arrow_extendr

arrow-extendr is a crate that facilitates the transfer of Apache Arrow memory between R and Rust. It utilizes extendr, the {nanoarrow} R package, and arrow-rs.

Motivating Example

Say we have the following DBI connection which we will send requests to using arrow. The result of dbGetQueryArrow() is a nanoarrow_array_stream. We want to count the number of rows in each batch of the steam using Rust.

# adapted from https://github.com/r-dbi/DBI/blob/main/vignettes/DBI-arrow.Rmd

library(DBI)
con <- dbConnect(RSQLite::SQLite())
data <- data.frame(
  a = runif(10000, 0, 10),
  b = rnorm(10000, 4.5),
  c = sample(letters, 10000, TRUE)
)

dbWriteTable(con, "tbl", data)

We can write an extendr function which creates an ArrowArrayStreamReader from an &Robj. In the function we instantiate a counter to keep track of the number of rows per chunk. For each chunk we print the number of rows.

use extendr_api::prelude::*;
use arrow_extendr::FromArrowRobj;
use arrow::ffi_stream::ArrowArrayStreamReader;

#[extendr]
/// @export
fn process_stream(stream: Robj) -> i32 {
    let rb = ArrowArrayStreamReader::from_arrow_robj(&stream)
        .unwrap();

    let mut n = 0;

    rprintln!("Processing `ArrowArrayStreamReader`...");
    for chunk in rb {
        let chunk_rows = chunk.unwrap().num_rows();
        rprintln!("Found {chunk_rows} rows");
        n += chunk_rows as i32;
    }

    n
}

With this function we can use it on the output of dbGetQueryArrow() or other Arrow related DBI functions.

query <- dbGetQueryArrow(con, "SELECT * FROM tbl WHERE a < 3")
process_stream(query)
#> Processing `ArrowArrayStreamReader`...
#> Found 256 rows
#> Found 256 rows
#> Found 256 rows
#> ... truncated ...
#> Found 256 rows
#> Found 256 rows
#> Found 143 rows
#> [1] 2959

Polars interop

arrow-extendr provides optional interop with Polars via versioned feature flags. Use the feature that matches your polars-core version:

Feature	polars-core version
polars	0.53 (alias for polars-53)
polars-53	0.53
polars-51	0.51

These features are mutually exclusive — enabling more than one will produce a compile error.

polars-core 0.53

arrow_extendr = { version = "58.0.1", features = ["polars-53"], default-features = false }
polars-core = "0.53.0"
anyhow = "1"

polars-core 0.51

arrow_extendr = { version = "58.0.1", features = ["polars-51"], default-features = false }
polars-core = "0.51.0"
anyhow = "1"

This enables the following conversions via the Arrow C Stream interface:

Type	Direction	R object
polars_core::frame::DataFrame	IntoArrowRobj	nanoarrow_array_stream
polars_core::frame::DataFrame	FromArrowRobj	nanoarrow_array_stream
polars_arrow::ffi::ArrowArrayStream	IntoArrowRobj	nanoarrow_array_stream
polars_arrow::ffi::ArrowArrayStreamReader	FromArrowRobj	nanoarrow_array_stream

Example: round-trip a Polars DataFrame through R

use extendr_api::prelude::*;
use anyhow::anyhow;
use arrow_extendr::{FromArrowRobj, IntoArrowRobj};
use polars_core::frame::DataFrame;

#[extendr]
/// @export
fn polars_round_trip(x: Robj) -> anyhow::Result<Robj> {
    let df = DataFrame::from_arrow_robj(&x)?;
    rprintln!("{df:?}");
    df.into_arrow_robj().map_err(|e| anyhow!("{e:?}"))
}

library(nanoarrow)

df <- data.frame(a = 1:5, b = letters[1:5])
stream <- as_nanoarrow_array_stream(df)
polars_round_trip(stream)

Using arrow-extendr in a package

To use arrow-extendr in an R package first create an R package and make it an extendr package with:

usethis::create_package("my_package")
rextendr::use_extendr();

Next, you have to ensure that nanoarrow is a dependency of the package since arrow-extendr will call functions from nanoarrow to convert between R and Arrow memory. To do this run usethis::use_package("nanoarrow") to add it to your Imports field in the DESCRIPTION file.

Versioning

At present, versions of arrow-rs are not compatible with each other. This means if your crate uses arrow-rs version 48.0.1, then the arrow-extendr must also use that same version. As such, arrow-extendr uses the same versions as arrow-rs so that it is easy to match the required versions you need.

Versions:

• 58.0.0
• 55.1.0
• 54.0.0
• 53.0.0
• 52.0.0
• 51.0.0
• 50.0.0 (compatible with geoarrow-rs 0.1.0)
• 49.0.0-geoarrow (not available on crates.io but is the current Git version)
• 48.0.1
• 49.0.0