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Add example reading data from an
mmaped IPC file
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| // 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. | ||
|
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| //! Example of reading arrow IPC files and streams using "zero copy" API | ||
| //! | ||
| //! Zero copy in this case means the Arrow arrays refer directly to a user | ||
| //! provided buffer or memory region. | ||
| use arrow::array::{record_batch, RecordBatch}; | ||
| use arrow::error::Result; | ||
| use arrow_buffer::Buffer; | ||
| use arrow_cast::pretty::pretty_format_batches; | ||
| use arrow_ipc::convert::fb_to_schema; | ||
| use arrow_ipc::reader::{read_footer_length, FileDecoder}; | ||
| use arrow_ipc::writer::FileWriter; | ||
| use arrow_ipc::{root_as_footer, Block}; | ||
| use std::path::PathBuf; | ||
| use std::sync::Arc; | ||
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| /// This example shows how to read data from an Arrow IPC file without copying | ||
| /// using `mmap` and the [`FileDecoder`] API | ||
| fn main() { | ||
| // Create a temporary file with 3 record batches | ||
| let ipc_path = ipc_file(); | ||
| // Open the file and memory map it using the mmap2 crate: | ||
| let ipc_file = std::fs::File::open(&ipc_path.path).expect("failed to open file"); | ||
| let mmap = unsafe { memmap2::Mmap::map(&ipc_file).expect("failed to mmap file") }; | ||
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| // Convert the mmap region to an Arrow `Buffer` to back the arrow arrays. We | ||
| // do this by first creating a `bytes::Bytes` (which is zero copy) and then | ||
| // creating a Buffer from the `Bytes` (which is also zero copy) | ||
| let bytes = bytes::Bytes::from_owner(mmap); | ||
| let buffer = Buffer::from(bytes); | ||
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| // Now, use the FileDecoder API (wrapped by `IPCBufferDecoder` for | ||
| // convenience) to crate Arrays re-using the data in the underlying buffer | ||
| let decoder = IPCBufferDecoder::new(buffer); | ||
| assert_eq!(decoder.num_batches(), 3); | ||
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| // Create the Arrays and print them | ||
| for i in 0..decoder.num_batches() { | ||
| let batch = decoder.get_batch(i).unwrap().expect("failed to read batch"); | ||
| assert_eq!(3, batch.num_rows()); | ||
| println!("Batch {i}\n{}", pretty_format_batches(&[batch]).unwrap()); | ||
| } | ||
| } | ||
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| /// Return 3 [`RecordBatch`]es with a single column of type `Int32` | ||
| fn example_data() -> Vec<RecordBatch> { | ||
| vec![ | ||
| record_batch!(("my_column", Int32, [1, 2, 3])).unwrap(), | ||
| record_batch!(("my_column", Int32, [4, 5, 6])).unwrap(), | ||
| record_batch!(("my_column", Int32, [7, 8, 9])).unwrap(), | ||
| ] | ||
| } | ||
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| /// Return a temporary file that contains an IPC file with 3 [`RecordBatch`]es | ||
| fn ipc_file() -> TempFile { | ||
| let path = PathBuf::from("example.arrow"); | ||
| let file = std::fs::File::create(&path).unwrap(); | ||
| let data = example_data(); | ||
| let mut writer = FileWriter::try_new(file, &data[0].schema()).unwrap(); | ||
| for batch in &data { | ||
| writer.write(batch).unwrap(); | ||
| } | ||
| writer.finish().unwrap(); | ||
| TempFile { path } | ||
| } | ||
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| /// Incrementally decodes [`RecordBatch`]es from an IPC file stored in a Arrow | ||
| /// [`Buffer`] using the [`FileDecoder`] API. | ||
| /// | ||
| /// This is a wrapper around the example in the `FileDecoder` which handles the | ||
| /// low level interaction with the Arrow IPC format. | ||
| struct IPCBufferDecoder { | ||
| /// Memory (or memory mapped) Buffer with the data | ||
| buffer: Buffer, | ||
| /// Decoder that reads Arrays that refers to the underlying buffers | ||
| decoder: FileDecoder, | ||
| /// Location of the batches within the buffer | ||
| batches: Vec<Block>, | ||
| } | ||
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| impl IPCBufferDecoder { | ||
| fn new(buffer: Buffer) -> Self { | ||
| let trailer_start = buffer.len() - 10; | ||
| let footer_len = read_footer_length(buffer[trailer_start..].try_into().unwrap()).unwrap(); | ||
| let footer = root_as_footer(&buffer[trailer_start - footer_len..trailer_start]).unwrap(); | ||
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| let schema = fb_to_schema(footer.schema().unwrap()); | ||
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| let mut decoder = FileDecoder::new(Arc::new(schema), footer.version()); | ||
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| // Read dictionaries | ||
| for block in footer.dictionaries().iter().flatten() { | ||
| let block_len = block.bodyLength() as usize + block.metaDataLength() as usize; | ||
| let data = buffer.slice_with_length(block.offset() as _, block_len); | ||
| decoder.read_dictionary(block, &data).unwrap(); | ||
| } | ||
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| // convert to Vec from the flatbuffers Vector to avoid having a direct dependency on flatbuffers | ||
| let batches = footer | ||
| .recordBatches() | ||
| .map(|b| b.iter().copied().collect()) | ||
| .unwrap_or_default(); | ||
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| Self { | ||
| buffer, | ||
| decoder, | ||
| batches, | ||
| } | ||
| } | ||
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| /// Return the number of [`RecordBatch`]es in this buffer | ||
| fn num_batches(&self) -> usize { | ||
| self.batches.len() | ||
| } | ||
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| /// Return the [`RecordBatch`] at message index `i`. | ||
| /// | ||
| /// This may return `None` if the IPC message was None | ||
| fn get_batch(&self, i: usize) -> Result<Option<RecordBatch>> { | ||
| let block = &self.batches[i]; | ||
| let block_len = block.bodyLength() as usize + block.metaDataLength() as usize; | ||
| let data = self | ||
| .buffer | ||
| .slice_with_length(block.offset() as _, block_len); | ||
| self.decoder.read_record_batch(block, &data) | ||
| } | ||
| } | ||
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| /// This structure deletes the file when it is dropped | ||
| struct TempFile { | ||
| path: PathBuf, | ||
| } | ||
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| impl Drop for TempFile { | ||
| fn drop(&mut self) { | ||
| if let Err(e) = std::fs::remove_file(&self.path) { | ||
| println!("Error deleting '{:?}': {:?}", self.path, e); | ||
| } | ||
| } | ||
| } |