SIMD JSON for Rust  

Rust port of extremely fast simdjson JSON parser with Serde compatibility.

---

simd-json is a Rust port of the simdjson c++ library. It follows most of the design closely with a few exceptions to make it better fit into the Rust ecosystem.

Goals

The goal of the Rust port of simdjson is not to create a one-to-one copy, but to integrate the principles of the C++ library into a Rust library that plays well with the Rust ecosystem. As such we provide both compatibility with Serde as well as parsing to a DOM to manipulate data.

Performance

As a rule of thumb this library tries to get as close as possible to the performance of the C++ implementation (currently tracking 0.2.x, work in progress). However, in some design decisions—such as parsing to a DOM or a tape—ergonomics is prioritized over performance. In other places Rust makes it harder to achieve the same level of performance.

To take advantage of this library your system needs to support SIMD instructions. On x86, it will select the best available supported instruction set (avx2 or sse4.2) when the runtime-detection feature is enabled (default). On aarch64 this library uses the NEON instruction set. On wasm this library uses the simd128 instruction set when available. When no supported SIMD instructions are found, this library will use a fallback implementation, but this is significantly slower.

Allocator

For best performance, we highly suggest using mimalloc or jemalloc instead of the system allocator used by default. Another recent allocator that works well (but we have yet to test it in production) is snmalloc.

Safety

simd-json uses a lot of unsafe code.

There are a few reasons for this:

• SIMD intrinsics are inherently unsafe. These uses of unsafe are inescapable in a library such as simd-json.
• We work around some performance bottlenecks imposed by safe rust. These are avoidable, but at a performance cost. This is a more considered path in simd-json.

simd-json goes through extra scrutiny for unsafe code. These steps are:

• Unit tests - to test 'the obvious' cases, edge cases, and regression cases
• Structural constructive property based testing - We generate random valid JSON objects to exercise the full simd-json codebase stochastically. Floats are currently excluded since slightly different parsing algorithms lead to slightly different results here. In short "is simd-json correct".
• Data-oriented property-based testing of string-like data - to assert that sequences of legal printable characters don't panic or crash the parser (they might and often error so - they are not valid JSON!)
• Destructive Property based testing - make sure that no illegal byte sequences crash the parser in any way
• Fuzzing - fuzz based on upstream & jsonorg simd pass/fail cases
• Miri testing for UB

This doesn't ensure complete safety nor is at a bulletproof guarantee, but it does go a long way to assert that the library is of high production quality and fit for purpose for practical industrial applications.

Features

Various features can be enabled or disabled to tweak various parts of this library. Any features not mentioned here are for internal configuration and testing.

runtime-detection (default)

This feature allows selecting the optimal algorithm based on available features during runtime. It has no effect on non-x86 platforms. When neither AVX2 nor SSE4.2 is supported, it will fall back to a native Rust implementation.

Disabling this feature (with default-features = false) and setting RUSTFLAGS="-C target-cpu=native will result in better performance but the resulting binary will not be portable across x86 processors.

serde_impl (default)

Enable Serde support. This consist of implementing serde::Serializer and serde::Deserializer, allowing types that implement serde::Serialize/serde::Deserialize to be constructed/serialized to BorrowedValue/OwnedValue. In addition, this provides the same convenience functions that serde_json provides.

Disabling this feature (with default-features = false) will remove serde and serde_json from the dependencies.

swar-number-parsing (default)

Enables a parsing method that will parse 8 digits at a time for floats. This is a common pattern but comes at a slight performance hit if most of the float have less than 8 digits.

known-key

The known-key feature changes the hash mechanism for the DOM representation of the underlying JSON object from ahash to fxhash. The ahash hasher is faster at hashing and provides protection against DOS attacks by forcing multiple keys into a single hashing bucket. The fxhash hasher allows for repeatable hashing results, which in turn allows memoizing hashes for well known keys and saving time on lookups. In workloads that are heavy on accessing some well-known keys, this can be a performance advantage.

The known-key feature is optional and disabled by default and should be explicitly configured.

value-no-dup-keys

This flag has no effect on simd-json itself but purely affects the Value structs.

The value-no-dup-keys feature flag enables stricter behavior for objects when deserializing into a Value. When enabled, the Value deserializer will remove duplicate keys in a JSON object and only keep the last one. If not set duplicate keys are considered undefined behavior and Value will not make guarantees on its behavior.

big-int-as-float

The big-int-as-float feature flag treats very large integers that won't fit into u64 as f64 floats. This prevents parsing errors if the JSON you are parsing contains very large integers. Keep in mind that f64 loses some precision when representing very large numbers.

128bit

Add support for parsing and serializing 128-bit integers. This feature is disabled by default because such large numbers are rare in the wild and adding the support incurs a performance penalty.

beef

Enabling this feature can break dependencies in your dependency tree that are using simd-json.

Replace std::borrow::Cow with beef::lean::Cow This feature is disabled by default, because it is a breaking change in the API.

ordered-float

By default the representation of Floats used in borrowed::Value and owned::Value is simply a value of f64. This however has the normally-not-a-big-deal side effect of not having these Value types be std::cmp::Eq. This does, however, introduce some incompatibilities when offering simd-json as a quasi-drop-in replacement for serde-json.

So, this feature changes the internal representation of Floats to be an f64 wrapped by an Eq-compatible adapter.

This probably carries with it some small performance trade-offs, hence its enablement by feature rather than by default.

portable

Currently disabled

An highly experimental implementation of the algorithm using std::simd and up to 512 byte wide registers.

Usage

simd-json offers three main entry points for usage:

Values API

The values API is a set of optimized DOM objects that allow parsed JSON to JSON data that has no known variable structure. simd-json has two versions of this:

Borrowed Values

use simd_json;
let mut d = br#"{"some": ["key", "value", 2]}"#.to_vec();
let v: simd_json::BorrowedValue = simd_json::to_borrowed_value(&mut d).unwrap();

Owned Values

use simd_json;
let mut d = br#"{"some": ["key", "value", 2]}"#.to_vec();
let v: simd_json::OwnedValue = simd_json::to_owned_value(&mut d).unwrap();

Serde Compatible API

use simd_json;
use serde_json::Value;

let mut d = br#"{"some": ["key", "value", 2]}"#.to_vec();
let v: Value = simd_json::serde::from_slice(&mut d).unwrap();

Tape API

use simd_json;

let mut d = br#"{"the_answer": 42}"#.to_vec();
let tape = simd_json::to_tape(&mut d).unwrap();
let value = tape.as_value();
// try_get treats value like an object, returns Ok(Some(_)) because the key is found
assert!(value.try_get("the_answer").unwrap().unwrap() == 42);
// returns Ok(None) because the key is not found but value is an object
assert!(value.try_get("does_not_exist").unwrap() == None);
// try_get_idx treats value like an array, returns Err(_) because value is not an array
assert!(value.try_get_idx(0).is_err());

Other interesting things

There are also bindings for upstream simdjson available here

License

simd-json itself is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE)
• MIT license (LICENSE-MIT)

at your option.

However it ports a lot of code from simdjson so their work and copyright on that should also be respected.

The Serde integration is based on serde-json so their copyright should as well be respected.

All Thanks To Our Contributors: