Problem Statement

The Spark WriteSupport class for Parquet is hardcoded to use org.apache.spark.sql.execution.datasources.parquet.ParquetWriteSupport, which is not configurable. Currently, this class doesn’t carry over the field metadata in StructType to MessageType. However, Parquet column encryption (Parquet-1396, Parquet-1178) requires the field metadata inside MessageType of Parquet, so that the metadata can be used to control column encryption.

Technical Solution

1. Extend SparkToParquetSchemaConverter class and override convert() method to add the functionality of carrying over the field metadata
2. Extend ParquetWriteSupport and use the extended converter in #1. The extension avoids changing the built-in WriteSupport to mitigate the risk.
3. Change Spark code to make the WriteSupport class configurable to let the user configure to use the extended WriteSupport in #2.  The default WriteSupport is still org.apache.spark.sql.execution.datasources.parquet.ParquetWriteSupport.

Technical Details

Note: The code below kind of in messy format. The link below shows correct format. 

Extend SparkToParquetSchemaConverter class

 SparkToParquetMetadataSchemaConverter extends SparkToParquetSchemaConverter {

 

      override def convert(catalystSchema: StructType): MessageType =

{                   Types                   ._buildMessage_()                  .addFields(catalystSchema.map(*convertFieldWithMetadata*): _*)                 .named(ParquetSchemaConverter._SPARK_PARQUET_SCHEMA_NAME_)                       }

 

      private def convertFieldWithMetadata(field: StructField) : Type =

{               val extField  = new ExtType[Any](convertField(field))               val metaBuilder = new MetadataBuilder().withMetadata(field.metadata)               val metaData = metaBuilder.getMap              extField.setMetadata(metaData)              return extField             }

 }

Extend ParquetWriteSupport

class CryptoParquetWriteSupport extends ParquetWriteSupport {

  override def init(configuration: Configuration): WriteContext =

{               val converter = new *SparkToParquetMetadataSchemaConverter*(configuration)       createContext(configuration, converter)    }

}

Make WriteSupport configurable

class ParquetFileFormat{

 

   **    override def prepareWrite(...) {

       …

       if (conf.get(ParquetOutputFormat.WRITE_SUPPORT_CLASS) == null) {

           ParquetOutputFormat.setWriteSupportClass(job, classOf[ParquetWriteSupport])

       ** 

      ...

   }

}

Verification

The ParquetHelloWorld.java in the github repository parquet-writesupport-extensions has a sample verification of passing down the field metadata and perform column encryption.

Dependency

• Parquet-1178
• Parquet-1396
• Parquet-1397