etseidl commented on code in PR #8080: URL: https://github.com/apache/arrow-rs/pull/8080#discussion_r2277889858
########## parquet/src/file/metadata/push_decoder.rs: ########## @@ -0,0 +1,553 @@ +// 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 crate::errors::ParquetError; +use crate::file::metadata::{ParquetMetaData, ParquetMetaDataReader}; +use crate::DecodeResult; + +/// A push decoder for [`ParquetMetaData`]. +/// +/// This structure implements a push API based version of the [`ParquetMetaDataReader`], which +/// decouples the IO from the metadata decoding logic. +/// +/// You can use this decoder to customize your IO operations, as shown in the +/// examples below for minimizing bytes read, prefetching data, or +/// using async IO. +/// +/// # Example +/// +/// The most basic usage is to feed the decoder with the necessary byte ranges +/// as requested as shown below. +/// +/// ```rust +/// # use std::ops::Range; +/// # use bytes::Bytes; +/// # use arrow_array::record_batch; +/// # use parquet::DecodeResult; +/// # use parquet::arrow::ArrowWriter; +/// # use parquet::errors::ParquetError; +/// # use parquet::file::metadata::{ParquetMetaData, ParquetMetaDataPushDecoder}; +/// # +/// # fn decode_metadata() -> Result<ParquetMetaData, ParquetError> { +/// # let file_bytes = { +/// # let mut buffer = vec![0]; +/// # let batch = record_batch!(("a", Int32, [1, 2, 3])).unwrap(); +/// # let mut writer = ArrowWriter::try_new(&mut buffer, batch.schema(), None).unwrap(); +/// # writer.write(&batch).unwrap(); +/// # writer.close().unwrap(); +/// # Bytes::from(buffer) +/// # }; +/// # // mimic IO by returning a function that returns the bytes for a given range +/// # let get_range = |range: &Range<u64>| -> Bytes { +/// # let start = range.start as usize; +/// # let end = range.end as usize; +/// # file_bytes.slice(start..end) +/// # }; +/// # +/// # let file_len = file_bytes.len() as u64; +/// // The `ParquetMetaDataPushDecoder` needs to know the file length. +/// let mut decoder = ParquetMetaDataPushDecoder::try_new(file_len).unwrap(); +/// // try to decode the metadata. If more data is needed, the decoder will tell you what ranges +/// loop { +/// match decoder.try_decode() { +/// Ok(DecodeResult::Data(metadata)) => { return Ok(metadata); } // decode successful +/// Ok(DecodeResult::NeedsData(ranges)) => { +/// // The decoder needs more data +/// // +/// // In this example, we call a function that returns the bytes for each given range. +/// // In a real application, you would likely read the data from a file or network. +/// let data = ranges.iter().map(|range| get_range(range)).collect(); +/// // Push the data into the decoder and try to decode again on the next iteration. +/// decoder.push_ranges(ranges, data).unwrap(); +/// } +/// Ok(DecodeResult::Finished) => { unreachable!("returned metadata in previous match arm") } +/// Err(e) => return Err(e), +/// } +/// } +/// # } +/// ``` +/// +/// # Example with "prefetching" +/// +/// By default, the [`ParquetMetaDataPushDecoder`] will requests only the exact byte +/// ranges it needs. This minimizes the number of bytes read, however it +/// requires at least two IO operations to read the metadata - one to read the +/// footer and then one to read the metadata. +/// +/// If the file has a "Page Index" (see [Self::with_page_indexes]), three +/// IO operations are required to read the metadata, as the page index is +/// not part of the normal metadata footer. +/// +/// To reduce the number of IO operations in systems with high per operation +/// overhead (e.g. cloud storage), you can "prefetch" the data and then push +/// the data into the decoder before calling [`Self::try_decode`]. If you do +/// not push enough bytes, the decoder will return the ranges that are still +/// needed. +/// +/// This approach can also be used when you the entire file already in memory +/// for other reasons. +/// +/// ```rust +/// # use std::ops::Range; +/// # use bytes::Bytes; +/// # use arrow_array::record_batch; +/// # use parquet::DecodeResult; +/// # use parquet::arrow::ArrowWriter; +/// # use parquet::errors::ParquetError; +/// # use parquet::file::metadata::{ParquetMetaData, ParquetMetaDataPushDecoder}; +/// # +/// # fn decode_metadata() -> Result<ParquetMetaData, ParquetError> { +/// # let file_bytes = { +/// # let mut buffer = vec![0]; +/// # let batch = record_batch!(("a", Int32, [1, 2, 3])).unwrap(); +/// # let mut writer = ArrowWriter::try_new(&mut buffer, batch.schema(), None).unwrap(); +/// # writer.write(&batch).unwrap(); +/// # writer.close().unwrap(); +/// # Bytes::from(buffer) +/// # }; +/// # +/// let file_len = file_bytes.len() as u64; +/// // for this example, we "prefetch" all the bytes which we have in memory, +/// // but in a real application, you would likely read a chunk from the end +/// // for example 1MB. +/// let prefetched_bytes = file_bytes.clone(); +/// let mut decoder = ParquetMetaDataPushDecoder::try_new(file_len).unwrap(); +/// // push the prefetched bytes into the decoder +/// decoder.push_ranges(vec![0..file_len], vec![prefetched_bytes]).unwrap(); +/// // The decoder will now be able to decode the metadata. Note in a real application, +/// // unless you can guarantee that the pushed data is enough to decode the metadata, +/// // you still need to call `try_decode` in a loop until it returns `DecodeResult::Data` +/// // as shown in the previous example +/// match decoder.try_decode() { +/// Ok(DecodeResult::Data(metadata)) => { return Ok(metadata); } // decode successful] +/// other @ _ => { panic!("expected DecodeResult::Data, got: {other:?}") } +/// } +/// # } +/// ``` +/// +/// # Example using [`AsyncRead`] +/// +/// [`ParquetMetaDataPushDecoder`] is designed to work with any data source that can +/// provide byte ranges, including async IO sources. However, it does not +/// implement async IO itself. To use async IO, you simply write an async +/// wrapper around it that reads the required byte ranges and pushes them into the +/// decoder. +/// +/// ```rust +/// # use std::ops::Range; +/// # use bytes::Bytes; +/// use tokio::io::{AsyncRead, AsyncReadExt, AsyncSeek, AsyncSeekExt}; +/// # use arrow_array::record_batch; +/// # use parquet::DecodeResult; +/// # use parquet::arrow::ArrowWriter; +/// # use parquet::errors::ParquetError; +/// # use parquet::file::metadata::{ParquetMetaData, ParquetMetaDataPushDecoder}; +/// # +/// // this function decodes Parquet Metadata from anything that implements +/// // [`AsyncRead`] and [`AsyncSeek`] such as a tokio::fs::File +/// async fn decode_metadata( +/// file_len: u64, +/// mut async_source: impl AsyncRead + AsyncSeek + Unpin +/// ) -> Result<ParquetMetaData, ParquetError> { +/// // We need a ParquetMetaDataPushDecoder to decode the metadata. +/// let mut decoder = ParquetMetaDataPushDecoder::try_new(file_len).unwrap(); +/// loop { +/// match decoder.try_decode() { +/// Ok(DecodeResult::Data(metadata)) => { return Ok(metadata); } // decode successful +/// Ok(DecodeResult::NeedsData(ranges)) => { +/// // The decoder needs more data +/// // +/// // In this example we use the AsyncRead and AsyncSeek traits to read the +/// // required ranges from the async source. +/// let mut data = Vec::with_capacity(ranges.len()); +/// for range in &ranges { +/// let mut buffer = vec![0; (range.end - range.start) as usize]; +/// async_source.seek(std::io::SeekFrom::Start(range.start)).await?; +/// async_source.read_exact(&mut buffer).await?; +/// data.push(Bytes::from(buffer)); +/// } +/// // Push the data into the decoder and try to decode again on the next iteration. +/// decoder.push_ranges(ranges, data).unwrap(); +/// } +/// Ok(DecodeResult::Finished) => { unreachable!("returned metadata in previous match arm") } +/// Err(e) => return Err(e), +/// } +/// } +/// } +/// ``` +/// [`AsyncRead`]: tokio::io::AsyncRead +#[derive(Debug)] +pub struct ParquetMetaDataPushDecoder { + done: bool, + metadata_reader: ParquetMetaDataReader, Review Comment: I wholeheartedly endorse this idea. 😄 -- This is an automated message from the Apache Git Service. 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