scylladb/docs/dev/wasm.md

WASM support for user-defined functions

This document describes the details of WASM language support in user-defined functions (UDF). The language wasm is one of the possible languages to use, besides Lua, to implement these functions.

Experimental status

Before the design of WebAssembly integration and ABI is finalized, it's only available in experimental mode. User-defined functions are already experimental at the time of this writing, but in order to be ready for backward incompatible changes, the language accepted by CQL is currently named "xwasm". Once the ABI is set in stone, it should be changed to "wasm".

ABI versions

Different programming languages may require different ABIs. To support that, the wasm program is required to export the symbol "_scylla_abi", that is a WebAssembly global with a 32-bit value of the offset in memory, where the version number can be read (that's the only method of exporting a constant in Rust).

Currently, the only available ABI versions are 1 and 2. Both of them use the same protocol for passing parameters and returning values, but they differ in approaches to memory management.

Memory management

The memory management differs depending on the used ABI verison:

• version 1 - There are no requirements of the usage of memory by the user. The host grows memory for each of parameters and does not free the memory in any way.
• version 2 - The user program is required to export "_scylla_malloc" and "_scylla_free" methods, which are then used by the host for allocating memory for parameters and freeing memory for the returned value. The user is required to free the memory allocated for parameters using the "_scylla_free" method, and allocate the memory for result using the "_scylla_malloc" method (both can be achieved by using the provided helper libraries). Alternatively, the user may return one of the arguments, shifting the responsibility of freeing it to the host. The "_scylla_malloc" and "_scylla_free" methods may be simple wrappers of "malloc" and "free" methods implemented by default when compiling with WASI.

Supported types

Due to the limitations imposed by WebAssembly specification, the following types can be natively supported with CQL:

• int
• bigint
• smallint
• tinyint
• bool
• float
• double Specifically, the int, bigint, float and double types are represented as WASM i32, i64, f32 and f64 types, while the smallint, tinyint and bool are represented as the WASM i32.

The rest of CQL types (text, date, timestamp, etc.) are implemented by putting their serialized representation into wasm module memory and passing for each parameter a 64-bit value, of which top 32 bits are its size and its bottom 32 bits are a pointer to its serialized representation, like below:

{
    int32_t size;
    int32_t ptr;
    int64_t param = ((int64_t)size << 32) | ptr;
}

Support for NULL values

Native WebAssembly types can only be represented directly if the function does not operate on NULL values. Fortunately, user-defined functions explicitly specify whether they accept NULL or not.

If the function is specified not to accept NULL, all parameters and return values are represented as in the description above.

If the function is specified to accept NULL, parameters and return values of both natively and non-natively supported types are represented using their serialized representation, also decribed above.

The important distinction is that size equal to -1 (minus one or 0xffffffff) indicates that the value is NULL and should not be parsed.

NOTE: CQL syntax extensions and new helper libraries may be deployed together with new ABI versions, the description below only refers to ABI versions 1 and 2.

Currently, returning NULL values is possible only for functions declared to be CALLED ON NULL INPUT. This decision allows returning some values as native WebAssembly types without having to allocate memory for them and serialize them first. Alternative ways of expressing whether a function can return null should be considered - perhaps as CQL syntax extension.

How to generate a correct wasm UDF source code

Scylla accepts UDF's source code in WebAssembly text format - also known as wat. The source can use and define whatever's needed for execution, including multiple helper functions and symbols. The requirements for it to be accepted as correct UDF source is that the WebAssembly module exports a symbol with the same name as the function, this symbol's type is indeed a function with correct signature, the module exports a _scylla_abi global and all symbols related to the selected ABI version.

UDF's source code can be, naturally, simply coded by hand in wat. It is not often very convenient to program directly in assembly, so here are a few tips.

Compiling to wasm

Rust

The main supported language for WASM UDFs is Rust. To generate WebAssembly from rust, it's best to use the scylla-udf Rust helper library, and follow the instructions present there.

As a short example, here's a sample Rust code which can be compiled to WebAssembly:

use scylla_udf::export_udf;

#[export_udf]
pub fn fib(n: i32) -> i32 {
    if n < 2 {
        n
    } else {
        fib(n - 1) + fib(n - 2)
    }
}

The compilation instructions are described at https://github.com/scylladb/scylla-rust-udf but the commands will generally be:

 cargo build --target=wasm32-wasi
 wasm2wat target/wasm32-wasi/debug/fib.wasm > fib.wat

C

Clang is capable of compiling C source code to wasm and it also supports useful built-ins for using wasm-specific interfaces, like __builtin_wasm_memory_size and __builtin_wasm_memory_grow for memory management.

However, there is no C helper library yet, so implementing UDFs in C is in general much more difficult than in Rust. Just to implement the fib() function, you need something like this:

#include<stdlib.h>

const int WASM_PAGE_SIZE = 64 * 1024;

const int _scylla_abi = 2;

void* _scylla_malloc(int size) {
    return malloc(size);
}

void _scylla_free(void* ptr) {
    free(ptr);
}

static long long swap_int64(long long val) {
    val = ((val << 8) & 0xFF00FF00FF00FF00ULL ) | ((val >> 8) & 0x00FF00FF00FF00FFULL );
    val = ((val << 16) & 0xFFFF0000FFFF0000ULL ) | ((val >> 16) & 0x0000FFFF0000FFFFULL );
    return (val << 32) | ((val >> 32) & 0xFFFFFFFFULL);
}

long long fib_aux(long long n) {
    if (n < 2) {
        return n;
    }
    return fib_aux(n-1) + fib_aux(n-2);
}

long long fib(long long p) {
    int size = p >> 32;
    long long* p_val = (long long*)(p & 0xffffffff);
    // Initialize memory for the return value
    long long* ret_val = _scylla_malloc(sizeof(long long));
    if (size == -1) {
        *ret_val = swap_int64(42);
    } else {
        *ret_val = swap_int64(fib_aux(swap_int64(*p_val)));
    }
    _scylla_free(p_val);
    // 8 is the size of a bigint
    return (long long)(8ll << 32) | (long long)ret_val;
}
// using wasi in c/c++ requires adding a main function to the program
int main(){}

And compile it with:

 /path/to/wasm/supporting/c/compiler --sysroot=/path/to/wasi/sysroot -O2  --target=wasm32-wasi -Wl,--export=fib -Wl,--export=_scylla_abi -Wl,--export=_scylla_malloc -Wl,--export=_scylla_free -Wl,--no-entry fibnull.c -o fibnull.wasm
 wasm2wat fibnull.wasm > fibnull.wat

The main case when C may be the most convenient language of an UDF is when the UDF RETURNS NULL ON NULL INPUT and only takes WASM-compatible types (ints/doubles) as parameters and return values. In that case, the _scylla_free and _scylla_malloc don't need to be exported, _scylla_abi can be set to 1, and parameters/returns are not serialized:

const int _scylla_abi = 1;

long long fib(int n) {
    if (n < 2) {
        return n;
    }
    return fib(n-1) + fib(n-2);
}

// using wasi in c/c++ requires adding a main function to the program
int main(){}

Compilation instructions:

 /path/to/wasm/supporting/c/compiler --sysroot=/path/to/wasi/sysroot -O2  --target=wasm32-wasi -Wl,--export=fib -Wl,--export=_scylla_abi -Wl,--no-entry fib.c -o fib.wasm
 wasm2wat fibnull.wasm > fibnull.wat

The compiled example can be viewed at test/cql-pytest/test_wasm.py::test_docs_c

AssemblyScript

AssemblyScript is a TypeScript-like language that compiles to WebAsembly.

Install via npm:

npm install -g assemblyscript

Example source code:

export const _scylla_abi = [1]
export function fib(n: i32): i32 {
  if (n < 2) {
    return n
  }
  return fib(n - 1) + fib(n - 2)
}

Compile directly to WebAssembly Text Format with:

asc fib.ts --textFile fib.wat --optimize

The compiled example can be viewed at test/cql-pytest/test_wasm.py::test_docs_assemblyscript

Similarly to C, the AssemblyScript can only be conveniently used with "RETURNS NULL ON NULL INPUT" UDFs that only have WASM-compatible arguments/returns.

Generating wat from wasm

For those who want to use precompiled WASM modules, it's enough to translate WASM bytecode to wat representation. On Linux, it can be achieved by a wasm2wat tool, available in most distributions in the wabt package.

Example

Here's how a wasm function can be declared:

CREATE FUNCTION ks.fib (input bigint) RETURNS NULL ON NULL INPUT RETURNS bigint LANGUAGE xwasm
AS '(module
  (func $fib (param $n i64) (result i64)
    (if
      (i64.lt_s (local.get $n) (i64.const 2))
      (return (local.get $n))
    )
    (i64.add
      (call $fib (i64.sub (local.get $n) (i64.const 1)))
      (call $fib (i64.sub (local.get $n) (i64.const 2)))
    )
  )
  (export "fib" (func $fib))
  (global (;0;) i32 (i32.const 1024))
  (export "_scylla_abi" (global 0))
  (data $.rodata (i32.const 1024) "\\01")
)'

and it can be invoked just like a regular UDF:

scylla@cqlsh:ks> CREATE TABLE t(id int, n bigint, PRIMARY KEY(id,n));
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 0);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 1);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 2);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 3);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 4);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 5);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 6);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 7);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 8);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 9);
scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 10);
scylla@cqlsh:ks> SELECT n, ks.fib(n) FROM t;

 n  | ks.fib(n)
----+-----------
  0 |         0
  1 |         1
  2 |         1
  3 |         2
  4 |         3
  5 |         5
  6 |         8
  7 |        13
  8 |        21
  9 |        34
 10 |        55

(11 rows)