redux-scuttlebutt

scuttlebutt

This seems like a silly name, but I assure you, this is real science. — dominictarr/scuttlebutt

Efficient peer to peer reconciliation. We use it as the underlying protocol to share dispatched redux actions among peers, and eventually agree on their order in time. As actions from the past arrive, we replay history as if they had always existed.

A sample "server" peer is included, which might sync state changes to a database, write a persistent log, or manage system/world/NPC actors.

While it works great in a traditional client-server set up, you can flexibly upgrade/downgrade to peer-to-peer connections, go offline for minutes or days, and changes will sync when you next connect to another scuttlebutt instance.

Note, by default, scuttlebutt itself does not make any guarantees of security or identity: peer Bob is able to lie to Jane about Amy's actions. Security guarantees can added using the signAsync and verifyAsync] dispatcher options.

For more, read the Scuttlebutt paper.

dispatcher

Dispatcher is our Scuttlebutt model. It handles remote syncing of local actions, local dispatching of remote actions, and altering action history (rolling back to past checkpoints and replaying actions) as required.

Redux store enhancer

Our default export is the store enhancer. You use it like this:

// configureStore.js

import { createStore, applyMiddleware } from 'redux'

import rootReducer from '../reducers'
import scuttlebutt from 'redux-scuttlebutt'

export default (initialState) => {
  return createStore(rootReducer, initialState, scuttlebutt({
    uri: 'http://localhost:3000',
  }))
}

It wraps your store's root reducer (to store history), getState (to return the current state in history) and dispatch (to connect to peers).

If you're using the redux dev-tools enhancer, it must come after the redux- scuttlebutt enhancer (or scuttlebutt will emit PERFORM_ACTION actions over the network).

options

The store enhancer takes an options object, including the key dispatcherOptions which is passed directly through to the internal dispatcher:

scuttlebutt({
  // uri of a scuttlebutt peer or server
  uri: `${window.location.protocol}//${window.location.host}`,

  // options for primus.io <https://github.com/primus/primus#getting-started>
  primusOptions: {},

  // the Primus object, can be switched out with any compatible transport.
  primus: (typeof window === 'object' && window.Primus),

  // options passed through to the dispatcher (and their defaults)
  dispatcherOptions: {
    customDispatch: function getDelayedDispatch(dispatcher) {
      return function (action) {
        // the default will batch-reduce actions by the hundred, firing redux's
        // subscribe method on the last one, triggering the actual rendering on
        // the next animationFrame.
        // see: https://github.com/grrowl/redux-scuttlebutt/blob/master/src/dispatcher.js#L22
      }
    },

    isGossipType: function(actionType) {
      // returns a boolean representing whether an action's type should be
      // broadcast to the network.
      // (by default, returns false for actions prefixed with @@, such as @@INIT
      // and internal @@scuttlebutt-prefixed action types)
    },

    verifyAsync: function(callback, action, getStateHistory) {
      // if specified, the verifyAsync function must call callback(false) if the
      // action is invalid, or callback(true) if the action is valid.
      // getStateHistory() will return an array of ordered updates
    },

    signAsync: function(callback, action, getStateHistory) {
      // if specified, the signAsync will be called for every locally dispatched
      // action. must call callback(action) and can mutate the action if
      // desired.
      // getStateHistory() will return an array of ordered updates
    },
  }
})

signAsync & verifyAsync

The dispatcher options signAsync and verifyAsync allows you to add arbitrary metadata to actions as they are dispatched, and filter remote actions which are received from peers. This means you can validate any action against itself or the redux state, other actions in history, a cryptographic signature, rate limit, or any arbitrary rule.

For security, you can use redux-signatures to add Ed25519 signatures to your actions. This could be used to verify authors in a peering or mesh structure.

import { Ed25519, verifyAction, signAction } from 'redux-signatures'

const identity = new Ed25519()

scuttlebutt({
  uri: 'http://localhost:3000',
  signAsync: signAction.bind(this, identity),
  verifyAsync: verifyAction.bind(this, identity),
}))

The getStateHistory parameter returns an array of the form [UPDATE_ACTION, UPDATE_TIMESTAMP, UPDATE_SOURCE, UPDATE_SNAPSHOT]. These UPDATE_* constants are exported from scuttlebutt.

Note, if your verification is computationally expensive, you are responsible for throttling/delay (like you might for getDelayedDispatch).

conflict-free reducers

While redux-scuttlebutt facilitates action sharing and enhancing the store, it's the responsiblity of the app's reducers to apply actions. Overall your app must be strictly pure, without side effects or non-deterministic mutations.

In a complex real-time multi-user app, this is easier said than done. Some strategies may be,

• Avoid preconditions. The Game Of Life example only dispatches TOGGLE and STEP. Neither have preconditions, there's no "illegal" way to dispatch them, and they'll always successfully mutate state.
• Only allow peers (action sources) control over their own domain (entity). An entity might request something of another entity, which that entity would then dispatch its own action to mutate its own domain.
• Implement a Conflict-free data type, which only allows certain operations in exchange for never conflicting. See: https://github.com/pfrazee/crdt_notes#portfolio-of-basic-crdts
  • We'd love to expose the most useful and common ones from this library to assist with development.

example

Examples are found under examples/.

• counter: redux counter example with the addition of redux-scuttlebutt.
• chat: A very basic chat application.
• grrowl/redux-game-of-life-scuttlebutt: Conway's Game Of Life multiplayer edition.

roadmap and thoughts

• message validation on top of our existing scuttlebutt library
  • robust crypto in the browser comes with a number of performance and security tradeoffs, which we don't want to bake into the library itself.
  • our recommendation is to implement what's right for your implementation in userland.
  • have released an example of message signing with ed25519 signatures and asyncronous message validation in this gist.
  • released redux-signatures which plugs directly into the dispatcher.
    • Allows flexible implementation, e.g. in a client-server topology you may only want to use sign on the client and verify on the server only. This avoids running the most processor intensive part on the clients with no loss of security.
• underlying scuttlebutt implementation
  • currently depends on our own scuttlebutt fork, not yet published to npm, I'm not sure if dominictarr wants to accept these changes upstream.
  • should probably republish as scuttlebutt-logical
• add a @@scuttlebutt/COMPACTION action
  • reducers would receive the whole history array as state
  • enables removing multiple actions from history which are inconsequential — such as multiple "SET_VALUE" actions, when only the last one applies.
  • also enables forgetting, and therefore not replaying to other clients, actions after a certain threshold.
• implement CRDT helpers for reducers to easily implement complex shared data types.
• tests
  • simulate a multi-hop distributed network with delay, ensure consistency
  • ensure rewind/reordering works
  • ensure API
• allow pluggable socket library/transport
• more example applications! something real-time, event driven.
• WebRTC support
  • Genericize server into websockets and webrtc versions
  • Write client modules to support either

contributions

Contributions very welcomed. This project is still in its very early, experimental stages.

A major aim of this project is to be able to drop this middleware into an existing, compatible project and have it "just work". Additional features should be configurable in redux-scuttlebutt itself or at the highest level of the application without heavy modification with the redux application's structure/actions/reducers

licence

MIT. Without open source projects like React, Redux, Scuttlebutt, and all the amazing technology which has been the bedrock and inspiration for this project, many wonderful things in this world wouldn't exist.