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https://github.com/davidje13/shared-reducer

Shared state management via websockets.
https://github.com/davidje13/shared-reducer

reducer websocket

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Shared state management via websockets.

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README 

        
# Shared Reducer



Shared state management via websockets.



Designed to work with

[json-immutability-helper](https://github.com/davidje13/json-immutability-helper).



## Install dependency



```bash

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

```



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



## Usage (Backend)



This project is compatible with [websocket-express](https://github.com/davidje13/websocket-express),

but can also be used in isolation.



### With websocket-express



```javascript

import {

  Broadcaster,

  WebsocketHandlerFactory,

  InMemoryModel,

  ReadWrite,

} from 'shared-reducer/backend';

import context from 'json-immutability-helper';

import { WebSocketExpress } from 'websocket-express';



const model = new InMemoryModel();

const broadcaster = new Broadcaster(model, context);

model.set('a', { foo: 'v1' });



const app = new WebSocketExpress();

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



const handlerFactory = new WebsocketHandlerFactory(broadcaster);

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



const server = app.listen();



// later, to shutdown gracefully:

// send a close signal to all clients and wait up to 1 second for acknowledgement:

await handlerFactory.close(1000);

server.close();

```



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



```javascript

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

import context from 'json-immutability-helper';



const model = new InMemoryModel();

const broadcaster = new Broadcaster(model, context);

model.set('a', { foo: 'v1' });



// ...



const subscription = await broadcaster.subscribe('a');



const begin = subscription.getInitialData();

subscription.listen((change, meta) => { /*...*/ });

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](https://github.com/davidje13/collection-storage), or you can write your own

implementation of the `Model` interface to link any backend.



```javascript

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 = new Broadcaster(model, context);

```



Note that the provided validator MUST verify structural integrity (e.g. ensuring no unexpected

fields are added or types are changed).



## Usage (Frontend)



```javascript

import { SharedReducer } from 'shared-reducer/frontend';

import context from 'json-immutability-helper';



const reducer = new SharedReducer(context, () => ({

  url: 'ws://destination',

  token: 'my-token',

}));



reducer.addStateListener((state) => {

  console.log('latest state is', state);

});



reducer.addEventListener('connected', () => {

  console.log('connected / reconnected');

});



reducer.addEventListener('disconnected', (e) => {

  console.log('connection lost', e.detail.code, e.detail.reason);

});



reducer.addEventListener('warning', (e) => {

  console.log('latest change failed', e.detail);

});



const dispatch = reducer.dispatch;



dispatch([

  { a: ['=', 8] },

]);



dispatch([

  (state) => {

    return [{ a: ['=', Math.pow(2, state.a)] }];

  },

]);



dispatch([

  (state) => {

    console.log('state after handling is', state);

    return [];

  },

]);



dispatch(

  [{ a: ['add', 1] }],

  (state) => console.log('state after syncing is', state),

  (message) => console.warn('failed to sync', message),

);



dispatch([

  { a: ['add', 1] },

  { a: ['add', 1] },

]);

```



### Specs



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

[json-immutability-helper](https://github.com/davidje13/json-immutability-helper).



## WebSocket protocol



The websocket protocol is minimal:



### Client-to-server



- ``: The authentication token is sent as the first message when the connection is

  established. This is plaintext. The server should respond by either terminating the connection (if

  the token is deemed invalid), or with an `init` event which defines the latest state in its

  entirety. If no token is specified using `withToken`, no message will be sent (when not using

  authentication, it is assumed the server will send the `init` event unprompted).



- `P` (ping): Can be sent periodically to keep the connection alive. The server sends a "Pong"

  message in response immediately.



- `{"change": , "id": }`: Defines a delta. This may contain the aggregate result of many

  operations performed on the client. The ID is an opaque identifier which is reflected back to the

  same client in the confirmation message. Other clients will not receive the ID.



- `x` (close ack): Sent by the client in response to `X` (closing). Indicates that the client will

  not send any more messages on this connection (but may still be expecting some responses to

  existing messages).



### Server-to-client



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



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



- `{"change": }`: Sent whenever another client has changed the server state.



- `{"change": , "id": }`: Sent whenever the current client has changed the server state.

  Note that the spec and ID will match the client-sent values.



  The IDs sent by different clients can coincide, so the ID is only reflected to the client which

  sent the spec.



- `{"error": , "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).



- `X` (closing): Sent when the server is about to shut down. The client should respond with `x` and

  not send any more messages on the current connection. Any currently in-flight messages will be

  acknowledged on a best-effort basis by the server. The server might not wait for the acknowledging

  `x` message before closing the connection.



### Specs



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

[json-immutability-helper](https://github.com/davidje13/json-immutability-helper).



## Alternative reducer



To enable different features of `json-immutability-helper`, you can customise it before passing it

to the constructor. For example, to enable list commands such as `updateWhere` and mathematical

commands such as Reverse Polish Notation (`rpn`):



```javascript

// Backend

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 = new Broadcaster(

  new InMemoryModel(),

  context.with(listCommands, mathCommands),

);

```



```javascript

// Frontend

import { SharedReducer } from 'shared-reducer/frontend';

import listCommands from 'json-immutability-helper/commands/list';

import mathCommands from 'json-immutability-helper/commands/math';

import context from 'json-immutability-helper';



const reducer = new SharedReducer(

  context.with(listCommands, mathCommands),

  () => ({ url: 'ws://destination' }),

);

```



If you want to use an entirely different reducer, create a wrapper:



```javascript

import context from 'json-immutability-helper';



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

  },

};



// Backend

const broadcaster = new Broadcaster(new InMemoryModel(), myReducer);



// Frontend

const reducer = new SharedReducer(myReducer, () => ({ url: 'ws://destination' }));

```



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 (Backend)



The `Broadcaster` constructor can also take some optional arguments:



```javascript

new Broadcaster(model, reducer[, options]);

```



- `options.subscribers`: specify a custom keyed broadcaster, used for communicating changes to all

consumers. Required interface:



  ```javascript

  {

    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.



- `options.taskQueues`: specify a custom task queue, used to ensure operations happen in the correct

  order. Required interface:



  ```javascript

  {

    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 `subscribers`, the main reason to override this is to provide consistency if multiple

  servers are able to modify the same document simultaneously.



- `options.idProvider`: specify a custom unique ID provider. Must be a function which returns a

  unique string ID when called. Can be asynchronous.



  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.



## Other customisations (Frontend)



If the connection is lost, the frontend will attempt to reconnect automatically. By default this

uses an exponential backoff with a small amount of randomness, as well as attempting to connect if

the page regains focus or the computer rejoins a network. You can fully customise this behaviour:



```javascript

import { SharedReducer, OnlineScheduler, exponentialDelay } from 'shared-reducer/frontend';



const reducer = new SharedReducer(context, () => ({ url: 'ws://destination' }), {

  scheduler: new OnlineScheduler(

    exponentialDelay({

      base: 2,

      initialDelay: 200,

      maxDelay: 10 * 60 * 1000,

      randomness: 0.3,

    }),

    20 * 1000, // timeout for each connection attempt

  ),

});

```



The `exponentialDelay` helper returns:



```

min(initialDelay * (base ^ attempt), maxDelay) * (1 - random(randomness))

```



All delay values are in milliseconds.



You can also provide a custom function instead of `exponentialDelay`; it will be given the current

attempt number (0-based), and should return the number of milliseconds to wait before triggering the

attempt.



Finally, by default when reconnecting `SharedReducer` will replay all messages which have not been

confirmed (`AT_LEAST_ONCE` delivery). You can change this to `AT_MOST_ONCE` or a custom mechanism:



```javascript

import { SharedReducer, AT_MOST_ONCE } from 'shared-reducer/frontend';



const reducer = new SharedReducer(context, () => ({ url: 'ws://destination' }), {

  deliveryStrategy: AT_MOST_ONCE,

});

```



Custom strategies can be defined as functions:



```javascript

function myCustomDeliveryStrategy(serverState, spec, hasSent) {

  return true; // re-send all (equivalent to AT_LEAST_ONCE)

}

```



- `serverState` is the new state from the server after reconnecting.

- `spec` is a spec that has not been confirmed as delivered to the server.

- `hasSent` is `true` if the spec has already been sent to the server (but no delivery confirmation

  was received). It is `false` if the message was never sent to the server.



Note that the function will be invoked multiple times (once for each change that is pending). It

should return `true` for messages to resend, and `false` for messages to drop.



## Older versions



For older versions of this library, see the separate

[backend](https://github.com/davidje13/shared-reducer-backend) and

[frontend](https://github.com/davidje13/shared-reducer-frontend) repositories.