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# Apache Arrow Official Native Rust Implementation

[![crates.io](https://img.shields.io/crates/v/arrow.svg)](https://crates.io/crates/arrow)
[![docs.rs](https://img.shields.io/docsrs/arrow.svg)](https://docs.rs/arrow/latest/arrow/)

This crate contains the official Native Rust implementation of [Apache Arrow][arrow] in memory format, governed by the Apache Software Foundation.

The [crate documentation](https://docs.rs/arrow/latest/arrow/) contains examples and full API.
There are several [examples](https://github.com/apache/arrow-rs/tree/master/arrow/examples) to start from as well.

## Rust Version Compatibility

This crate is tested with the latest stable version of Rust. We do not currently test against other, older versions.

## Versioning / Releases

The arrow crate follows the [SemVer standard](https://doc.rust-lang.org/cargo/reference/semver.html) defined by Cargo and works well within the Rust crate ecosystem.

However, for historical reasons, this crate uses versions with major numbers greater than `0.x` (e.g. `29.0.0`), unlike many other crates in the Rust ecosystem which spend extended time releasing versions `0.x` to signal planned ongoing API changes. Minor arrow releases contain only compatible changes, while major releases may contain breaking API changes.

## Feature Flags

The `arrow` crate provides the following features which may be enabled in your `Cargo.toml`:

- `csv` (default) - support for reading and writing Arrow arrays to/from csv files
- `json` (default) - support for reading and writing Arrow array to/from json files
- `ipc` (default) - support for reading [Arrow IPC Format](https://arrow.apache.org/docs/format/Columnar.html#serialization-and-interprocess-communication-ipc), also used as the wire protocol in [arrow-flight](https://crates.io/crates/arrow-flight)
- `ipc_compression`  - Enables reading and writing compressed IPC streams (also enables `ipc`)
- `prettyprint` - support for formatting record batches as textual columns
- `js` - support for building arrow for WebAssembly / JavaScript
- `simd` - (_Requires Nightly Rust_) Use alternate hand optimized
  implementations of some [compute](https://github.com/apache/arrow-rs/tree/master/arrow/src/compute/kernels)
  kernels using explicit SIMD instructions via [packed_simd_2](https://docs.rs/packed_simd_2/latest/packed_simd_2/).
- `chrono-tz` - support of parsing timezone using [chrono-tz](https://docs.rs/chrono-tz/0.6.0/chrono_tz/)
- `ffi` - bindings for the Arrow C [C Data Interface](https://arrow.apache.org/docs/format/CDataInterface.html)
- `pyarrow` - bindings for pyo3 to call arrow-rs from python
- `dyn_cmp_dict` - enables comparison of dictionary arrays within dyn comparison kernels
- `dyn_arith_dict` - enables arithmetic on dictionary arrays within dyn arithmetic kernels

## Arrow Feature Status

The [Apache Arrow Status](https://arrow.apache.org/docs/status.html) page lists which features of Arrow this crate supports.

## Safety

Arrow seeks to uphold the Rust Soundness Pledge as articulated eloquently [here](https://raphlinus.github.io/rust/2020/01/18/soundness-pledge.html). Specifically:

> The intent of this crate is to be free of soundness bugs. The developers will do their best to avoid them, and welcome help in analyzing and fixing them

Where soundness in turn is defined as:

> Code is unable to trigger undefined behavior using safe APIs

One way to ensure this would be to not use `unsafe`, however, as described in the opening chapter of the [Rustonomicon](https://doc.rust-lang.org/nomicon/meet-safe-and-unsafe.html) this is not a requirement, and flexibility in this regard is one of Rust's great strengths.

In particular there are a number of scenarios where `unsafe` is largely unavoidable:

- Invariants that cannot be statically verified by the compiler and unlock non-trivial performance wins, e.g. values in a StringArray are UTF-8, [TrustedLen](https://doc.rust-lang.org/std/iter/trait.TrustedLen.html) iterators, etc...
- FFI
- SIMD

Additionally, this crate exposes a number of `unsafe` APIs, allowing downstream crates to explicitly opt-out of potentially expensive invariant checking where appropriate.

We have a number of strategies to help reduce this risk:

- Provide strongly-typed `Array` and `ArrayBuilder` APIs to safely and efficiently interact with arrays
- Extensive validation logic to safely construct `ArrayData` from untrusted sources
- All commits are verified using [MIRI](https://github.com/rust-lang/miri) to detect undefined behaviour
- Use a `force_validate` feature that enables additional validation checks for use in test/debug builds
- There is ongoing work to reduce and better document the use of unsafe, and we welcome contributions in this space

## Building for WASM

Arrow can compile to WebAssembly using the `wasm32-unknown-unknown` and `wasm32-wasi` targets.

In order to compile Arrow for `wasm32-unknown-unknown` you will need to disable default features, then include the desired features, but exclude test dependencies (the `test_utils` feature). For example, use this snippet in your `Cargo.toml`:

```toml
[dependencies]
arrow = { version = "5.0", default-features = false, features = ["csv", "ipc", "simd"] }
```

## Examples

The examples folder shows how to construct some different types of Arrow
arrays, including dynamic arrays:

Examples can be run using the `cargo run --example` command. For example:

```bash
cargo run --example builders
cargo run --example dynamic_types
cargo run --example read_csv
```

[arrow]: https://arrow.apache.org/

## Performance Tips

Arrow aims to be as fast as possible out of the box, whilst not compromising on safety. However,
it relies heavily on LLVM auto-vectorisation to achieve this. Unfortunately the LLVM defaults,
particularly for x86_64, favour portability over performance, and LLVM will consequently avoid
using more recent instructions that would result in errors on older CPUs.

To address this it is recommended that you override the LLVM defaults either
by setting the `RUSTFLAGS` environment variable, or by setting `rustflags` in your
[Cargo configuration](https://doc.rust-lang.org/cargo/reference/config.html)

Enable all features supported by the current CPU

```ignore
RUSTFLAGS="-C target-cpu=native"
```

Enable all features supported by the current CPU, and enable full use of AVX512

```ignore
RUSTFLAGS="-C target-cpu=native -C target-feature=-prefer-256-bit"
```

Enable all features supported by CPUs more recent than haswell (2013)

```ignore
RUSTFLAGS="-C target-cpu=haswell"
```

For a full list of features and target CPUs use

```shell
$ rustc --print target-cpus
$ rustc --print target-features
```
