Shared Reducer Backend

Shared state management via websockets.

Designed to work with shared-reducer-frontend and json-immutability-helper.

Install dependency

npm install --save shared-reducer-backend json-immutability-helper

(if you want to use an alternative reducer, see the instructions below).

When using this with shared-reducer-frontend, ensure both dependencies are at the same major version (e.g. both are 2.x or both are 3.x). The API may change between major versions.

Usage

This project is compatible with websocket-express, but can also be used in isolation.

With websocket-express

import {
  Broadcaster,
  websocketHandler,
  InMemoryModel,
  ReadWrite,
} from 'shared-reducer-backend';
import context from 'json-immutability-helper';
import WebSocketExpress from 'websocket-express';

const model = new InMemoryModel();
const broadcaster = Broadcaster.for(model)
  .withReducer(context)
  .build();
model.set('a', { foo: 'v1' });

const app = new WebSocketExpress();
const server = app.listen(0, 'localhost');

const handler = websocketHandler(broadcaster);
app.ws('/:id', handler((req) => req.params.id, () => ReadWrite));

For real use-cases, you will probably want to add authentication middleware to the expressjs chain, and you may want to give some users read-only and others read-write access, which can be achieved in the second lambda.

Alone

import { Broadcaster, InMemoryModel } from 'shared-reducer-backend';
import context from 'json-immutability-helper';

const model = new InMemoryModel();
const broadcaster = Broadcaster.for(model)
  .withReducer(context)
  .build();
model.set('a', { foo: 'v1' });

// ...

const subscription = await broadcaster.subscribe(
  'a',
  (change, meta) => { /*...*/ },
);

const begin = subscription.getInitialData();
await subscription.send(['=', { foo: 'v2' }]);
// callback provided earlier is invoked

await subscription.close();

Persisting data

A convenience wrapper is provided for use with collection-storage, or you can write your own implementation of the Model interface to link any backend.

import {
  Broadcaster,
  CollectionStorageModel,
} from 'shared-reducer-backend';
import context from 'json-immutability-helper';
import CollectionStorage from 'collection-storage';

const db = await CollectionStorage.connect('memory://something');
const model = new CollectionStorageModel(
  db.getCollection('foo'),
  'id',
  // a function which takes in an object and returns it if valid,
  // or throws if invalid (protects stored data from malicious changes)
  MY_VALIDATOR,
);
const broadcaster = Broadcaster.for(model)
  .withReducer(context)
  .build();

Note that the provided validator MUST verify structural integrity (e.g. ensuring no unexpected fields are added or types are changed).

WebSocket protocol

The websocket protocol is minimal:

Messages received

P (ping): Can be sent periodically to keep the connection alive. Sends a "Pong" message in response immediately.

{"change": <spec>, "id": <id>}: Defines a delta. The ID will be reflected back once the change has been applied. Other clients will not receive the ID.

Messages sent

p (pong): Reponse to a ping. May also be sent unsolicited.

{"init": <state>}: The first message sent by the server, in response to a successful connection.

{"change": <spec>}: Sent whenever another client has changed the server state.

{"change": <spec>, "id": <id>}: Sent whenever the current client has changed the server state. Note that the spec and ID will match the client-sent values.

{"error": <message>, "id": <id>}: Sent if the server rejects a client-initiated change.

If this is returned, the server state will not have changed (i.e. the entire spec failed).

Specs

The specs need to match whichever reducer you are using. In the examples above, that is json-immutability-helper.

Alternative reducer

To enable different features of json-immutability-helper, you can customise it before passing it to withReducer. For example, to enable list commands such as updateWhere and mathematical commands such as Reverse Polish Notation (rpn):

import { Broadcaster, InMemoryModel } from 'shared-reducer-backend';
import listCommands from 'json-immutability-helper/commands/list';
import mathCommands from 'json-immutability-helper/commands/math';
import context from 'json-immutability-helper';

const broadcaster = Broadcaster.for(new InMemoryModel())
  .withReducer(context.with(listCommands, mathCommands))
  .build();

If you want to use an entirely different reducer, create a wrapper and pass it to withReducer:

import { Broadcaster, InMemoryModel } from 'shared-reducer-backend';

const myReducer = {
  update: (value, spec) => {
    // return a new value which is the result of applying
    // the given spec to the given value (or throw an error)
  },
  combine: (specs) => {
    // return a new spec which is equivalent to applying
    // all the given specs in order
    // (this is not used by the backend but is used by
    // shared-reducer-frontend to reduce data transfers)
  },
};

const broadcaster = Broadcaster.for(new InMemoryModel())
  .withReducer(myReducer)
  .build();

Be careful when using your own reducer to avoid introducing security vulnerabilities; the functions will be called with untrusted input, so should be careful to avoid attacks such as code injection or prototype pollution.

Other customisations

The Broadcaster builder has other settable properties:

• withSubscribers: specify a custom keyed broadcaster, used for communicating changes to all consumers. Required interface:

  {
    add(key, listener) {
      // add the listener function to key
    },
    remove(key, listener) {
      // remove the listener function from key
    },
    broadcast(key, message) {
      // call all current listener functions for key with
      // the parameter message
    },
  }

  All functions can be asynchronous or synchronous.

  The main use-case for overriding this would be to share messages between multiple servers for load balancing, but note that in most cases you probably want to load balance documents rather than users for better scalability.

• withTaskQueues: specify a custom task queue, used to ensure operations happen in the correct order. Required interface:

  {
    push(key, task) {
      // add the (possibly asynchronous) task to the queue
      // for the given key
    },
  }

  The default implementation will execute the task if it is the first task in a particular queue. If there is already a task in the queue, it will be stored and executed once the existing tasks have finished. Once all tasks for a particular key have finished, it will remove the queue.

  As with withSubscribers, the main reason to override this is to provide consistency if multiple servers are able to modify the same document simultaneously.

• withIdProvider: specify a custom unique ID provider. Required interface:

  {
    get() {
      // return a unique string (must be synchronous)
    },
  }

  The returned ID is used internally and passed through the configured taskQueues to identify the source of a change. It is not revealed to users. The default implementation uses a fixed random prefix followed by an incrementing number, which should be sufficient for most use cases.