parquets

parquets

TODO: TypeScript ...

fully asynchronous, pure node.js implementation of the Parquet file format

This package contains a fully asynchronous, pure TypeScript implementation of the Parquet file format. The implementation conforms with the Parquet specification and is tested for compatibility with Apache's Java reference implementation.

What is Parquet?: Parquet is a column-oriented file format; it allows you to write a large amount of structured data to a file, compress it and then read parts of it back out efficiently. The Parquet format is based on Google's Dremel paper.

Installation

To use parquets with node.js, install it using npm:

  $ npm install parquets

parquets requires node.js >= 7.6.0

Usage: Writing files

Once you have installed the parquets library, you can import it as a single module:

import { ParquetSchema, ParquetWriter, ParquetReader } from 'parquets';

Parquet files have a strict schema, similar to tables in a SQL database. So, in order to produce a Parquet file we first need to declare a new schema. Here is a simple example that shows how to instantiate a ParquetSchema object:

// declare a schema for the `fruits` table
let schema = new ParquetSchema({
  name: { type: 'UTF8' },
  quantity: { type: 'INT64' },
  price: { type: 'DOUBLE' },
  date: { type: 'TIMESTAMP_MILLIS' },
  in_stock: { type: 'BOOLEAN' }
});

Note that the Parquet schema supports nesting, so you can store complex, arbitrarily nested records into a single row (more on that later) while still maintaining good compression.

Once we have a schema, we can create a ParquetWriter object. The writer will take input rows as JSON objects, convert them to the Parquet format and store them on disk.

// create new ParquetWriter that writes to 'fruits.parquet`
let writer = await ParquetWriter.openFile(schema, 'fruits.parquet');

// append a few rows to the file
await writer.appendRow({name: 'apples', quantity: 10, price: 2.5, date: new Date(), in_stock: true});
await writer.appendRow({name: 'oranges', quantity: 10, price: 2.5, date: new Date(), in_stock: true});

Once we are finished adding rows to the file, we have to tell the writer object to flush the metadata to disk and close the file by calling the close() method:

Usage: Reading files

A parquet reader allows retrieving the rows from a parquet file in order. The basic usage is to create a reader and then retrieve a cursor/iterator which allows you to consume row after row until all rows have been read.

You may open more than one cursor and use them concurrently. All cursors become invalid once close() is called on the reader object.

// create new ParquetReader that reads from 'fruits.parquet`
let reader = await ParquetReader.openFile('fruits.parquet');

// create a new cursor
let cursor = reader.getCursor();

// read all records from the file and print them
let record = null;
while (record = await cursor.next()) {
  console.log(record);
}

When creating a cursor, you can optionally request that only a subset of the columns should be read from disk. For example:

// create a new cursor that will only return the `name` and `price` columns
let cursor = reader.getCursor(['name', 'price']);

It is important that you call close() after you are finished reading the file to avoid leaking file descriptors.

await reader.close();

Encodings

Internally, the Parquet format will store values from each field as consecutive arrays which can be compressed/encoded using a number of schemes.

Plain Encoding (PLAIN)

The most simple encoding scheme is the PLAIN encoding. It simply stores the values as they are without any compression. The PLAIN encoding is currently the default for all types except BOOLEAN:

let schema = new ParquetSchema({
  name: { type: 'UTF8', encoding: 'PLAIN' },
});

Run Length Encoding (RLE)

The Parquet hybrid run length and bitpacking encoding allows to compress runs of numbers very efficiently. Note that the RLE encoding can only be used in combination with the BOOLEAN, INT32 and INT64 types. The RLE encoding requires an additional bitWidth parameter that contains the maximum number of bits required to store the largest value of the field.

let schema = new ParquetSchema({
  age: { type: 'UINT_32', encoding: 'RLE', bitWidth: 7 },
});

Optional Fields

By default, all fields are required to be present in each row. You can also mark a field as 'optional' which will let you store rows with that field missing:

let schema = new ParquetSchema({
  name: { type: 'UTF8' },
  quantity: { type: 'INT64', optional: true },
});

let writer = await ParquetWriter.openFile(schema, 'fruits.parquet');
await writer.appendRow({name: 'apples', quantity: 10 });
await writer.appendRow({name: 'banana' }); // not in stock

Nested Rows & Arrays

Parquet supports nested schemas that allow you to store rows that have a more complex structure than a simple tuple of scalar values. To declare a schema with a nested field, omit the type in the column definition and add a fields list instead:

Consider this example, which allows us to store a more advanced "fruits" table where each row contains a name, a list of colours and a list of "stock" objects.

// advanced fruits table
let schema = new ParquetSchema({
  name: { type: 'UTF8' },
  colours: { type: 'UTF8', repeated: true },
  stock: {
    repeated: true,
    fields: {
      price: { type: 'DOUBLE' },
      quantity: { type: 'INT64' },
    }
  }
});

// the above schema allows us to store the following rows:
let writer = await ParquetWriter.openFile(schema, 'fruits.parquet');

await writer.appendRow({
  name: 'banana',
  colours: ['yellow'],
  stock: [
    { price: 2.45, quantity: 16 },
    { price: 2.60, quantity: 420 }
  ]
});

await writer.appendRow({
  name: 'apple',
  colours: ['red', 'green'],
  stock: [
    { price: 1.20, quantity: 42 },
    { price: 1.30, quantity: 230 }
  ]
});

await writer.close();

// reading nested rows with a list of explicit columns
let reader = await ParquetReader.openFile('fruits.parquet');

let cursor = reader.getCursor([['name'], ['stock', 'price']]);
let record = null;
while (record = await cursor.next()) {
  console.log(record);
}

await reader.close();

It might not be obvious why one would want to implement or use such a feature when the same can - in principle - be achieved by serializing the record using JSON (or a similar scheme) and then storing it into a UTF8 field:

Putting aside the philosophical discussion on the merits of strict typing, knowing about the structure and subtypes of all records (globally) means we do not have to duplicate this metadata (i.e. the field names) for every record. On top of that, knowing about the type of a field allows us to compress the remaining data more efficiently.

List of Supported Types & Encodings

We aim to be feature-complete and add new features as they are added to the Parquet specification; this is the list of currently implemented data types and encodings:

Logical Type	Primitive Type	Encodings
UTF8	BYTE_ARRAY	PLAIN
JSON	BYTE_ARRAY	PLAIN
BSON	BYTE_ARRAY	PLAIN
BYTE_ARRAY	BYTE_ARRAY	PLAIN
TIME_MILLIS	INT32	PLAIN, RLE
TIME_MICROS	INT64	PLAIN, RLE
TIMESTAMP_MILLIS	INT64	PLAIN, RLE
TIMESTAMP_MICROS	INT64	PLAIN, RLE
BOOLEAN	BOOLEAN	PLAIN, RLE
FLOAT	FLOAT	PLAIN
DOUBLE	DOUBLE	PLAIN
INT32	INT32	PLAIN, RLE
INT64	INT64	PLAIN, RLE
INT96	INT96	PLAIN
INT_8	INT32	PLAIN, RLE
INT_16	INT32	PLAIN, RLE
INT_32	INT32	PLAIN, RLE
INT_64	INT64	PLAIN, RLE
UINT_8	INT32	PLAIN, RLE
UINT_16	INT32	PLAIN, RLE
UINT_32	INT32	PLAIN, RLE
UINT_64	INT64	PLAIN, RLE

Buffering & Row Group Size

When writing a Parquet file, the ParquetWriter will buffer rows in memory until a row group is complete (or close() is called) and then write out the row group to disk.

The size of a row group is configurable by the user and controls the maximum number of rows that are buffered in memory at any given time as well as the number of rows that are co-located on disk:

let writer = await ParquetWriter.openFile(schema, 'fruits.parquet');
writer.setRowGroupSize(8192);

Depdendencies

Parquet uses thrift to encode the schema and other metadata, but the actual data does not use thrift.

Contributions

Please make sure you sign the contributor license agreement in order for us to be able to accept your contribution. We thank you very much!

License

Copyright (c) 2017 ironSource Ltd.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.