:Exception => LogHandler.log.error("Exception while converting to Avro" + e.printStackTrace())
      System.exit(-1)
      Row(0)  //This is just to allow compiler to accept. On exception, the application will exit before this point
} 
} 
}


  public static List avroToListWithAudit(byte[] kfkBytes, String kfkOffset, String loaddate, String loadtime ) throws IOException {
        AvroData av = getAvroData(kfkBytes);
        av.setLoaddate(loaddate);
        av.setLoadtime(loadtime);
        av.setKafkaOffset(kfkOffset);
        return avroToList(av);
    }



 public static List avroToList(AvroData a) throws UnsupportedEncodingException{
        List<Object> l = new ArrayList<>();
        for (Schema.Field f : a.getSchema().getFields()) {
            String field = f.name().toString();
            Object value = a.get(f.name());
            if (value == null) {
                //System.out.println("Adding null");
                l.add(""); 
            }
            else {
                switch (f.schema().getType().getName()){
                    case "union"://System.out.println("Adding union");
                        l.add(value.toString());
                        break;

                    default:l.add(value);
                        break;
                }

            }
        }
        return l;
    }

The getAvroData method needs to have code to construct the avro object from raw bytes. I am also trying to figure out a way to do that without having to specifying each attribute setter explicitly, but seems like there isnt one.

public static AvroData getAvroData (bytes)
{
AvroData av = AvroData.newBuilder().build();
        try {
            av.setAttr(String.valueOf("xyz"));
        .....
    }
   } 

Hope it helps

来源：https://stackoverflow.com/questions/41450739/reading-writing-with-avro-schemas-and-parquet-format-in-sparksql