Reading and Writing the Apache Parquet Format¶
The Apache Parquet project provides a standardized open-source columnar storage format for use in data analysis systems. It was created originally for use in Apache Hadoop with systems like Apache Drill, Apache Hive, Apache Impala, and Apache Spark adopting it as a shared standard for high performance data IO.
Apache Arrow is an ideal in-memory transport layer for data that is being read or written with Parquet files. We have been concurrently developing the C++ implementation of Apache Parquet, which includes a native, multithreaded C++ adapter to and from in-memory Arrow data. PyArrow includes Python bindings to this code, which thus enables reading and writing Parquet files with pandas as well.
Obtaining pyarrow with Parquet Support¶
If you installed pyarrow with pip or conda, it should be built with Parquet
support bundled:
In [1]: import pyarrow.parquet as pq
If you are building pyarrow from source, you must use -DARROW_PARQUET=ON
when compiling the C++ libraries and enable the Parquet extensions when
building pyarrow. If you want to use Parquet Encryption, then you must
use -DPARQUET_REQUIRE_ENCRYPTION=ON too when compiling the C++ libraries.
See the Python Development page for more details.
Reading and Writing Single Files¶
The functions read_table() and write_table()
read and write the pyarrow.Table object, respectively.
Let’s look at a simple table:
In [2]: import numpy as np
In [3]: import pandas as pd
In [4]: import pyarrow as pa
In [5]: df = pd.DataFrame({'one': [-1, np.nan, 2.5],
...: 'two': ['foo', 'bar', 'baz'],
...: 'three': [True, False, True]},
...: index=list('abc'))
...:
In [6]: table = pa.Table.from_pandas(df)
We write this to Parquet format with write_table:
In [7]: import pyarrow.parquet as pq
In [8]: pq.write_table(table, 'example.parquet')
This creates a single Parquet file. In practice, a Parquet dataset may consist
of many files in many directories. We can read a single file back with
read_table:
In [9]: table2 = pq.read_table('example.parquet')
In [10]: table2.to_pandas()
Out[10]:
one two three
a -1.0 foo True
b NaN bar False
c 2.5 baz True
You can pass a subset of columns to read, which can be much faster than reading the whole file (due to the columnar layout):
In [11]: pq.read_table('example.parquet', columns=['one', 'three'])
Out[11]:
pyarrow.Table
one: double
three: bool
----
one: [[-1,null,2.5]]
three: [[true,false,true]]
When reading a subset of columns from a file that used a Pandas dataframe as the
source, we use read_pandas to maintain any additional index column data:
In [12]: pq.read_pandas('example.parquet', columns=['two']).to_pandas()
Out[12]:
two
a foo
b bar
c baz
We do not need to use a string to specify the origin of the file. It can be any of:
A file path as a string
A NativeFile from PyArrow
A Python file object
In general, a Python file object will have the worst read performance, while a
string file path or an instance of NativeFile (especially memory
maps) will perform the best.
Reading Parquet and Memory Mapping¶
Because Parquet data needs to be decoded from the Parquet format
and compression, it can’t be directly mapped from disk.
Thus the memory_map option might perform better on some systems
but won’t help much with resident memory consumption.
>>> pq_array = pa.parquet.read_table("area1.parquet", memory_map=True)
>>> print("RSS: {}MB".format(pa.total_allocated_bytes() >> 20))
RSS: 4299MB
>>> pq_array = pa.parquet.read_table("area1.parquet", memory_map=False)
>>> print("RSS: {}MB".format(pa.total_allocated_bytes() >> 20))
RSS: 4299MB
If you need to deal with Parquet data bigger than memory, the Tabular Datasets and partitioning is probably what you are looking for.
Parquet file writing options¶
write_table() has a number of options to
control various settings when writing a Parquet file.
version, the Parquet format version to use.'1.0'ensures compatibility with older readers, while'2.4'and greater values enable more Parquet types and encodings.data_page_size, to control the approximate size of encoded data pages within a column chunk. This currently defaults to 1MB.flavor, to set compatibility options particular to a Parquet consumer like'spark'for Apache Spark.
See the write_table() docstring for more details.
There are some additional data type handling-specific options described below.
Omitting the DataFrame index¶
When using pa.Table.from_pandas to convert to an Arrow table, by default
one or more special columns are added to keep track of the index (row
labels). Storing the index takes extra space, so if your index is not valuable,
you may choose to omit it by passing preserve_index=False
In [13]: df = pd.DataFrame({'one': [-1, np.nan, 2.5],
....: 'two': ['foo', 'bar', 'baz'],
....: 'three': [True, False, True]},
....: index=list('abc'))
....:
In [14]: df
Out[14]:
one two three
a -1.0 foo True
b NaN bar False
c 2.5 baz True
In [15]: table = pa.Table.from_pandas(df, preserve_index=False)
Then we have:
In [16]: pq.write_table(table, 'example_noindex.parquet')
In [17]: t = pq.read_table('example_noindex.parquet')
In [18]: t.to_pandas()
Out[18]:
one two three
0 -1.0 foo True
1 NaN bar False
2 2.5 baz True
Here you see the index did not survive the round trip.
Finer-grained Reading and Writing¶
read_table uses the ParquetFile class, which has other features:
In [19]: parquet_file = pq.ParquetFile('example.parquet')
In [20]: parquet_file.metadata
Out[20]:
<pyarrow._parquet.FileMetaData object at 0x7f6d7b12de50>
created_by: parquet-cpp-arrow version 13.0.0
num_columns: 4
num_rows: 3
num_row_groups: 1
format_version: 2.6
serialized_size: 2576
In [21]: parquet_file.schema
Out[21]:
<pyarrow._parquet.ParquetSchema object at 0x7f6d7b0f2340>
required group field_id=-1 schema {
optional double field_id=-1 one;
optional binary field_id=-1 two (String);
optional boolean field_id=-1 three;
optional binary field_id=-1 __index_level_0__ (String);
}
As you can learn more in the Apache Parquet format, a Parquet file consists of
multiple row groups. read_table will read all of the row groups and
concatenate them into a single table. You can read individual row groups with
read_row_group:
In [22]: parquet_file.num_row_groups
Out[22]: 1
In [23]: parquet_file.read_row_group(0)
Out[23]:
pyarrow.Table
one: double
two: string
three: bool
__index_level_0__: string
----
one: [[-1,null,2.5]]
two: [["foo","bar","baz"]]
three: [[true,false,true]]
__index_level_0__: [["a","b","c"]]
We can similarly write a Parquet file with multiple row groups by using
ParquetWriter:
In [24]: with pq.ParquetWriter('example2.parquet', table.schema) as writer:
....: for i in range(3):
....: writer.write_table(table)
....:
In [25]: pf2 = pq.ParquetFile('example2.parquet')
In [26]: pf2.num_row_groups
Out[26]: 3
Inspecting the Parquet File Metadata¶
The FileMetaData of a Parquet file can be accessed through
ParquetFile as shown above:
In [27]: parquet_file = pq.ParquetFile('example.parquet')
In [28]: metadata = parquet_file.metadata
or can also be read directly using read_metadata():
In [29]: metadata = pq.read_metadata('example.parquet')
In [30]: metadata
Out[30]:
<pyarrow._parquet.FileMetaData object at 0x7f6d7b079b80>
created_by: parquet-cpp-arrow version 13.0.0
num_columns: 4
num_rows: 3
num_row_groups: 1
format_version: 2.6
serialized_size: 2576
The returned FileMetaData object allows to inspect the
Parquet file metadata,
such as the row groups and column chunk metadata and statistics:
In [31]: metadata.row_group(0)
Out[31]:
<pyarrow._parquet.RowGroupMetaData object at 0x7f6d7b079ef0>
num_columns: 4
num_rows: 3
total_byte_size: 282
In [32]: metadata.row_group(0).column(0)
Out[32]:
<pyarrow._parquet.ColumnChunkMetaData object at 0x7f6d7b0816d0>
file_offset: 108
file_path:
physical_type: DOUBLE
num_values: 3
path_in_schema: one
is_stats_set: True
statistics:
<pyarrow._parquet.Statistics object at 0x7f6d7b081810>
has_min_max: True
min: -1.0
max: 2.5
null_count: 1
distinct_count: 0
num_values: 2
physical_type: DOUBLE
logical_type: None
converted_type (legacy): NONE
compression: SNAPPY
encodings: ('PLAIN', 'RLE', 'RLE_DICTIONARY')
has_dictionary_page: True
dictionary_page_offset: 4
data_page_offset: 36
total_compressed_size: 104
total_uncompressed_size: 100
Data Type Handling¶
Reading types as DictionaryArray¶
The read_dictionary option in read_table and ParquetDataset will
cause columns to be read as DictionaryArray, which will become
pandas.Categorical when converted to pandas. This option is only valid for
string and binary column types, and it can yield significantly lower memory use
and improved performance for columns with many repeated string values.
pq.read_table(table, where, read_dictionary=['binary_c0', 'stringb_c2'])
Storing timestamps¶
Some Parquet readers may only support timestamps stored in millisecond
('ms') or microsecond ('us') resolution. Since pandas uses nanoseconds
to represent timestamps, this can occasionally be a nuisance. By default
(when writing version 1.0 Parquet files), the nanoseconds will be cast to
microseconds (‘us’).
In addition, We provide the coerce_timestamps option to allow you to select
the desired resolution:
pq.write_table(table, where, coerce_timestamps='ms')
If a cast to a lower resolution value may result in a loss of data, by default
an exception will be raised. This can be suppressed by passing
allow_truncated_timestamps=True:
pq.write_table(table, where, coerce_timestamps='ms',
allow_truncated_timestamps=True)
Timestamps with nanoseconds can be stored without casting when using the more recent Parquet format version 2.6:
pq.write_table(table, where, version='2.6')
However, many Parquet readers do not yet support this newer format version, and therefore the default is to write version 1.0 files. When compatibility across different processing frameworks is required, it is recommended to use the default version 1.0.
Older Parquet implementations use INT96 based storage of
timestamps, but this is now deprecated. This includes some older
versions of Apache Impala and Apache Spark. To write timestamps in
this format, set the use_deprecated_int96_timestamps option to
True in write_table.
pq.write_table(table, where, use_deprecated_int96_timestamps=True)
Compression, Encoding, and File Compatibility¶
The most commonly used Parquet implementations use dictionary encoding when
writing files; if the dictionaries grow too large, then they “fall back” to
plain encoding. Whether dictionary encoding is used can be toggled using the
use_dictionary option:
pq.write_table(table, where, use_dictionary=False)
The data pages within a column in a row group can be compressed after the encoding passes (dictionary, RLE encoding). In PyArrow we use Snappy compression by default, but Brotli, Gzip, ZSTD, LZ4, and uncompressed are also supported:
pq.write_table(table, where, compression='snappy')
pq.write_table(table, where, compression='gzip')
pq.write_table(table, where, compression='brotli')
pq.write_table(table, where, compression='zstd')
pq.write_table(table, where, compression='lz4')
pq.write_table(table, where, compression='none')
Snappy generally results in better performance, while Gzip may yield smaller files.
These settings can also be set on a per-column basis:
pq.write_table(table, where, compression={'foo': 'snappy', 'bar': 'gzip'},
use_dictionary=['foo', 'bar'])
Partitioned Datasets (Multiple Files)¶
Multiple Parquet files constitute a Parquet dataset. These may present in a number of ways:
A list of Parquet absolute file paths
A directory name containing nested directories defining a partitioned dataset
A dataset partitioned by year and month may look like on disk:
dataset_name/
year=2007/
month=01/
0.parq
1.parq
...
month=02/
0.parq
1.parq
...
month=03/
...
year=2008/
month=01/
...
...
Writing to Partitioned Datasets¶
You can write a partitioned dataset for any pyarrow file system that is a
file-store (e.g. local, HDFS, S3). The default behaviour when no filesystem is
added is to use the local filesystem.
# Local dataset write
pq.write_to_dataset(table, root_path='dataset_name',
partition_cols=['one', 'two'])
The root path in this case specifies the parent directory to which data will be saved. The partition columns are the column names by which to partition the dataset. Columns are partitioned in the order they are given. The partition splits are determined by the unique values in the partition columns.
To use another filesystem you only need to add the filesystem parameter, the
individual table writes are wrapped using with statements so the
pq.write_to_dataset function does not need to be.
# Remote file-system example
from pyarrow.fs import HadoopFileSystem
fs = HadoopFileSystem(host, port, user=user, kerb_ticket=ticket_cache_path)
pq.write_to_dataset(table, root_path='dataset_name',
partition_cols=['one', 'two'], filesystem=fs)
Compatibility Note: if using pq.write_to_dataset to create a table that
will then be used by HIVE then partition column values must be compatible with
the allowed character set of the HIVE version you are running.
Writing _metadata and _common_metadata files¶
Some processing frameworks such as Spark or Dask (optionally) use _metadata
and _common_metadata files with partitioned datasets.
Those files include information about the schema of the full dataset (for
_common_metadata) and potentially all row group metadata of all files in the
partitioned dataset as well (for _metadata). The actual files are
metadata-only Parquet files. Note this is not a Parquet standard, but a
convention set in practice by those frameworks.
Using those files can give a more efficient creation of a parquet Dataset, since it can use the stored schema and and file paths of all row groups, instead of inferring the schema and crawling the directories for all Parquet files (this is especially the case for filesystems where accessing files is expensive).
The write_to_dataset() function does not automatically
write such metadata files, but you can use it to gather the metadata and
combine and write them manually:
# Write a dataset and collect metadata information of all written files
metadata_collector = []
pq.write_to_dataset(table, root_path, metadata_collector=metadata_collector)
# Write the ``_common_metadata`` parquet file without row groups statistics
pq.write_metadata(table.schema, root_path / '_common_metadata')
# Write the ``_metadata`` parquet file with row groups statistics of all files
pq.write_metadata(
table.schema, root_path / '_metadata',
metadata_collector=metadata_collector
)
When not using the write_to_dataset() function, but
writing the individual files of the partitioned dataset using
write_table() or ParquetWriter,
the metadata_collector keyword can also be used to collect the FileMetaData
of the written files. In this case, you need to ensure to set the file path
contained in the row group metadata yourself before combining the metadata, and
the schemas of all different files and collected FileMetaData objects should be
the same:
metadata_collector = []
pq.write_table(
table1, root_path / "year=2017/data1.parquet",
metadata_collector=metadata_collector
)
# set the file path relative to the root of the partitioned dataset
metadata_collector[-1].set_file_path("year=2017/data1.parquet")
# combine and write the metadata
metadata = metadata_collector[0]
for _meta in metadata_collector[1:]:
metadata.append_row_groups(_meta)
metadata.write_metadata_file(root_path / "_metadata")
# or use pq.write_metadata to combine and write in a single step
pq.write_metadata(
table1.schema, root_path / "_metadata",
metadata_collector=metadata_collector
)
Reading from Partitioned Datasets¶
The ParquetDataset class accepts either a directory name or a list
of file paths, and can discover and infer some common partition structures,
such as those produced by Hive:
dataset = pq.ParquetDataset('dataset_name/')
table = dataset.read()
You can also use the convenience function read_table exposed by
pyarrow.parquet that avoids the need for an additional Dataset object
creation step.
table = pq.read_table('dataset_name')
Note: the partition columns in the original table will have their types
converted to Arrow dictionary types (pandas categorical) on load. Ordering of
partition columns is not preserved through the save/load process. If reading
from a remote filesystem into a pandas dataframe you may need to run
sort_index to maintain row ordering (as long as the preserve_index
option was enabled on write).
Note
The ParquetDataset is being reimplemented based on the new generic Dataset
API (see the Tabular Datasets docs for an overview). This is not yet the
default, but can already be enabled by passing the use_legacy_dataset=False
keyword to ParquetDataset or read_table():
pq.ParquetDataset('dataset_name/', use_legacy_dataset=False)
Enabling this gives the following new features:
Filtering on all columns (using row group statistics) instead of only on the partition keys.
More fine-grained partitioning: support for a directory partitioning scheme in addition to the Hive-like partitioning (e.g. “/2019/11/15/” instead of “/year=2019/month=11/day=15/”), and the ability to specify a schema for the partition keys.
General performance improvement and bug fixes.
It also has the following changes in behaviour:
The partition keys need to be explicitly included in the
columnskeyword when you want to include them in the result while reading a subset of the columns
This new implementation is already enabled in read_table, and in the
future, this will be turned on by default for ParquetDataset. The new
implementation does not yet cover all existing ParquetDataset features (e.g.
specifying the metadata, or the pieces property API). Feedback is
very welcome.
Using with Spark¶
Spark places some constraints on the types of Parquet files it will read. The
option flavor='spark' will set these options automatically and also
sanitize field characters unsupported by Spark SQL.
Multithreaded Reads¶
Each of the reading functions by default use multi-threading for reading columns in parallel. Depending on the speed of IO and how expensive it is to decode the columns in a particular file (particularly with GZIP compression), this can yield significantly higher data throughput.
This can be disabled by specifying use_threads=False.
Note
The number of threads to use concurrently is automatically inferred by Arrow
and can be inspected using the cpu_count() function.
Reading from cloud storage¶
In addition to local files, pyarrow supports other filesystems, such as cloud
filesystems, through the filesystem keyword:
from pyarrow import fs
s3 = fs.S3FileSystem(region="us-east-2")
table = pq.read_table("bucket/object/key/prefix", filesystem=s3)
Currently, HDFS and
Amazon S3-compatible storage are
supported. See the Filesystem Interface docs for more details. For those
built-in filesystems, the filesystem can also be inferred from the file path,
if specified as a URI:
table = pq.read_table("s3://bucket/object/key/prefix")
Other filesystems can still be supported if there is an fsspec-compatible implementation available. See Using fsspec-compatible filesystems with Arrow for more details. One example is Azure Blob storage, which can be interfaced through the adlfs package.
from adlfs import AzureBlobFileSystem
abfs = AzureBlobFileSystem(account_name="XXXX", account_key="XXXX", container_name="XXXX")
table = pq.read_table("file.parquet", filesystem=abfs)
Parquet Modular Encryption (Columnar Encryption)¶
Columnar encryption is supported for Parquet files in C++ starting from Apache Arrow 4.0.0 and in PyArrow starting from Apache Arrow 6.0.0.
Parquet uses the envelope encryption practice, where file parts are encrypted with “data encryption keys” (DEKs), and the DEKs are encrypted with “master encryption keys” (MEKs). The DEKs are randomly generated by Parquet for each encrypted file/column. The MEKs are generated, stored and managed in a Key Management Service (KMS) of user’s choice.
Reading and writing encrypted Parquet files involves passing file encryption
and decryption properties to ParquetWriter and to
ParquetFile, respectively.
Writing an encrypted Parquet file:
encryption_properties = crypto_factory.file_encryption_properties(
kms_connection_config, encryption_config)
with pq.ParquetWriter(filename, schema,
encryption_properties=encryption_properties) as writer:
writer.write_table(table)
Reading an encrypted Parquet file:
decryption_properties = crypto_factory.file_decryption_properties(
kms_connection_config)
parquet_file = pq.ParquetFile(filename,
decryption_properties=decryption_properties)
In order to create the encryption and decryption properties, a
pyarrow.parquet.encryption.CryptoFactory should be created and
initialized with KMS Client details, as described below.
KMS Client¶
The master encryption keys should be kept and managed in a production-grade
Key Management System (KMS), deployed in the user’s organization. Using Parquet
encryption requires implementation of a client class for the KMS server.
Any KmsClient implementation should implement the informal interface
defined by pyarrow.parquet.encryption.KmsClient as following:
import pyarrow.parquet.encryption as pe
class MyKmsClient(pe.KmsClient):
"""An example KmsClient implementation skeleton"""
def __init__(self, kms_connection_configuration):
pe.KmsClient.__init__(self)
# Any KMS-specific initialization based on
# kms_connection_configuration comes here
def wrap_key(self, key_bytes, master_key_identifier):
wrapped_key = ... # call KMS to wrap key_bytes with key specified by
# master_key_identifier
return wrapped_key
def unwrap_key(self, wrapped_key, master_key_identifier):
key_bytes = ... # call KMS to unwrap wrapped_key with key specified by
# master_key_identifier
return key_bytes
The concrete implementation will be loaded at runtime by a factory function
provided by the user. This factory function will be used to initialize the
pyarrow.parquet.encryption.CryptoFactory for creating file encryption
and decryption properties.
For example, in order to use the MyKmsClient defined above:
def kms_client_factory(kms_connection_configuration):
return MyKmsClient(kms_connection_configuration)
crypto_factory = CryptoFactory(kms_client_factory)
An example
of such a class for an open source
KMS can be found in the Apache
Arrow GitHub repository. The production KMS client should be designed in
cooperation with an organization’s security administrators, and built by
developers with experience in access control management. Once such a class is
created, it can be passed to applications via a factory method and leveraged
by general PyArrow users as shown in the encrypted parquet write/read sample
above.
KMS connection configuration¶
Configuration of connection to KMS (pyarrow.parquet.encryption.KmsConnectionConfig
used when creating file encryption and decryption properties) includes the
following options:
kms_instance_url, URL of the KMS instance.kms_instance_id, ID of the KMS instance that will be used for encryption (if multiple KMS instances are available).key_access_token, authorization token that will be passed to KMS.custom_kms_conf, a string dictionary with KMS-type-specific configuration.
Encryption configuration¶
pyarrow.parquet.encryption.EncryptionConfiguration (used when
creating file encryption properties) includes the following options:
footer_key, the ID of the master key for footer encryption/signing.column_keys, which columns to encrypt with which key. Dictionary with master key IDs as the keys, and column name lists as the values, e.g.{key1: [col1, col2], key2: [col3]}.encryption_algorithm, the Parquet encryption algorithm. Can beAES_GCM_V1(default) orAES_GCM_CTR_V1.plaintext_footer, whether to write the file footer in plain text (otherwise it is encrypted).double_wrapping, whether to use double wrapping - where data encryption keys (DEKs) are encrypted with key encryption keys (KEKs), which in turn are encrypted with master encryption keys (MEKs). If set tofalse, single wrapping is used - where DEKs are encrypted directly with MEKs.cache_lifetime, the lifetime of cached entities (key encryption keys, local wrapping keys, KMS client objects) represented as adatetime.timedelta.internal_key_material, whether to store key material inside Parquet file footers; this mode doesn’t produce additional files. If set tofalse, key material is stored in separate files in the same folder, which enables key rotation for immutable Parquet files.data_key_length_bits, the length of data encryption keys (DEKs), randomly generated by Parquet key management tools. Can be 128, 192 or 256 bits.
Note
When double_wrapping is true, Parquet implements a “double envelope
encryption” mode that minimizes the interaction of the program with a KMS
server. In this mode, the DEKs are encrypted with “key encryption keys”
(KEKs, randomly generated by Parquet). The KEKs are encrypted with “master
encryption keys” (MEKs) in the KMS; the result and the KEK itself are
cached in the process memory.
An example encryption configuration:
encryption_config = pq.EncryptionConfiguration(
footer_key="footer_key_name",
column_keys={
"column_key_name": ["Column1", "Column2"],
},
)
Decryption configuration¶
pyarrow.parquet.encryption.DecryptionConfiguration (used when creating
file decryption properties) is optional and it includes the following options:
cache_lifetime, the lifetime of cached entities (key encryption keys, local wrapping keys, KMS client objects) represented as adatetime.timedelta.