How to read data from AVRO file using C++ interface?

Question

I'm attempting to write a simple program to extract some data from a bunch of AVRO files. The schema for each file may be different so I would like to read the files generically (i.e. without having to pregenerate and then compile in the schema for each) using the C++ interface.

I have been attempting to follow the generic.cc example but it assumes a separate schema where I would like to read the schema from each AVRO file.

Here is my code:

#include <fstream>
#include <iostream>

#include "Compiler.hh"
#include "DataFile.hh"
#include "Decoder.hh"
#include "Generic.hh"
#include "Stream.hh"

const std::string BOLD("\033[1m");
const std::string ENDC("\033[0m");
const std::string RED("\033[31m");
const std::string YELLOW("\033[33m");

int main(int argc, char**argv)
{
    std::cout << "AVRO Test\n" << std::endl;

    if (argc < 2)
    {
        std::cerr << BOLD << RED << "ERROR: " << ENDC << "please provide an "
                  << "input file\n" << std::endl;
        return -1;
    }

    avro::DataFileReaderBase dataFile(argv[1]);
    auto dataSchema = dataFile.dataSchema();

    // Write out data schema in JSON for grins
    std::ofstream output("data_schema.json");
    dataSchema.toJson(output);
    output.close();

    avro::DecoderPtr decoder = avro::binaryDecoder();
    auto inStream = avro::fileInputStream(argv[1]);
    decoder->init(*inStream);

    avro::GenericDatum datum(dataSchema);
    avro::decode(*decoder, datum);
    std::cout << "Type: " << datum.type() << std::endl;

    return 0;
}

Everytime I run the code, no matter what file I use, I get this:

$ ./avrotest twitter.avro
AVRO Test

terminate called after throwing an instance of 'avro::Exception'
what(): Cannot have negative length: -40 Aborted

In addition to my own data files, I have tried using the data files located here: https://github.com/miguno/avro-cli-examples, with the same result.

I tried using the avrocat utility on all of the same files and it works fine. What am I doing wrong?

(NOTE: outputting the data schema for each file in JSON works correctly as expected)

Answer 1

After a bunch more fooling around, I figured it out. You're supposed to use DataFileReader templated with GenericDatum. With the end result being something like this:

#include <fstream>
#include <iostream>

#include "Compiler.hh"
#include "DataFile.hh"
#include "Decoder.hh"
#include "Generic.hh"
#include "Stream.hh"

const std::string BOLD("\033[1m");
const std::string ENDC("\033[0m");
const std::string RED("\033[31m");
const std::string YELLOW("\033[33m");

int main(int argc, char**argv)
{
    std::cout << "AVRO Test\n" << std::endl;

    if (argc < 2)
    {
        std::cerr << BOLD << RED << "ERROR: " << ENDC << "please provide an "
                  << "input file\n" << std::endl;
        return -1;
    }

    avro::DataFileReader<avro::GenericDatum> reader(argv[1]);
    auto dataSchema = reader.dataSchema();

    // Write out data schema in JSON for grins
    std::ofstream output("data_schema.json");
    dataSchema.toJson(output);
    output.close();

    avro::GenericDatum datum(dataSchema);
    while (reader.read(datum)) 
    {
        std::cout << "Type: " << datum.type() << std::endl;
        if (datum.type() == avro::AVRO_RECORD) 
        {
            const avro::GenericRecord& r = datum.value<avro::GenericRecord>();
            std::cout << "Field-count: " << r.fieldCount() << std::endl;

            // TODO: pull out each field
        }
    }

    return 0;
}

Perhaps an example like this should be included with libavro...