Native Rust implementation of Apache Arrow

Welcome to the implementation of Arrow, the popular in-memory columnar format, in Rust.

This part of the Arrow project is divided in 4 main components:

Independently, they support a vast array of functionality for in-memory computations.

Together, they allow users to write an SQL query or a DataFrame (using datafusion crate), run it against a parquet file (using parquet crate), evaluate it in-memory using Arrow's columnar format (using the arrow crate), and send to another process (using arrow-flight crate).

Generally speaking, the arrow crate offers the functionality to develop code that uses Arrow arrays, and datafusion offers most operations typically found in SQL, with the notable exceptions of:

• join
• window functions

There are too many features to enumerate here, but some notable mentions:

• Arrow implements all formats in the specification except certain dictionaries
• Arrow supports SIMD operations to some of its vertical operations
• DataFusion supports async execution
• DataFusion supports user-defined functions, aggregates, and whole execution nodes

You can find more details about each crate on their respective READMEs.

Developer's guide to Arrow Rust

Before running tests and examples, it is necessary to set up the local development environment.

Git Submodules

The tests rely on test data that is contained in git submodules.

To pull down this data run the following:

git submodule update --init

This populates data in two git submodules:

• ../cpp/submodules/parquet_testing/data (sourced from https://github.com/apache/parquet-testing.git)
• ../testing (sourced from https://github.com/apache/arrow-testing)

To run the tests of the whole crate, create two new environment variables to point to these directories as follows:

export PARQUET_TEST_DATA=../cpp/submodules/parquet-testing/data
export ARROW_TEST_DATA=../testing/data

To run the tests of an individual crate within the project (e.g. in datafusion/), adjust the path accordingly:

export PARQUET_TEST_DATA=../../cpp/submodules/parquet-testing/data
export ARROW_TEST_DATA=../../testing/data

from here on, this is a pure Rust project and cargo can be used to run tests, benchmarks, docs and examples as usual.

Code Formatting

Our CI uses rustfmt to check code formatting. Although the project is built and tested against nightly rust we use the stable version of rustfmt. So before submitting a PR be sure to run the following and check for lint issues:

cargo +stable fmt --all -- --check

Clippy Lints

We recommend using clippy for checking lints during development. While we do not yet enforce clippy checks, we recommend not introducing new clippy errors or warnings.

Run the following to check for clippy lints.

cargo clippy

If you use Visual Studio Code with the rust-analyzer plugin, you can enable clippy to run each time you save a file. See https://users.rust-lang.org/t/how-to-use-clippy-in-vs-code-with-rust-analyzer/41881.

One of the concerns with clippy is that it often produces a lot of false positives, or that some recommendations may hurt readability. We do not have a policy of which lints are ignored, but if you disagree with a clippy lint, you may disable the lint and briefly justify it.

Search for allow(clippy:: in the codebase to identify lints that are ignored/allowed. We currently prefer ignoring lints on the lowest unit possible.

• If you are introducing a line that returns a lint warning or error, you may disable the lint on that line.
• If you have several lints on a function or module, you may disable the lint on the function or module.
• If a lint is pervasive across multiple modules, you may disable it at the crate level.

CI and Dockerized builds

There are currently multiple CI systems that build the project and they all use the same docker image. It is possible to run the same build locally.

From the root of the Arrow project, run the following command to build the Docker image that the CI system uses to build the project.

docker-compose build debian-rust

Run the following command to build the project in the same way that the CI system will build the project. Note that this currently does cause some files to be written to your local workspace.

docker-compose run --rm debian-rust bash