Example of reading and writing parquet metadata outside the file

parquet/Cargo.toml

@@ -118,6 +118,12 @@ zstd = ["dep:zstd", "zstd-sys"]
 # Display memory in example/write_parquet.rs
 sysinfo = ["dep:sysinfo"]
 
+
+[[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 +1,217 @@
+// 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 parquet::arrow::arrow_reader::{
+    ArrowReaderMetadata, ArrowReaderOptions, ParquetRecordBatchReaderBuilder,
+};
+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.
+///
+/// # Usecase
+///
+/// 1. Read Parquet metadata from an existing Parquet file.
+///
+/// 2. Store that metadata somewhere other than the rest of the parquet data
+///    (e.g. a cache) that is not necessarily in RAM
+///
+/// 3. Use the metadata to determine of the file should be read, and if so,
+///   read the data stored in that Parquet file, without re-reading / reparsing
+///   the metadata.
+///
+/// Note that Parquet metadata is not necessarily contiguous in the files: part
+/// is  stored in the footer (the last bytes of the file), but other portions
+/// (such as the PageIndex) can be stored elsewhere,
+///
+/// You can use the these APIs to store a copy of the metadata for parquet files
+/// stored on remote object storage (e.g. S3) in 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 those  files with a single
+/// object store request.
+///
+/// # Specifically, this example:
+/// 1. Reads the metadata of a Parquet file
+/// 2. Removes some column statistics from the metadata (to make them smaller)
+/// 3. Stores the metadata in a separate file
+/// 4. Reads the metadata from the separate file and uses that to read the Parquet file
+///
+/// Without this API, to implement the functionality you would need to implement
+/// a conversion of the `ParquetMetaData` and related structures to/from some
+/// other structs that can be serialized/deserialized.
+
+#[tokio::main(flavor = "current_thread")]
+async fn main() -> parquet::errors::Result<()> {
+    let tempdir = TempDir::new().unwrap();
+    println!("data in {tempdir:?}");
+    let parquet_path = create_parquet_file(&tempdir);
+    let metadata_path = tempdir.path().join("thrift_metadata.dat");
+    // temp: don't clean up tempdir
+    std::mem::forget(tempdir);
+
+    let metadata = get_metadata_from_parquet_file(&parquet_path).await;
+    println!(
+        "Read metadata from Parquet file into memory: {} bytes",
+        metadata.memory_size()
+    );
+    let metadata = prepare_metadata(metadata);
+    write_metadata_to_file(metadata, &metadata_path);
+
+    // now read the metadata from the file and use it to read the Parquet file
+    let metadata = read_metadata_from_file(&metadata_path);
+    println!("Read metadata from file: {metadata:#?}");
+
+    let batches = read_parquet_file_with_metadata(&parquet_path, metadata);
+
+    // 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 | grames      |",
+            "| 33  | pears       |",
+            "+-----+-------------+",
+        ],
+        "actual output:\n\n{batches_lines:#?}"
+    );
+
+    Ok(())
+}
+
+/// Reads the metadata from a parquet file
+async fn get_metadata_from_parquet_file(file: impl AsRef<Path>) -> ParquetMetaData {
+    // pretend we are reading the metadata from a remote object store
+    let file = std::fs::File::open(file).unwrap();
+    let file = tokio::fs::File::from_std(file);
+
+    // tell the reader to read the page index
+    let reader_options = ArrowReaderOptions::new().with_page_index(true);
+
+    let builder = ParquetRecordBatchStreamBuilder::new_with_options(file, reader_options)
+        .await
+        .unwrap();
+
+    // The metadata is Arc'd -- so unwrap it after dropping the builder
+    let metadata = Arc::clone(builder.metadata());
+    drop(builder);
+    Arc::try_unwrap(metadata).unwrap()
+}
+
+/// modifies the metadata to reduce its size
+fn prepare_metadata(metadata: ParquetMetaData) -> ParquetMetaData {
+    // maybe we will do this
+    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_file(metadata: ParquetMetaData, file: impl AsRef<Path>) {
+    let file = File::create(file).unwrap();
+    let writer = ParquetMetaDataWriter::new(file, &metadata);
+    writer.finish().unwrap()
+}
+
+/// Reads the metadata from a file
+///
+/// This function reads the format written by `write_metadata_to_file`
+fn read_metadata_from_file(file: impl AsRef<Path>) -> ParquetMetaData {
+    let file = File::open(file).unwrap();
+    ParquetMetaDataReader::new()
+        .with_column_indexes(true)
+        .with_offset_indexes(true)
+        .parse_and_finish(&file)
+        .unwrap()
+}
+
+/// Reads the 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.
+///
+/// In this example, we read the results as Arrow record batches
+fn read_parquet_file_with_metadata(
+    file: impl AsRef<Path>,
+    metadata: ParquetMetaData,
+) -> Vec<RecordBatch> {
+    let file = std::fs::File::open(file).unwrap();
+    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 = ParquetRecordBatchReaderBuilder::new_with_metadata(file, arrow_reader_metadata)
+        .build()
+        .unwrap();
+
+    reader.collect::<arrow::error::Result<Vec<_>>>().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",
+                "grames",
+                "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
+}