Arroyo SQL can be extended with user-defined functions. Currently, only Rust UDFs are supported. UDFs can be defined as part of the SQL API call, or via the Web UI.

UDFs are defined as Rust functions. The parameters and return type of the UDF are determined by the definition of the function. All types must be valid SQL data types.

Arroyo UDFs are annotated with the #[udf] in the arroyo_udf_plugin crate.

Here’s an example of a simple UDF that squares an integer:

use arroyo_udf_plugin::udf;

#[udf]
fn my_square(x: i64) -> i64 {
  x * x
}

Once registered, this UDF can be used in SQL queries:

SELECT my_square(auction.bid)
FROM nexmark;

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Creating UDFs

UDFs are developed and managed in the Web UI, within the Pipeline editor. To create a new UDF, navigate to the UDFs tab and click “New.”

The UDF creation interface

The name for the UDF is determined automatically from the function name that is annotated with #[udf]. You may include helper functions, structs, and other definitions so long as only one function has the #[udf] annotation.

As you’re developing, you may make use of the “Check” button to validate the UDF definition. Any Rust compilation errors will be included in the errors box at the bottom of the UI.

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Nullability

Nullability handling is controlled via the type signature of the UDF parameters and return types. If a parameter is an Option type (for example Option<i64>), then it will be called with all inputs, even if they are NULL. If the parameter is not an Option type (for example i64), then it will only be called with non-NULL inputs.

Similarly, if the return type is an Option type, then the output type is nullable, otherwise it is non-nullable.

In table form:

InputParameter typeReturn typeCalled onNullability
NullableTTNon-null valuesNullable
NullableOption<T>TAll valuesNon-null
NullableTOption<T>Non-null valuesNullable
NullableOption<T>Option<T>All valuesNullable
Non-nullTTAll valuesNon-null
Non-nullOption<T>TAll valuesNon-null
Non-nullTOption<T>All valuesNullable
Non-nullOption<T>Option<T>All valuesNullable

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Dependencies

UDFs can depend on external crates. To add dependencies, you can define a special comment in the UDF definition like this:

/*
[dependencies]
regex = "1.10.2"
*/

use arroyo_udf_plugin::udf;
use regex::Regex;

#[udf]
fn my_regex_matches(s: String) -> bool {
    let re = Regex::new(r"test").unwrap();
    re.is_match(&s)
}

Internally, the contents of the [dependencies] comment are used to generate a Cargo.toml file for the UDF. See the Cargo.toml reference for more details on the syntax.

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Global UDFs

UDFs may be either local—associated with a particular pipeline—or global, in which case they are stored in the database and may be used in any pipeline.

All UDFs begin as local. If you’ve written a UDF that will benefit from being globally available, you can make it global by hovering over the tab containing the UDF, and clicking the “Globalize” button in the drop down.

The process of making a global UDF

Global UDFs are available in all pipelines, but will be shadowed by local UDFs with the same name.

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Performance considerations

UDFs are expected to run and return quickly (think microseconds, not milliseconds). This means they shouldn’t do any blocking work (like making network requests) or CPU-intensive computations. Within the Arroyo dataflow, UDFs are treated as normal scalar functions. They run serially against a batch of data, and while they run forward progress is blocked.

If you need to use IO, CPU-intensive computations, or other usecases for long-running UDFs, see async UDFs.

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Debugging UDFs

If UDFs panic, they will produce log lines in the worker logs. These are not currently forwarded to the Web UI, so you will need to find the logs where the workers are running. For the default process scheduler, this will be in the controller logs. For the kubernetes scheduler, this will be within the actual worker pods.

Window FunctionsUDAFs