parquetjs0.10.1 • Public • Published
fully asynchronous, pure node.js implementation of the Parquet file format
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.
To use parquet.js with node.js, install it using npm:
$ npm install parquetjs
parquet.js requires node.js >= 7.6.0
Usage: Writing files
Once you have installed the parquet.js library, you can import it as a single module:
var parquet = ;
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
// declare a schema for the `fruits` tablevar schema =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`var writer = await parquetParquetWriter;// append a few rows to the fileawait writer;await writer;
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
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 parquetParquetReader;// create a new cursorlet cursor = reader;// read all records from the file and print themlet record = null;while record = await cursornextconsole;
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` columnslet cursor = reader;
It is important that you call close() after you are finished reading the file to avoid leaking file descriptors.
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
var schema =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
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.
var schema =age: type: 'UINT_32' encoding: 'RLE' bitWidth: 7;
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:
var schema =name: type: 'UTF8'quantity: type: 'INT64' optional: true;var writer = await parquetParquetWriter;await writer;await writer; // 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
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 tablevar schema =name: type: 'UTF8'colours: type: 'UTF8' repeated: truestock:repeated: truefields:price: type: 'DOUBLE'quantity: type: 'INT64';// the above schema allows us to store the following rows:var writer = await parquetParquetWriter;await writer;await writer;await writer;// reading nested rows with a list of explicit columnslet reader = await parquetParquetReader;let cursor = reader;let record = null;while record = await cursornextconsole;await reader;
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|
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:
var writer = await parquetParquetWriter;writer;
Parquet uses thrift to encode the schema and other metadata, but the actual data does not use thrift.
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