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Reading JSON files#

Line-separated JSON files can either be read as a single Arrow Table with a TableReader or streamed as RecordBatches with a StreamingReader.

Both of these readers require an arrow::io::InputStream instance representing the input file. Their behavior can be customized using a combination of ReadOptions, ParseOptions, and other parameters.

TableReader#

TableReader reads an entire file in one shot as a Table. Each independent JSON object in the input file is converted to a row in the output table.

#include "arrow/json/api.h"

{
   // ...
   arrow::MemoryPool* pool = default_memory_pool();
   std::shared_ptr<arrow::io::InputStream> input = ...;

   auto read_options = arrow::json::ReadOptions::Defaults();
   auto parse_options = arrow::json::ParseOptions::Defaults();

   // Instantiate TableReader from input stream and options
   auto maybe_reader = arrow::json::TableReader::Make(pool, input, read_options, parse_options);
   if (!maybe_reader.ok()) {
      // Handle TableReader instantiation error...
   }
   auto reader = *maybe_reader;

   // Read table from JSON file
   auto maybe_table = reader->Read();
   if (!maybe_table.ok()) {
      // Handle JSON read error
      // (for example a JSON syntax error or failed type conversion)
   }
   auto table = *maybe_table;
}

StreamingReader#

StreamingReader reads a file incrementally from blocks of a roughly equal byte size, each yielding a RecordBatch. Each independent JSON object in a block is converted to a row in the output batch.

All batches adhere to a consistent Schema, which is derived from the first loaded batch. Alternatively, an explicit schema may be passed via ParseOptions.

#include "arrow/json/api.h"

{
   // ...
   auto read_options = arrow::json::ReadOptions::Defaults();
   auto parse_options = arrow::json::ParseOptions::Defaults();

   std::shared_ptr<arrow::io::InputStream> stream;
   auto result = arrow::json::StreamingReader::Make(stream,
                                                    read_options,
                                                    parse_options);
   if (!result.ok()) {
      // Handle instantiation error
   }
   std::shared_ptr<arrow::json::StreamingReader> reader = *result;

   for (arrow::Result<std::shared_ptr<arrow::RecordBatch>> maybe_batch : *reader) {
      if (!maybe_batch.ok()) {
         // Handle read/parse error
      }
      std::shared_ptr<arrow::RecordBatch> batch = *maybe_batch;
      // Operate on each batch...
   }
}

Data types#

Since JSON values are typed, the possible Arrow data types on output depend on the input value types. Top-level JSON values should always be objects. The fields of top-level objects are taken to represent columns in the Arrow data. For each name/value pair in a JSON object, there are two possible modes of deciding the output data type:

if the name is in ParseOptions::explicit_schema, conversion of the JSON value to the corresponding Arrow data type is attempted;
otherwise, the Arrow data type is determined via type inference on the JSON value, trying out a number of Arrow data types in order.

The following tables show the possible combinations for each of those two modes.

Explicit conversions from JSON to Arrow#
JSON value type	Allowed Arrow data types
Null	Any (including Null)
Number	All Integer types, Float32, Float64, Date32, Date64, Time32, Time64
Boolean	Boolean
String	Binary, LargeBinary, String, LargeString, Timestamp
Array	List
Object (nested)	Struct

Implicit type inference from JSON to Arrow#
JSON value type	Inferred Arrow data types (in order)
Null	Null, any other
Number	Int64, Float64
Boolean	Boolean
String	Timestamp (with seconds unit), String
Array	List
Object (nested)	Struct

Reading and Writing CSV files

Tabular Datasets