colyseus/schema

An incremental binary state serializer with delta encoding for games.
Although it was born to be used on Colyseus, this library can be used as standalone.

Defining Schema

As Colyseus is written in TypeScript, the schema is defined as type annotations inside the state class. Additional server logic may be added to that class, but client-side generated (not implemented) files will consider only the schema itself.

import { Schema, type, ArraySchema, MapSchema } from '@colyseus/schema';

export class Player extends Schema {
  @type("string")
  name: string;

  @type("number")
  x: number;

  @type("number")
  y: number;
}

export class State extends Schema {
  @type('string')
  fieldString: string;

  @type('number') // varint
  fieldNumber: number;

  @type(Player)
  player: Player;

  @type([ Player ])
  arrayOfPlayers: ArraySchema<Player>;

  @type({ map: Player })
  mapOfPlayers: MapSchema<Player>;
}

See example.

Supported types

Primitive Types

Declaration:

Primitive types (string, number, boolean, etc)

@type("string")
name: string;

@type("int32")
name: number;

Custom Schema type

@type(Player)
player: Player;

Array of custom Schema type

@type([ Player ])
arrayOfPlayers: ArraySchema<Player>;

Array of a primitive type

You can't mix types inside arrays.

@type([ "number" ])
arrayOfNumbers: ArraySchema<number>;

@type([ "string" ])
arrayOfStrings: ArraySchema<string>;

Map of custom Schema type

@type({ map: Player })
mapOfPlayers: MapSchema<Player>;

Map of a primitive type

You can't mix types inside maps.

@type({ map: "number" })
mapOfNumbers: MapSchema<number>;

@type({ map: "string" })
mapOfStrings: MapSchema<string>;

Backwards/forwards compability

Backwards/fowards compatibility is possible by declaring new fields at the end of existing structures, and earlier declarations to not be removed, but be marked @deprecated() when needed.

This is particularly useful for native-compiled targets, such as C#, C++, Haxe, etc - where the client-side can potentially not have the most up-to-date version of the schema definitions.

Reflection

The Schema definitions can encode itself through Reflection. You can have the definition implementation in the server-side, and just send the encoded reflection to the client-side, for example:

import { Schema, type, Reflection } from "@colyseus/schema";

class MyState extends Schema {
  @type("string")
  currentTurn: string;

  // more definitions relating to more Schema types.
}

// send `encodedStateSchema` across the network
const encodedStateSchema = Reflection.encode(new MyState());

// instantiate `MyState` in the client-side, without having its definition:
const myState = Reflection.decode(encodedStateSchema);

Data filters

On the example below, considering we're making a card game, we are filtering the cards to be available only for the owner of the cards, or if the card has been flagged as "revealed".

import { Schema, type, filter } from "@colyseus/schema";

export class State extends Schema {
  @filterChildren(function(client: any, key: string, value: Card, root: State) {
      return (value.ownerId === client.sessionId) || value.revealed;
  })
  @type({ map: Card })
  cards = new MapSchema<Card>();
}

Limitations and best practices

• Each Schema structure can hold up to 64 fields. If you need more fields, use nested structures.
• NaN or null numbers are encoded as 0
• null strings are encoded as ""
• Infinity numbers are encoded as Number.MAX_SAFE_INTEGER
• Multi-dimensional arrays are not supported.
• Items inside Arrays and Maps must be all instance of the same type.
• @colyseus/schema encodes only field values in the specified order.
  • Both encoder (server) and decoder (client) must have same schema definition.
  • The order of the fields must be the same.
• Avoid manipulating indexes of an array. This result in at least 2 extra bytes for each index change. Example: If you have an array of 20 items, and remove the first item (through shift()) this means 38 extra bytes to be serialized.

Generating client-side schema files (for strictly typed languages)

If you're using JavaScript or LUA, there's no need to bother about this. Interpreted programming languages are able to re-build the Schema locally through the use of Reflection.

You can generate the client-side schema files based on the TypeScript schema definitions automatically.

# C#/Unity
schema-codegen ./schemas/State.ts --output ./unity-project/ --csharp

# C/C++
schema-codegen ./schemas/State.ts --output ./cpp-project/ --cpp

# Haxe
schema-codegen ./schemas/State.ts --output ./haxe-project/ --haxe

Benchmarks:

Decoder implementations

Decoders for each target language are located at /decoders/. They have no third party dependencies.

Why

Initial thoghts/assumptions, for Colyseus:

• little to no bottleneck for detecting state changes.
• have a schema definition on both server and client
• better experience on staticaly-typed languages (C#, C++)
• mutations should be cheap.

Practical Colyseus issues this should solve:

• Avoid decoding large objects that haven't been patched
• Allow to send different patches for each client
• Better developer experience on statically-typed languages

Inspiration:

• Protocol Buffers
• flatbuffers
• schemapack
• avro

License

MIT