Rust helper library for Scylla UDFs

This crate allows writing pure Rust functions that can be used as Scylla UDFs.

Note: this crate is officially supported and ready to use. However, UDFs are still an experimental feature in ScyllaDB, and the crate has not been widely used, which is why it's still in beta and its API is subject to change. We appreciate bug reports and pull requests!

Usage

Prerequisites

To use this helper library in Scylla you'll need:

• Standard library for Rust wasm32-wasi
  • Can be added in rustup installations using rustup target add wasm32-wasi
  • For non rustup setups, you can try following the steps at https://rustwasm.github.io/docs/wasm-pack/prerequisites/non-rustup-setups.html
  • Also available as an rpm: rust-std-static-wasm32-wasi
• wasm2wat parser
  • Available in many distributions in the wabt package

Compilation

We recommend a setup with cargo.

1. Start with a library package

cargo new --lib

2. Add the following lines to the Cargo.toml to set the crate-type to cdylib

[lib]
crate-type = ["cdylib"]

3. Implement your package, exporting Scylla UDFs using the scylla_udf::export_udf macro.
4. Build the package using the wasm32-wasi target:

cargo build --target=wasm32-wasi

5. Find the compiled .wasm binary. Let's assume it's target/wasm32-wasi/debug/abc.wasm.
6. (optional) Optimize the binary using wasm-opt -O3 target/wasm32-wasi/debug/abc.wasm (can be combined with using cargo build --release profile)
7. Translate the binary into wat:

wasm2wat target/wasm32-wasi/debug/abc.wasm > target/wasm32/wasi/debug/abc.wat

CQL Statement

The resulting target/wasm32/wasi/debug/abc.wat code can now be used directly in a CREATE FUNCTION statement. The resulting code will most likely contain ' characters, so it may be necessary to first replace them with '', so that they're usable in a CQL string.

For example, if you have an Rust UDF that joins a list of words using commas, you can create a Scylla UDF using the following statement:

CREATE FUNCTION commas(string list<text>) CALLED ON NULL INPUT RETURNS text AS ' (module ...) '

CQL Type Mapping

The argument and return value types used in functions annotated with #[export_udf] must all map to CQL types used in the CREATE FUNCTION statements used in Scylla, according to the tables below.

If the Scylla function is created with types that do not match the types used in the Rust function, calling the UDF will fail or produce arbitrary results.

Native types

Collections

If a CQL type T maps to Rust type RustT, you can use it as a collection parameter:

Tuples

If CQL types T1, T2, ... map to Rust types RustT1, RustT2, ..., you can use them in tuples:

Nulls

If a CQL Value of type T, that's mapped to type RustT, may be a null (possible in non-RETURNS NULL ON NULL INPUT UDFs), the type used in the Rust function should be Option<RustT>.

Contributing

In general, try to follow the same rules as in https://github.com/scylladb/scylla-rust-driver/blob/main/CONTRIBUTING.md

Testing

This crate is meant to be compiled to a wasm32-wasi target and ran in a WASM runtime. The tests that use WASM-specific code will most likely not succeed when executed in a different way (in particular, with a simple cargo test command).

For example, if you have the wasmtime runtime installed and in PATH, you can use the following command to run tests:

CARGO_TARGET_WASM32_WASI_RUNNER="wasmtime --allow-unknown-exports" cargo test --target=wasm32-wasi