kiwi-schema

Kiwi Message Format

Kiwi is a schema-based binary format for efficiently encoding trees of data. It's inspired by Google's Protocol Buffer format but is simpler, has a more compact encoding, and has better support for optional fields.

Goals:

• Efficient encoding of common values: Variable-length encoding is used for numeric values where small values take up less space.
• Efficient encoding of compound objects: The struct feature supports nested objects with zero encoding overhead.
• Presence of optional fields is detectable: This is not possible with Protocol Buffers, especially for repeated fields.
• Linearly serializable: Reading and writing are both single-scan operations so they are cache-efficient and have guaranteed time complexity.
• Backwards compatibility: New versions of the schema can still read old data.
• Simple implementation: The API is very minimal and the generated C++ code only depends on a single file.

Non-goals:

• Forwards compatibility: Old versions of the schema cannot read new data.
• Optimal bit-packing: Compression can be used after encoding for more space savings if needed.

Native Types

• bool: A value that stores either true or false. Will use 1 byte.
• byte: An unsigned 8-bit integer value. Uses 1 byte, obviously.
• int: A 32-bit integer value stored using a variable-length encoding optimized for storing numbers with a small magnitude. Will use at most 5 bytes.
• uint: A 32-bit integer value stored using a variable-length encoding optimized for storing small non-negative numbers. Will use at most 5 bytes.
• float: A 32-bit floating-point number. Normally uses 4 bytes but a value of zero uses 1 byte (denormal numbers become zero when encoded).
• string: A UTF-8 null-terminated string. Will use at least 1 byte.
• T[]: Any type can be made into an array using the [] suffix.

User Types

• enum: A uint with a restricted set of values that are identified by name. New fields can be added to any message while maintaining backwards compatibility.
• struct: A compound value with a fixed set of fields that are always required and written out in order. New fields cannot be added to a struct once that struct is in use.
• message: A compound value with optional fields. A field can be made required using the required keyword. New fields can be added to any message while maintaining backwards compatibility.

Example Schema

enum Type {
  FLAT = 0;
  ROUND = 1;
  POINTED = 2;
}

struct Color {
  byte red;
  byte green;
  byte blue;
  byte alpha;
}

message Example {
  required uint clientID = 1;
  Type type = 2;
  Color[] colors = 3;
}

Live Demo

See http://evanw.github.io/kiwi/ for a live demo of the schema compiler.

JavaScript Usage

Make sure to install the kiwi package beforehand using npm install kiwi-schema.

var kiwi = require('kiwi-schema');
var schema = kiwi.compileSchema([
  'message Test {',
  '  int x = 1;',
  '}',
].join('\n'));

var buffer = schema.encodeTest({x: 123});
var test = schema.decodeTest(buffer);

if (test.x !== undefined) {
  console.log('x is %d', test.x);
}

C++ Usage

Make sure to generate the C++ code beforehand using something like kiwic --schema test.kiwi --cpp test.h.

#define IMPLEMENT_KIWI_H
#define IMPLEMENT_SCHEMA_H

#include <stdio.h>
#include "test.h"

int main() {
  Test test;
  test.set_x(123);

  kiwi::ByteBuffer buffer;
  bool encode_success = test.encode(buffer);

  Test test2;
  kiwi::MemoryPool pool;
  bool decode_success = test2.decode(buffer, pool);

  if (test2.x()) {
    printf("x is %d\n", *test2.x());
  }

  return 0;
}

Skew Usage

Make sure to generate the Skew code beforehand using something like kiwic --schema test.kiwi --skew test.sk.

@import
var console dynamic

@entry
def main int {
  var test = Test.new
  test.x = 123

  var buffer = test.encode
  var test2 = Test.decode(buffer)

  if test2.has_x {
    console.log("x is \(test2.x)")
  }

  return 0
}