1 unstable release
0.1.0 | Feb 15, 2023 |
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#718 in WebAssembly
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31KB
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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
- Can be added in rustup installations using
wasm2wat
parser- Available in many distributions in the
wabt
package
- Available in many distributions in the
Compilation
We recommend a setup with cargo.
- Start with a library package
cargo new --lib
- Add the following lines to the Cargo.toml to set the crate-type to cdylib
[lib]
crate-type = ["cdylib"]
- Implement your package, exporting Scylla UDFs using the
scylla_udf::export_udf
macro. - Build the package using the wasm32-wasi target:
cargo build --target=wasm32-wasi
- Find the compiled
.wasm
binary. Let's assume it'starget/wasm32-wasi/debug/abc.wasm
. - (optional) Optimize the binary using
wasm-opt -O3 target/wasm32-wasi/debug/abc.wasm
(can be combined with usingcargo build --release
profile) - 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
CQL Type | Rust type |
---|---|
ASCII | String |
BIGINT | i64 |
BLOB | Vec<u8> |
BOOLEAN | bool |
COUNTER | scylla_udf::Counter |
DATE | chrono::NaiveDate |
DECIMAL | bigdecimal::Decimal |
DOUBLE | f64 |
DURATION | scylla_udf::CqlDuration |
FLOAT | f32 |
INET | std::net::IpAddr |
INT | i32 |
SMALLINT | i16 |
TEXT | String |
TIME | scylla_udf::Time |
TIMESTAMP | scylla_udf::Timestamp |
TIMEUUID | uuid::Uuid |
TINYINT | i8 |
UUID | uuid::Uuid |
VARCHAR | String |
VARINT | num_bigint::BigInt |
Collections
If a CQL type T
maps to Rust type RustT
, you can use it as a collection parameter:
CQL Type | Rust type |
---|---|
LIST<T> | Vec<RustT> |
MAP<T> | std::collections::BTreeMap<RustT>, std::collections::HashMap<RustT> |
SET<T> | Vec<RustT>, std::collections::BTreeSet<RustT>, std::collections::HashSet<RustT> |
Tuples
If CQL types T1
, T2
, ... map to Rust types RustT1
, RustT2
, ..., you can use them in tuples:
CQL Type | Rust type |
---|---|
TUPLE<T1, T2, ...> | (RustT1, RustT2, ...) |
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
Dependencies
~7.5MB
~141K SLoC