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.
- Forwards compatibility: Old versions of the schema can optionally read new data if a copy of the new schema is bundled with the data (the new schema lets the decoder skip over unknown fields).
- Simple implementation: The API is very minimal and the generated C++ code only depends on a single file.
Non-goals:
- 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.
- int64: A 64-bit integer value stored using a variable-length encoding optimized for storing numbers with a small magnitude. Will use at most 9 bytes.
- uint64: A 64-bit integer value stored using a variable-length encoding optimized for storing small non-negative numbers. Will use at most 9 bytes.
- 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. 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 {
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.
Usage Examples
Pick a language:
