Example of reading and writing parquet metadata outside the file

parquet/Cargo.toml

@@ -121,6 +121,12 @@ sysinfo = ["dep:sysinfo"]
 # Verify 32-bit CRC checksum when decoding parquet pages
 crc = ["dep:crc32fast"]
 
+
+[[example]]
+name = "external_metadata"
+required-features = ["arrow", "async"]
+path = "./examples/external_metadata.rs"
+
 [[example]]
 name = "read_parquet"
 required-features = ["arrow"]

parquet/examples/external_metadata.rs

@@ -0,0 +1,236 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use arrow_array::{ArrayRef, Int32Array, RecordBatch, StringArray};
+use arrow_cast::pretty::pretty_format_batches;
+use futures::TryStreamExt;
+use parquet::arrow::arrow_reader::{ArrowReaderMetadata, ArrowReaderOptions};
+use parquet::arrow::{ArrowWriter, ParquetRecordBatchStreamBuilder};
+use parquet::file::metadata::{ParquetMetaData, ParquetMetaDataReader, ParquetMetaDataWriter};
+use parquet::file::properties::{EnabledStatistics, WriterProperties};
+use std::fs::File;
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+use tempfile::TempDir;
+
+/// This example demonstrates advanced usage of the Parquet metadata APIs.
+///
+/// It is sometimes desired to copy the metadata for parquet files stored on
+/// remote object storage (e.g. S3) to a local file or an in-memory cache, use a
+/// query engine like DataFusion to analyze the metadata to determine which file
+/// to read, and then read any matching files with a single subsequent object
+/// store request.
+///
+/// # Example Overview
+///
+/// 1. Reads the metadata of a Parquet file using [`ParquetMetaDataReader`]
+///
+/// 2. Removes column statistics from the metadata (to make it smaller)
+///
+/// 3. Stores the metadata in a separate file using [`ParquetMetaDataWriter`]
+///
+/// 4. Reads the metadata from the separate file and uses that to read the
+///    Parquet file, thus avoiding a second IO to read metadata or reparsing
+///    the footer.
+///
+#[tokio::main(flavor = "current_thread")]
+async fn main() -> parquet::errors::Result<()> {
+    let tempdir = TempDir::new().unwrap();
+    let parquet_path = create_parquet_file(&tempdir);
+    let metadata_path = tempdir.path().join("thrift_metadata.dat");
+
+    // In this example, we use a tokio file to mimic an async remote data source
+    let mut remote_parquet_file = tokio::fs::File::open(&parquet_path).await?;
+
+    let metadata = get_metadata_from_remote_parquet_file(&mut remote_parquet_file).await;
+    println!(
+        "Metadata from 'remote' Parquet file into memory: {} bytes",
+        metadata.memory_size()
+    );
+
+    // now slim down the metadata and write it to a "local" file
+    let metadata = prepare_metadata(metadata);
+    write_metadata_to_local_file(metadata, &metadata_path);
+
+    // now read the metadata from the local file and use it to read the "remote" Parquet file
+    let metadata = read_metadata_from_local_file(&metadata_path);
+    println!("Read metadata from file");
+
+    let batches = read_remote_parquet_file_with_metadata(remote_parquet_file, metadata).await;
+
+    // display the results
+    let batches_string = pretty_format_batches(&batches).unwrap().to_string();
+    let batches_lines: Vec<_> = batches_string.split('\n').collect();
+
+    assert_eq!(
+        batches_lines,
+        [
+            "+-----+-------------+",
+            "| id  | description |",
+            "+-----+-------------+",
+            "| 100 | oranges     |",
+            "| 200 | apples      |",
+            "| 201 | grapefruit  |",
+            "| 300 | bannanas    |",
+            "| 102 | grapes      |",
+            "| 33  | pears       |",
+            "+-----+-------------+",
+        ],
+        "actual output:\n\n{batches_lines:#?}"
+    );
+
+    Ok(())
+}
+
+/// Reads the metadata from a "remote" parquet file
+///
+/// Note that this function models reading from a remote file source using a
+/// tokio file. In a real application, you would implement [`MetadataFetch`] for
+/// your own remote source.
+///
+/// [`MetadataFetch`]: parquet::arrow::async_reader::MetadataFetch
+async fn get_metadata_from_remote_parquet_file(
+    remote_file: &mut tokio::fs::File,
+) -> ParquetMetaData {
+    // the remote source must know the total file size (e.g. from an object store LIST operation)
+    let file_size = remote_file.metadata().await.unwrap().len();
+
+    // tell the reader to read the page index
+    ParquetMetaDataReader::new()
+        .with_page_indexes(true)
+        .load_and_finish(remote_file, file_size as usize)
+        .await
+        .unwrap()
+}
+
+/// modifies the metadata to reduce its size
+fn prepare_metadata(metadata: ParquetMetaData) -> ParquetMetaData {
+    let orig_size = metadata.memory_size();
+
+    let mut builder = metadata.into_builder();
+
+    // remove column statistics to reduce the size of the metadata by converting
+    // the various structures into their respective builders and modifying them
+    // as needed.
+    for row_group in builder.take_row_groups() {
+        let mut row_group_builder = row_group.into_builder();
+        for column in row_group_builder.take_columns() {
+            let column = column.into_builder().clear_statistics().build().unwrap();
+            row_group_builder = row_group_builder.add_column_metadata(column);
+        }
+        let row_group = row_group_builder.build().unwrap();
+        builder = builder.add_row_group(row_group);
+    }
+    let metadata = builder.build();
+
+    // verifiy that the size has indeed been reduced
+    let new_size = metadata.memory_size();
+    assert!(new_size < orig_size, "metadata size did not decrease");
+    println!("Reduced metadata size from {} to {}", orig_size, new_size);
+    metadata
+}
+
+/// writes the metadata to a file
+///
+/// The data is stored using the same thrift format as the Parquet file metadata
+fn write_metadata_to_local_file(metadata: ParquetMetaData, file: impl AsRef<Path>) {
+    let file = File::create(file).unwrap();
+    ParquetMetaDataWriter::new(file, &metadata)
+        .finish()
+        .unwrap()
+}
+
+/// Reads the metadata from a file
+///
+/// This function reads the format written by `write_metadata_to_file`
+fn read_metadata_from_local_file(file: impl AsRef<Path>) -> ParquetMetaData {
+    let file = File::open(file).unwrap();
+    ParquetMetaDataReader::new()
+        .with_page_indexes(true)
+        .parse_and_finish(&file)
+        .unwrap()
+}
+
+/// Reads the "remote" Parquet file using the metadata
+///
+/// This shows how to read the Parquet file using previously read metadata
+/// instead of the metadata in the Parquet file itself. This avoids an IO /
+/// having to fetch and decode the metadata from the Parquet file before
+/// beginning to read it.
+///
+/// Note that this function models reading from a remote file source using a
+/// tokio file. In a real application, you would implement [`AsyncFileReader`]
+/// for your own remote source.
+///
+/// In this example, we simply buffer the results in memory as Arrow record
+/// batches but a real application would likely process the batches as they are
+/// read.
+///
+/// [`AsyncFileReader`]: parquet::arrow::async_reader::AsyncFileReader
+async fn read_remote_parquet_file_with_metadata(
+    remote_file: tokio::fs::File,
+    metadata: ParquetMetaData,
+) -> Vec<RecordBatch> {
+    let options = ArrowReaderOptions::new()
+        // tell the reader to read the page index
+        .with_page_index(true);
+    // create a reader with pre-existing metadata
+    let arrow_reader_metadata = ArrowReaderMetadata::try_new(metadata.into(), options).unwrap();
+    let reader =
+        ParquetRecordBatchStreamBuilder::new_with_metadata(remote_file, arrow_reader_metadata)
+            .build()
+            .unwrap();
+
+    reader.try_collect::<Vec<_>>().await.unwrap()
+}
+
+/// Make a new parquet file in the temporary directory, and returns the path
+fn create_parquet_file(tmpdir: &TempDir) -> PathBuf {
+    let path = tmpdir.path().join("example.parquet");
+    let new_file = File::create(&path).unwrap();
+
+    let batch = RecordBatch::try_from_iter(vec![
+        (
+            "id",
+            Arc::new(Int32Array::from(vec![100, 200, 201, 300, 102, 33])) as ArrayRef,
+        ),
+        (
+            "description",
+            Arc::new(StringArray::from(vec![
+                "oranges",
+                "apples",
+                "grapefruit",
+                "bannanas",
+                "grapes",
+                "pears",
+            ])),
+        ),
+    ])
+    .unwrap();
+
+    let props = WriterProperties::builder()
+        // ensure we write the page index level statistics
+        .set_statistics_enabled(EnabledStatistics::Page)
+        .build();
+
+    let mut writer = ArrowWriter::try_new(new_file, batch.schema(), Some(props)).unwrap();
+
+    writer.write(&batch).unwrap();
+    writer.finish().unwrap();
+
+    path
+}

parquet/src/file/metadata/mod.rs

@@ -36,30 +36,27 @@
 //!
 //! # APIs for working with Parquet Metadata
 //!
-//! The Parquet readers and writers in this crate read and write
+//! The Parquet readers and writers in this crate handle reading and writing
 //! metadata into parquet files. To work with metadata directly,
-//! the following APIs are available.
+//! the following APIs are available:
 //!
-//! Reading:
-//! * Read from bytes to `ParquetMetaData`: [`decode_footer`]
-//!   and [`decode_metadata`]
-//! * Read from an `async` source to `ParquetMetaData`: [`MetadataLoader`]
-//! * Read from bytes or from an async source to `ParquetMetaData`: [`ParquetMetaDataReader`]
+//! * [`ParquetMetaDataReader`] for reading
+//! * [`ParquetMetaDataWriter`] for writing.
 //!
-//! [`MetadataLoader`]: https://docs.rs/parquet/latest/parquet/arrow/async_reader/struct.MetadataLoader.html
-//! [`decode_footer`]: crate::file::footer::decode_footer
-//! [`decode_metadata`]: crate::file::footer::decode_metadata
+//! [`ParquetMetaDataReader`]: https://docs.rs/parquet/latest/parquet/file/metadata/struct.ParquetMetaDataReader.html
+//! [`ParquetMetaDataWriter`]: https://docs.rs/parquet/latest/parquet/file/metadata/struct.ParquetMetaDataWriter.html
 //!
-//! Writing:
-//! * Write `ParquetMetaData` to bytes in memory: [`ParquetMetaDataWriter`]
-//! * Writes `ParquetMetaData` to an async target: Not yet supported
+//! # Examples
 //!
+//! Please see [`external_metadata.rs`]
+//!
+//! [`external_metadata.rs`]: https://github.com/apache/arrow-rs/tree/master/parquet/examples/external_metadata.rs
 //!
 //! # Metadata Encodings and Structures
 //!
 //! There are three different encodings of Parquet Metadata in this crate:
 //!
-//! 1. `bytes`:encoded with the Thrift TCompactProtocol as defined in
+//! 1. `bytes`:encoded with the Thrift `TCompactProtocol` as defined in
 //!    [parquet.thrift]
 //!
 //! 2. [`format`]: Rust structures automatically generated by the thrift compiler